Merge pull request #752 from quantified-uncertainty/reducer-typescript-wall

Reducer typescript wall
2022-06-24 05:43:21 -07:00 · 2022-06-24 05:43:21 -07:00 · aeea4806b4
commit aeea4806b4
parent 934608d971 3b84c2892c
50 changed files with 1107 additions and 772 deletions
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Category/Reducer_Category_Module_TypeChecker_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Category/Reducer_Category_Module_TypeChecker_test.res
@ -0,0 +1,4 @@
+open Jest
+open Expect
+
+test("todo", () => expect("1")->toBe("1"))
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltInMacros_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltInMacros_test.res
@ -19,23 +19,23 @@ describe("bindStatement", () => {
  testMacro(
    [],
    eBindStatement(eBindings([]), exampleStatementY),
-    "Ok((:$_setBindings_$ {} :y 1) context: {})",
+    "Ok((:$_setBindings_$ @{} :y 1) context: @{})",
  )
  // Then it answers the bindings for the next statement when reduced
-  testMacroEval([], eBindStatement(eBindings([]), exampleStatementY), "Ok({y: 1})")
+  testMacroEval([], eBindStatement(eBindings([]), exampleStatementY), "Ok(@{y: 1})")
  // Now let's feed a binding to see what happens
  testMacro(
    [],
-    eBindStatement(eBindings([("x", EvNumber(2.))]), exampleStatementX),
-    "Ok((:$_setBindings_$ {x: 2} :y 2) context: {x: 2})",
+    eBindStatement(eBindings([("x", IEvNumber(2.))]), exampleStatementX),
+    "Ok((:$_setBindings_$ @{x: 2} :y 2) context: @{x: 2})",
  )
  // An expression does not return a binding, thus error
  testMacro([], eBindStatement(eBindings([]), exampleExpression), "Assignment expected")
  // When bindings from previous statement are missing the context is injected. This must be the first statement of a block
  testMacro(
-    [("z", EvNumber(99.))],
+    [("z", IEvNumber(99.))],
    eBindStatementDefault(exampleStatementY),
-    "Ok((:$_setBindings_$ {z: 99} :y 1) context: {z: 99})",
+    "Ok((:$_setBindings_$ @{z: 99} :y 1) context: @{z: 99})",
  )
 })

@ -43,26 +43,26 @@ describe("bindExpression", () => {
  // x is simply bound in the expression
  testMacro(
    [],
-    eBindExpression(eBindings([("x", EvNumber(2.))]), eSymbol("x")),
-    "Ok(2 context: {x: 2})",
+    eBindExpression(eBindings([("x", IEvNumber(2.))]), eSymbol("x")),
+    "Ok(2 context: @{x: 2})",
  )
  // When an let statement is the end expression then bindings are returned
  testMacro(
    [],
-    eBindExpression(eBindings([("x", EvNumber(2.))]), exampleStatementY),
-    "Ok((:$_exportBindings_$ (:$_setBindings_$ {x: 2} :y 1)) context: {x: 2})",
+    eBindExpression(eBindings([("x", IEvNumber(2.))]), exampleStatementY),
+    "Ok((:$_exportBindings_$ (:$_setBindings_$ @{x: 2} :y 1)) context: @{x: 2})",
  )
  // Now let's reduce that expression
  testMacroEval(
    [],
-    eBindExpression(eBindings([("x", EvNumber(2.))]), exampleStatementY),
-    "Ok({x: 2,y: 1})",
+    eBindExpression(eBindings([("x", IEvNumber(2.))]), exampleStatementY),
+    "Ok(@{x: 2,y: 1})",
  )
  // When bindings are missing the context is injected. This must be the first and last statement of a block
  testMacroEval(
-    [("z", EvNumber(99.))],
+    [("z", IEvNumber(99.))],
    eBindExpressionDefault(exampleStatementY),
-    "Ok({y: 1,z: 99})",
+    "Ok(@{y: 1,z: 99})",
  )
 })

@ -72,7 +72,7 @@ describe("block", () => {
  testMacroEval([], eBlock(list{exampleExpression}), "Ok(1)")
  // Block with a single statement
  testMacro([], eBlock(list{exampleStatementY}), "Ok((:$$_bindExpression_$$ (:$_let_$ :y 1)))")
-  testMacroEval([], eBlock(list{exampleStatementY}), "Ok({y: 1})")
+  testMacroEval([], eBlock(list{exampleStatementY}), "Ok(@{y: 1})")
  // Block with a statement and an expression
  testMacro(
    [],
@ -86,7 +86,7 @@ describe("block", () => {
    eBlock(list{exampleStatementY, exampleStatementZ}),
    "Ok((:$$_bindExpression_$$ (:$$_bindStatement_$$ (:$_let_$ :y 1)) (:$_let_$ :z :y)))",
  )
-  testMacroEval([], eBlock(list{exampleStatementY, exampleStatementZ}), "Ok({y: 1,z: 1})")
+  testMacroEval([], eBlock(list{exampleStatementY, exampleStatementZ}), "Ok(@{y: 1,z: 1})")
  // Block inside a block
  testMacro([], eBlock(list{eBlock(list{exampleExpression})}), "Ok((:$$_bindExpression_$$ {1}))")
  testMacroEval([], eBlock(list{eBlock(list{exampleExpression})}), "Ok(1)")
@ -99,7 +99,7 @@ describe("block", () => {
  testMacroEval(
    [],
    eBlock(list{eLetStatement("z", eBlock(list{eBlock(list{exampleExpressionY})}))}),
-    "Ok({z: :y})",
+    "Ok(@{z: :y})",
  )
  // Empty block
  testMacro([], eBlock(list{}), "Ok(:undefined block)") //TODO: should be an error
@ -115,7 +115,7 @@ describe("block", () => {
    "Ok((:$$_bindExpression_$$ {(:$_let_$ :y (:add :x 1)); :y}))",
  )
  testMacroEval(
-    [("x", EvNumber(1.))],
+    [("x", IEvNumber(1.))],
    eBlock(list{
      eBlock(list{
        eLetStatement("y", eFunction("add", list{eSymbol("x"), eNumber(1.)})),
@ -135,12 +135,12 @@ describe("lambda", () => {
  testMacro([], callLambdaExpression, "Ok(((:$$_lambda_$$ [y] :y) 1))")
  testMacroEval([], callLambdaExpression, "Ok(1)")
  // Parameters shadow the outer scope
-  testMacroEval([("y", EvNumber(666.))], callLambdaExpression, "Ok(1)")
+  testMacroEval([("y", IEvNumber(666.))], callLambdaExpression, "Ok(1)")
  // When not shadowed by the parameters, the outer scope variables are available
  let lambdaExpression = eFunction(
    "$$_lambda_$$",
    list{eArrayString(["z"]), eFunction("add", list{eSymbol("y"), eSymbol("z")})},
  )
  let callLambdaExpression = eList(list{lambdaExpression, eNumber(1.)})
-  testMacroEval([("y", EvNumber(666.))], callLambdaExpression, "Ok(667)")
+  testMacroEval([("y", IEvNumber(666.))], callLambdaExpression, "Ok(667)")
 })
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res
@ -1,4 +1,4 @@
-module ExpressionValue = ReducerInterface.ExpressionValue
+module ExpressionValue = ReducerInterface.ExternalExpressionValue

 open Jest
 open Expect
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Helpers.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Helpers.res
@ -1,19 +1,22 @@
-module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+// Reducer_Helpers
 module ErrorValue = Reducer_ErrorValue
-module Bindings = Reducer_Category_Bindings
+module ExternalExpressionValue = ReducerInterface.ExternalExpressionValue
+module InternalExpressionValue = ReducerInterface.InternalExpressionValue
+module Module = Reducer_Category_Module

-let removeDefaults = (ev: ExpressionT.expressionValue): ExpressionT.expressionValue =>
-  switch ev {
-  | EvRecord(extbindings) => {
-      let bindings: Bindings.t = Bindings.fromRecord(extbindings)
-      let keys = Js.Dict.keys(Reducer.defaultExternalBindings)
-      Belt.Map.String.keep(bindings, (key, _value) => {
-        let removeThis = Js.Array2.includes(keys, key)
-        !removeThis
-      })->Bindings.toExpressionValue
-    }
+let removeDefaultsInternal = (iev: InternalExpressionValue.t) => {
+  switch iev {
+  | InternalExpressionValue.IEvModule(nameSpace) =>
+    Module.removeOther(
+      nameSpace,
+      ReducerInterface.StdLib.internalStdLib,
+    )->InternalExpressionValue.IEvModule
  | value => value
  }
+}

-let rRemoveDefaults = r => Belt.Result.map(r, ev => removeDefaults(ev))
+let removeDefaultsExternal = (ev: ExternalExpressionValue.t): ExternalExpressionValue.t =>
+  ev->InternalExpressionValue.toInternal->removeDefaultsInternal->InternalExpressionValue.toExternal
+
+let rRemoveDefaultsInternal = r => Belt.Result.map(r, removeDefaultsInternal)
+let rRemoveDefaultsExternal = r => Belt.Result.map(r, removeDefaultsExternal)
--- a/packages/squiggle-lang/tests/Reducer/Reducer_MathJs/Reducer_MathJsEval_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_MathJs/Reducer_MathJsEval_test.res
@ -1,4 +1,3 @@
-open ReducerInterface.ExpressionValue
 module MathJs = Reducer_MathJs
 module ErrorValue = Reducer.ErrorValue

@ -6,14 +5,14 @@ open Jest
 open ExpectJs

 describe("eval", () => {
-  test("Number", () => expect(MathJs.Eval.eval("1"))->toEqual(Ok(EvNumber(1.))))
-  test("Number expr", () => expect(MathJs.Eval.eval("1-1"))->toEqual(Ok(EvNumber(0.))))
-  test("String", () => expect(MathJs.Eval.eval("'hello'"))->toEqual(Ok(EvString("hello"))))
+  test("Number", () => expect(MathJs.Eval.eval("1"))->toEqual(Ok(IEvNumber(1.))))
+  test("Number expr", () => expect(MathJs.Eval.eval("1-1"))->toEqual(Ok(IEvNumber(0.))))
+  test("String", () => expect(MathJs.Eval.eval("'hello'"))->toEqual(Ok(IEvString("hello"))))
  test("String expr", () =>
-    expect(MathJs.Eval.eval("concat('hello ','world')"))->toEqual(Ok(EvString("hello world")))
+    expect(MathJs.Eval.eval("concat('hello ','world')"))->toEqual(Ok(IEvString("hello world")))
  )
-  test("Boolean", () => expect(MathJs.Eval.eval("true"))->toEqual(Ok(EvBool(true))))
-  test("Boolean expr", () => expect(MathJs.Eval.eval("2>1"))->toEqual(Ok(EvBool(true))))
+  test("Boolean", () => expect(MathJs.Eval.eval("true"))->toEqual(Ok(IEvBool(true))))
+  test("Boolean expr", () => expect(MathJs.Eval.eval("2>1"))->toEqual(Ok(IEvBool(true))))
 })

 describe("errors", () => {
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res
@ -1,6 +1,6 @@
 module Expression = Reducer_Expression
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface_ExpressionValue
+module ExpressionValue = ReducerInterface.InternalExpressionValue
 module Parse = Reducer_Peggy_Parse
 module Result = Belt.Result
 module ToExpression = Reducer_Peggy_ToExpression
@ -29,7 +29,7 @@ let expectToExpressionToBe = (expr, answer, ~v="_", ()) => {
          ExpressionValue.defaultEnvironment,
        )
      )
-      ->Reducer_Helpers.rRemoveDefaults
+      ->Reducer_Helpers.rRemoveDefaultsInternal
      ->ExpressionValue.toStringResultOkless
    (a1, a2)->expect->toEqual((answer, v))
  }
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression_test.res
@ -22,11 +22,11 @@ describe("Peggy to Expression", () => {

  describe("multi-line", () => {
    testToExpression("x=1; 2", "{(:$_let_$ :x {1}); 2}", ~v="2", ())
-    testToExpression("x=1; y=2", "{(:$_let_$ :x {1}); (:$_let_$ :y {2})}", ~v="{x: 1,y: 2}", ())
+    testToExpression("x=1; y=2", "{(:$_let_$ :x {1}); (:$_let_$ :y {2})}", ~v="@{x: 1,y: 2}", ())
  })

  describe("variables", () => {
-    testToExpression("x = 1", "{(:$_let_$ :x {1})}", ~v="{x: 1}", ())
+    testToExpression("x = 1", "{(:$_let_$ :x {1})}", ~v="@{x: 1}", ())
    testToExpression("x", "{:x}", ~v=":x", ()) //TODO: value should return error
    testToExpression("x = 1; x", "{(:$_let_$ :x {1}); :x}", ~v="1", ())
  })
@ -35,7 +35,7 @@ describe("Peggy to Expression", () => {
    testToExpression(
      "identity(x) = x",
      "{(:$_let_$ :identity (:$$_lambda_$$ [x] {:x}))}",
-      ~v="{identity: lambda(x=>internal code)}",
+      ~v="@{identity: lambda(x=>internal code)}",
      (),
    ) // Function definitions become lambda assignments
    testToExpression("identity(x)", "{(:identity :x)}", ()) // Note value returns error properly
@ -155,7 +155,7 @@ describe("Peggy to Expression", () => {
    testToExpression(
      "y=99; x={y=1; y}",
      "{(:$_let_$ :y {99}); (:$_let_$ :x {(:$_let_$ :y {1}); :y})}",
-      ~v="{x: 1,y: 99}",
+      ~v="@{x: 1,y: 99}",
      (),
    )
  })
@ -165,19 +165,19 @@ describe("Peggy to Expression", () => {
    testToExpression(
      "f={|x| x}",
      "{(:$_let_$ :f {(:$$_lambda_$$ [x] {:x})})}",
-      ~v="{f: lambda(x=>internal code)}",
+      ~v="@{f: lambda(x=>internal code)}",
      (),
    )
    testToExpression(
      "f(x)=x",
      "{(:$_let_$ :f (:$$_lambda_$$ [x] {:x}))}",
-      ~v="{f: lambda(x=>internal code)}",
+      ~v="@{f: lambda(x=>internal code)}",
      (),
    ) // Function definitions are lambda assignments
    testToExpression(
      "f(x)=x ? 1 : 0",
      "{(:$_let_$ :f (:$$_lambda_$$ [x] {(:$$_ternary_$$ :x 1 0)}))}",
-      ~v="{f: lambda(x=>internal code)}",
+      ~v="@{f: lambda(x=>internal code)}",
      (),
    )
  })
--- a/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression_type_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression_type_test.res
@ -6,7 +6,7 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "p: number",
      "{(:$_typeOf_$ :p #number)}",
-      ~v="{_typeReferences_: {p: #number}}",
+      ~v="@{_typeReferences_: {p: #number}}",
      (),
    )
  })
@ -14,7 +14,7 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "type index=number",
      "{(:$_typeAlias_$ #index #number)}",
-      ~v="{_typeAliases_: {index: #number}}",
+      ~v="@{_typeAliases_: {index: #number}}",
      (),
    )
  })
@ -22,7 +22,7 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "answer: number|string|distribution",
      "{(:$_typeOf_$ :answer (:$_typeOr_$ (:$_constructArray_$ (#number #string #distribution))))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeOr',typeOr: [#number,#string,#distribution]}}}",
+      ~v="@{_typeReferences_: {answer: {typeOr: [#number,#string,#distribution],typeTag: 'typeOr'}}}",
      (),
    )
  })
@ -30,13 +30,13 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "f: number=>number=>number",
      "{(:$_typeOf_$ :f (:$_typeFunction_$ (:$_constructArray_$ (#number #number #number))))}",
-      ~v="{_typeReferences_: {f: {typeTag: 'typeFunction',inputs: [#number,#number],output: #number}}}",
+      ~v="@{_typeReferences_: {f: {inputs: [#number,#number],output: #number,typeTag: 'typeFunction'}}}",
      (),
    )
    testToExpression(
      "f: number=>number",
      "{(:$_typeOf_$ :f (:$_typeFunction_$ (:$_constructArray_$ (#number #number))))}",
-      ~v="{_typeReferences_: {f: {typeTag: 'typeFunction',inputs: [#number],output: #number}}}",
+      ~v="@{_typeReferences_: {f: {inputs: [#number],output: #number,typeTag: 'typeFunction'}}}",
      (),
    )
  })
@ -44,37 +44,37 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "answer: number<-min(1)<-max(100)|string",
      "{(:$_typeOf_$ :answer (:$_typeOr_$ (:$_constructArray_$ ((:$_typeModifier_max_$ (:$_typeModifier_min_$ #number 1) 100) #string))))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeOr',typeOr: [{typeTag: 'typeIdentifier',typeIdentifier: #number,min: 1,max: 100},#string]}}}",
+      ~v="@{_typeReferences_: {answer: {typeOr: [{max: 100,min: 1,typeIdentifier: #number,typeTag: 'typeIdentifier'},#string],typeTag: 'typeOr'}}}",
      (),
    )
    testToExpression(
      "answer: number<-memberOf([1,3,5])",
      "{(:$_typeOf_$ :answer (:$_typeModifier_memberOf_$ #number (:$_constructArray_$ (1 3 5))))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeIdentifier',typeIdentifier: #number,memberOf: [1,3,5]}}}",
+      ~v="@{_typeReferences_: {answer: {memberOf: [1,3,5],typeIdentifier: #number,typeTag: 'typeIdentifier'}}}",
      (),
    )
    testToExpression(
      "answer: number<-min(1)",
      "{(:$_typeOf_$ :answer (:$_typeModifier_min_$ #number 1))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeIdentifier',typeIdentifier: #number,min: 1}}}",
+      ~v="@{_typeReferences_: {answer: {min: 1,typeIdentifier: #number,typeTag: 'typeIdentifier'}}}",
      (),
    )
    testToExpression(
      "answer: number<-max(10)",
      "{(:$_typeOf_$ :answer (:$_typeModifier_max_$ #number 10))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeIdentifier',typeIdentifier: #number,max: 10}}}",
+      ~v="@{_typeReferences_: {answer: {max: 10,typeIdentifier: #number,typeTag: 'typeIdentifier'}}}",
      (),
    )
    testToExpression(
      "answer: number<-min(1)<-max(10)",
      "{(:$_typeOf_$ :answer (:$_typeModifier_max_$ (:$_typeModifier_min_$ #number 1) 10))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeIdentifier',typeIdentifier: #number,min: 1,max: 10}}}",
+      ~v="@{_typeReferences_: {answer: {max: 10,min: 1,typeIdentifier: #number,typeTag: 'typeIdentifier'}}}",
      (),
    )
    testToExpression(
      "answer: number<-max(10)<-min(1)",
      "{(:$_typeOf_$ :answer (:$_typeModifier_min_$ (:$_typeModifier_max_$ #number 10) 1))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeIdentifier',typeIdentifier: #number,max: 10,min: 1}}}",
+      ~v="@{_typeReferences_: {answer: {max: 10,min: 1,typeIdentifier: #number,typeTag: 'typeIdentifier'}}}",
      (),
    )
  })
@ -82,7 +82,7 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "answer: number | string $ opaque",
      "{(:$_typeOf_$ :answer (:$_typeModifier_opaque_$ (:$_typeOr_$ (:$_constructArray_$ (#number #string)))))}",
-      ~v="{_typeReferences_: {answer: {typeTag: 'typeOr',typeOr: [#number,#string],opaque: true}}}",
+      ~v="@{_typeReferences_: {answer: {opaque: true,typeOr: [#number,#string],typeTag: 'typeOr'}}}",
      (),
    )
  })
@ -90,7 +90,7 @@ describe("Peggy Types to Expression", () => {
    testToExpression(
      "odds1 = [1,3,5]; odds2 = [7, 9]; type odds = number<-memberOf(concat(odds1, odds2))",
      "{(:$_let_$ :odds1 {(:$_constructArray_$ (1 3 5))}); (:$_let_$ :odds2 {(:$_constructArray_$ (7 9))}); (:$_typeAlias_$ #odds (:$_typeModifier_memberOf_$ #number (:concat :odds1 :odds2)))}",
-      ~v="{_typeAliases_: {odds: {typeTag: 'typeIdentifier',typeIdentifier: #number,memberOf: [1,3,5,7,9]}},odds1: [1,3,5],odds2: [7,9]}",
+      ~v="@{_typeAliases_: {odds: {memberOf: [1,3,5,7,9],typeIdentifier: #number,typeTag: 'typeIdentifier'}},odds1: [1,3,5],odds2: [7,9]}",
      (),
    )
  })
--- a/packages/squiggle-lang/tests/Reducer/Reducer_TestHelpers.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_TestHelpers.res
@ -1,7 +1,6 @@
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+module ExternalExpressionValue = ReducerInterface.ExternalExpressionValue
 module ErrorValue = Reducer_ErrorValue
-module Bindings = Reducer_Category_Bindings

 open Jest
 open Expect
@ -9,7 +8,7 @@ open Expect
 let unwrapRecord = rValue =>
  rValue->Belt.Result.flatMap(value =>
    switch value {
-    | ExpressionValue.EvRecord(aRecord) => Ok(aRecord)
+    | ExternalExpressionValue.EvRecord(aRecord) => Ok(aRecord)
    | _ => ErrorValue.RETodo("TODO: External bindings must be returned")->Error
    }
  )
@ -19,18 +18,18 @@ let expectParseToBe = (expr: string, answer: string) =>

 let expectEvalToBe = (expr: string, answer: string) =>
  Reducer.evaluate(expr)
-  ->Reducer_Helpers.rRemoveDefaults
-  ->ExpressionValue.toStringResult
+  ->Reducer_Helpers.rRemoveDefaultsExternal
+  ->ExternalExpressionValue.toStringResult
  ->expect
  ->toBe(answer)

 let expectEvalError = (expr: string) =>
-  Reducer.evaluate(expr)->ExpressionValue.toStringResult->expect->toMatch("Error\(")
+  Reducer.evaluate(expr)->ExternalExpressionValue.toStringResult->expect->toMatch("Error\(")

 let expectEvalBindingsToBe = (expr: string, bindings: Reducer.externalBindings, answer: string) =>
  Reducer.evaluateUsingOptions(expr, ~externalBindings=Some(bindings), ~environment=None)
-  ->Reducer_Helpers.rRemoveDefaults
-  ->ExpressionValue.toStringResult
+  ->Reducer_Helpers.rRemoveDefaultsExternal
+  ->ExternalExpressionValue.toStringResult
  ->expect
  ->toBe(answer)

--- a/packages/squiggle-lang/tests/Reducer/Reducer_TestMacroHelpers.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_TestMacroHelpers.res
@ -1,25 +1,27 @@
 open Jest
 open Expect

-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module Expression = Reducer_Expression
-module ExpressionValue = ReducerInterface_ExpressionValue
+// module ExpressionValue = ReducerInterface.ExpressionValue
+module InternalExpressionValue = ReducerInterface.InternalExpressionValue
 module ExpressionWithContext = Reducer_ExpressionWithContext
 module Macro = Reducer_Expression_Macro
 module T = Reducer_Expression_T
+module Module = Reducer_Category_Module

 let testMacro_ = (
  tester,
-  bindArray: array<(string, ExpressionValue.expressionValue)>,
+  bindArray: array<(string, InternalExpressionValue.t)>,
  expr: T.expression,
  expectedCode: string,
 ) => {
-  let bindings = Belt.Map.String.fromArray(bindArray)
+  let bindings = Module.fromArray(bindArray)
  tester(expr->T.toString, () =>
    expr
    ->Macro.expandMacroCall(
      bindings,
-      ExpressionValue.defaultEnvironment,
+      InternalExpressionValue.defaultEnvironment,
      Expression.reduceExpression,
    )
    ->ExpressionWithContext.toStringResult
@ -30,39 +32,43 @@ let testMacro_ = (

 let testMacroEval_ = (
  tester,
-  bindArray: array<(string, ExpressionValue.expressionValue)>,
+  bindArray: array<(string, InternalExpressionValue.t)>,
  expr: T.expression,
  expectedValue: string,
 ) => {
-  let bindings = Belt.Map.String.fromArray(bindArray)
+  let bindings = Module.fromArray(bindArray)
  tester(expr->T.toString, () =>
    expr
-    ->Macro.doMacroCall(bindings, ExpressionValue.defaultEnvironment, Expression.reduceExpression)
-    ->ExpressionValue.toStringResult
+    ->Macro.doMacroCall(
+      bindings,
+      InternalExpressionValue.defaultEnvironment,
+      Expression.reduceExpression,
+    )
+    ->InternalExpressionValue.toStringResult
    ->expect
    ->toEqual(expectedValue)
  )
 }

 let testMacro = (
-  bindArray: array<(string, ExpressionValue.expressionValue)>,
+  bindArray: array<(string, InternalExpressionValue.t)>,
  expr: T.expression,
  expectedExpr: string,
 ) => testMacro_(test, bindArray, expr, expectedExpr)
 let testMacroEval = (
-  bindArray: array<(string, ExpressionValue.expressionValue)>,
+  bindArray: array<(string, InternalExpressionValue.t)>,
  expr: T.expression,
  expectedValue: string,
 ) => testMacroEval_(test, bindArray, expr, expectedValue)

 module MySkip = {
  let testMacro = (
-    bindArray: array<(string, ExpressionValue.expressionValue)>,
+    bindArray: array<(string, InternalExpressionValue.t)>,
    expr: T.expression,
    expectedExpr: string,
  ) => testMacro_(Skip.test, bindArray, expr, expectedExpr)
  let testMacroEval = (
-    bindArray: array<(string, ExpressionValue.expressionValue)>,
+    bindArray: array<(string, InternalExpressionValue.t)>,
    expr: T.expression,
    expectedValue: string,
  ) => testMacroEval_(Skip.test, bindArray, expr, expectedValue)
@ -70,12 +76,12 @@ module MySkip = {

 module MyOnly = {
  let testMacro = (
-    bindArray: array<(string, ExpressionValue.expressionValue)>,
+    bindArray: array<(string, InternalExpressionValue.t)>,
    expr: T.expression,
    expectedExpr: string,
  ) => testMacro_(Only.test, bindArray, expr, expectedExpr)
  let testMacroEval = (
-    bindArray: array<(string, ExpressionValue.expressionValue)>,
+    bindArray: array<(string, InternalExpressionValue.t)>,
    expr: T.expression,
    expectedValue: string,
  ) => testMacroEval_(Only.test, bindArray, expr, expectedValue)
--- a/packages/squiggle-lang/tests/Reducer/Reducer_externalBindings_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_externalBindings_test.res
@ -1,11 +1,14 @@
-// TODO: Reimplement with usual parse
 open Jest
 open Reducer_TestHelpers

 describe("Eval with Bindings", () => {
-  testEvalBindingsToBe("x", list{("x", ExpressionValue.EvNumber(1.))}, "Ok(1)")
-  testEvalBindingsToBe("x+1", list{("x", ExpressionValue.EvNumber(1.))}, "Ok(2)")
+  testEvalBindingsToBe("x", list{("x", ExternalExpressionValue.EvNumber(1.))}, "Ok(1)")
+  testEvalBindingsToBe("x+1", list{("x", ExternalExpressionValue.EvNumber(1.))}, "Ok(2)")
  testParseToBe("y = x+1; y", "Ok({(:$_let_$ :y {(:add :x 1)}); :y})")
-  testEvalBindingsToBe("y = x+1; y", list{("x", ExpressionValue.EvNumber(1.))}, "Ok(2)")
-  testEvalBindingsToBe("y = x+1", list{("x", ExpressionValue.EvNumber(1.))}, "Ok({x: 1,y: 2})")
+  testEvalBindingsToBe("y = x+1; y", list{("x", ExternalExpressionValue.EvNumber(1.))}, "Ok(2)")
+  testEvalBindingsToBe(
+    "y = x+1",
+    list{("x", ExternalExpressionValue.EvNumber(1.))},
+    "Ok(@{x: 1,y: 2})",
+  )
 })
--- a/packages/squiggle-lang/tests/Reducer/Reducer_functionTricks_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_functionTricks_test.res
@ -39,15 +39,15 @@ describe("symbol not defined", () => {
 })

 describe("call and bindings", () => {
-  testEvalToBe("f(x)=x+1", "Ok({f: lambda(x=>internal code)})")
+  testEvalToBe("f(x)=x+1", "Ok(@{f: lambda(x=>internal code)})")
  testEvalToBe("f(x)=x+1; f(1)", "Ok(2)")
-  testEvalToBe("f=1;y=2", "Ok({f: 1,y: 2})")
-  testEvalToBe("f(x)=x+1; y=f(1)", "Ok({f: lambda(x=>internal code),y: 2})")
+  testEvalToBe("f=1;y=2", "Ok(@{f: 1,y: 2})")
+  testEvalToBe("f(x)=x+1; y=f(1)", "Ok(@{f: lambda(x=>internal code),y: 2})")
  testEvalToBe("f(x)=x+1; y=f(1); f(1)", "Ok(2)")
-  testEvalToBe("f(x)=x+1; y=f(1); z=f(1)", "Ok({f: lambda(x=>internal code),y: 2,z: 2})")
+  testEvalToBe("f(x)=x+1; y=f(1); z=f(1)", "Ok(@{f: lambda(x=>internal code),y: 2,z: 2})")
  testEvalToBe(
    "f(x)=x+1; g(x)=f(x)+1",
-    "Ok({f: lambda(x=>internal code),g: lambda(x=>internal code)})",
+    "Ok(@{f: lambda(x=>internal code),g: lambda(x=>internal code)})",
  )
  testParseToBe(
    "f=99; g(x)=f; g(2)",
@ -57,7 +57,7 @@ describe("call and bindings", () => {
  testEvalToBe("f(x)=x; g(x)=f(x); g(2)", "Ok(2)")
  testEvalToBe(
    "f(x)=x+1; g(x)=f(x)+1; y=g(2)",
-    "Ok({f: lambda(x=>internal code),g: lambda(x=>internal code),y: 4})",
+    "Ok(@{f: lambda(x=>internal code),g: lambda(x=>internal code),y: 4})",
  )
  testEvalToBe("f(x)=x+1; g(x)=f(x)+1; g(2)", "Ok(4)")
 })
@ -65,7 +65,7 @@ describe("call and bindings", () => {
 describe("function tricks", () => {
  testEvalError("f(x)=f(y)=2; f(2)") //Error because chain assignment is not allowed
  testEvalToBe("y=2;g(x)=y+1;g(2)", "Ok(3)")
-  testEvalToBe("y=2;g(x)=inspect(y)+1", "Ok({g: lambda(x=>internal code),y: 2})")
+  testEvalToBe("y=2;g(x)=inspect(y)+1", "Ok(@{g: lambda(x=>internal code),y: 2})")
  MySkip.testEvalToBe("f(x) = x(x); f(f)", "????") // TODO: Infinite loop. Any solution? Catching proper exception or timeout?
  MySkip.testEvalToBe("f(x, x)=x+x; f(1,2)", "????") // TODO: Duplicate parameters
  testEvalToBe("myadd(x,y)=x+y; z=myadd; z", "Ok(lambda(x,y=>internal code))")
@ -75,7 +75,7 @@ describe("function tricks", () => {
 describe("lambda in structures", () => {
  testEvalToBe(
    "myadd(x,y)=x+y; z=[myadd]",
-    "Ok({myadd: lambda(x,y=>internal code),z: [lambda(x,y=>internal code)]})",
+    "Ok(@{myadd: lambda(x,y=>internal code),z: [lambda(x,y=>internal code)]})",
  )
  testEvalToBe("myadd(x,y)=x+y; z=[myadd]; z[0]", "Ok(lambda(x,y=>internal code))")
  testEvalToBe("myadd(x,y)=x+y; z=[myadd]; z[0](3,2)", "Ok(5)")
--- a/packages/squiggle-lang/tests/Reducer/Reducer_test.res
+++ b/packages/squiggle-lang/tests/Reducer/Reducer_test.res
@ -39,7 +39,7 @@ describe("eval", () => {
    testEvalToBe("x=1; y=x+1; y+1", "Ok(3)")
    testEvalError("1; x=1")
    testEvalError("1; 1")
-    testEvalToBe("x=1; x=1", "Ok({x: 1})")
+    testEvalToBe("x=1; x=1", "Ok(@{x: 1})")
  })
 })

--- a/packages/squiggle-lang/tests/ReducerInterface/ReducerInterface_ExpressionValue_test.res
+++ b/packages/squiggle-lang/tests/ReducerInterface/ReducerInterface_ExpressionValue_test.res
@ -1,4 +1,4 @@
-open ReducerInterface.ExpressionValue
+open ReducerInterface.ExternalExpressionValue
 open Jest
 open Expect

--- a/packages/squiggle-lang/package.json
+++ b/packages/squiggle-lang/package.json
@ -5,6 +5,7 @@
  "license": "MIT",
  "scripts": {
    "peggy": "peggy --cache",
+    "rescript": "rescript",
    "build": "yarn build:peggy && yarn build:rescript && yarn build:typescript",
    "build:peggy": "find . -type f -name *.peggy -exec yarn peggy {} \\;",
    "build:rescript": "rescript build -with-deps",
--- a/packages/squiggle-lang/src/js/index.ts
+++ b/packages/squiggle-lang/src/js/index.ts
@ -147,30 +147,13 @@ function createTsExport(
      // case
      return tag(
        "array",
-        x.value.map((arrayItem): squiggleExpression => {
-          switch (arrayItem.tag) {
-            case "EvRecord":
-              return tag(
-                "record",
-                _.mapValues(arrayItem.value, (recordValue: unknown) =>
-                  convertRawToTypescript(
-                    recordValue as rescriptExport,
-                    environment
-                  )
-                )
-              );
-            case "EvArray":
-              let y = arrayItem.value as unknown as rescriptExport[];
-              return tag(
-                "array",
-                y.map((childArrayItem) =>
-                  convertRawToTypescript(childArrayItem, environment)
-                )
-              );
-            default:
-              return createTsExport(arrayItem, environment);
-          }
-        })
+        x.value.map(
+          (arrayItem): squiggleExpression =>
+            convertRawToTypescript(
+              arrayItem as unknown as rescriptExport,
+              environment
+            )
+        )
      );
    case "EvArrayString":
      return tag("arraystring", x.value);
--- a/packages/squiggle-lang/src/rescript/Distributions/PointSetDist/Discrete.res
+++ b/packages/squiggle-lang/src/rescript/Distributions/PointSetDist/Discrete.res
@ -1,3 +1,4 @@
+@@warning("-27") //TODO: Remove and fix the warning
 open Distributions

 type t = PointSetTypes.discreteShape
--- a/packages/squiggle-lang/src/rescript/Distributions/PointSetDist/Mixed.res
+++ b/packages/squiggle-lang/src/rescript/Distributions/PointSetDist/Mixed.res
@ -1,3 +1,5 @@
+@@warning("-27") //TODO: Remove and fix the warning
+@@warning("-32") //TODO: Remove and fix the warning
 open Distributions

 type t = PointSetTypes.mixedShape
--- a/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res
+++ b/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res
@ -1,4 +1,4 @@
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
+type internalExpressionValue = ReducerInterface_InternalExpressionValue.t

 /*
  Function Registry "Type". A type, without any other information.
@ -28,7 +28,7 @@ type rec frValue =
  | FRValueArray(array<frValue>)
  | FRValueDistOrNumber(frValueDistOrNumber)
  | FRValueRecord(frValueRecord)
-  | FRValueLambda(ReducerInterface_ExpressionValue.lambdaValue)
+  | FRValueLambda(ReducerInterface_InternalExpressionValue.lambdaValue)
  | FRValueString(string)
  | FRValueVariant(string)
  | FRValueAny(frValue)
@ -41,7 +41,7 @@ and frValueDistOrNumber = FRValueNumber(float) | FRValueDist(DistributionTypes.g
 type fnDefinition = {
  name: string,
  inputs: array<frType>,
-  run: (array<frValue>, DistributionOperation.env) => result<expressionValue, string>,
+  run: (array<frValue>, DistributionOperation.env) => result<internalExpressionValue, string>,
 }

 type function = {
@ -73,47 +73,47 @@ module FRType = {
    | FRTypeAny => `any`
    }

-  let rec toFrValue = (r: expressionValue): option<frValue> =>
+  let rec toFrValue = (r: internalExpressionValue): option<frValue> =>
    switch r {
-    | EvNumber(f) => Some(FRValueNumber(f))
-    | EvString(f) => Some(FRValueString(f))
-    | EvDistribution(f) => Some(FRValueDistOrNumber(FRValueDist(f)))
-    | EvLambda(f) => Some(FRValueLambda(f))
-    | EvArray(elements) =>
+    | IEvNumber(f) => Some(FRValueNumber(f))
+    | IEvString(f) => Some(FRValueString(f))
+    | IEvDistribution(f) => Some(FRValueDistOrNumber(FRValueDist(f)))
+    | IEvLambda(f) => Some(FRValueLambda(f))
+    | IEvArray(elements) =>
      elements->E.A2.fmap(toFrValue)->E.A.O.openIfAllSome->E.O2.fmap(r => FRValueArray(r))
-    | EvRecord(record) =>
-      Js.Dict.entries(record)
+    | IEvRecord(map) =>
+      Belt.Map.String.toArray(map)
      ->E.A2.fmap(((key, item)) => item->toFrValue->E.O2.fmap(o => (key, o)))
      ->E.A.O.openIfAllSome
      ->E.O2.fmap(r => FRValueRecord(r))
    | _ => None
    }

-  let rec matchWithExpressionValue = (t: t, r: expressionValue): option<frValue> =>
+  let rec matchWithExpressionValue = (t: t, r: internalExpressionValue): option<frValue> =>
    switch (t, r) {
    | (FRTypeAny, f) => toFrValue(f)
-    | (FRTypeString, EvString(f)) => Some(FRValueString(f))
-    | (FRTypeNumber, EvNumber(f)) => Some(FRValueNumber(f))
-    | (FRTypeDistOrNumber, EvNumber(f)) => Some(FRValueDistOrNumber(FRValueNumber(f)))
-    | (FRTypeDistOrNumber, EvDistribution(Symbolic(#Float(f)))) =>
+    | (FRTypeString, IEvString(f)) => Some(FRValueString(f))
+    | (FRTypeNumber, IEvNumber(f)) => Some(FRValueNumber(f))
+    | (FRTypeDistOrNumber, IEvNumber(f)) => Some(FRValueDistOrNumber(FRValueNumber(f)))
+    | (FRTypeDistOrNumber, IEvDistribution(Symbolic(#Float(f)))) =>
      Some(FRValueDistOrNumber(FRValueNumber(f)))
-    | (FRTypeDistOrNumber, EvDistribution(f)) => Some(FRValueDistOrNumber(FRValueDist(f)))
-    | (FRTypeNumeric, EvNumber(f)) => Some(FRValueNumber(f))
-    | (FRTypeNumeric, EvDistribution(Symbolic(#Float(f)))) => Some(FRValueNumber(f))
-    | (FRTypeLambda, EvLambda(f)) => Some(FRValueLambda(f))
-    | (FRTypeArray(intendedType), EvArray(elements)) => {
+    | (FRTypeDistOrNumber, IEvDistribution(f)) => Some(FRValueDistOrNumber(FRValueDist(f)))
+    | (FRTypeNumeric, IEvNumber(f)) => Some(FRValueNumber(f))
+    | (FRTypeNumeric, IEvDistribution(Symbolic(#Float(f)))) => Some(FRValueNumber(f))
+    | (FRTypeLambda, IEvLambda(f)) => Some(FRValueLambda(f))
+    | (FRTypeArray(intendedType), IEvArray(elements)) => {
        let el = elements->E.A2.fmap(matchWithExpressionValue(intendedType))
        E.A.O.openIfAllSome(el)->E.O2.fmap(r => FRValueArray(r))
      }
-    | (FRTypeDict(r), EvRecord(record)) =>
-      record
-      ->Js.Dict.entries
+    | (FRTypeDict(r), IEvRecord(map)) =>
+      map
+      ->Belt.Map.String.toArray
      ->E.A2.fmap(((key, item)) => matchWithExpressionValue(r, item)->E.O2.fmap(o => (key, o)))
      ->E.A.O.openIfAllSome
      ->E.O2.fmap(r => FRValueDict(Js.Dict.fromArray(r)))
-    | (FRTypeRecord(recordParams), EvRecord(record)) => {
+    | (FRTypeRecord(recordParams), IEvRecord(map)) => {
        let getAndMatch = (name, input) =>
-          E.Dict.get(record, name)->E.O.bind(matchWithExpressionValue(input))
+          Belt.Map.String.get(map, name)->E.O.bind(matchWithExpressionValue(input))
        //All names in the type must be present. If any are missing, the corresponding
        //value will be None, and this function would return None.
        let namesAndValues: array<option<(Js.Dict.key, frValue)>> =
@ -125,35 +125,38 @@ module FRType = {
    | _ => None
    }

-  let rec matchReverse = (e: frValue): expressionValue =>
+  let rec matchReverse = (e: frValue): internalExpressionValue =>
    switch e {
-    | FRValueNumber(f) => EvNumber(f)
-    | FRValueDistOrNumber(FRValueNumber(n)) => EvNumber(n)
-    | FRValueDistOrNumber(FRValueDist(n)) => EvDistribution(n)
-    | FRValueDist(dist) => EvDistribution(dist)
-    | FRValueArray(elements) => EvArray(elements->E.A2.fmap(matchReverse))
+    | FRValueNumber(f) => IEvNumber(f)
+    | FRValueDistOrNumber(FRValueNumber(n)) => IEvNumber(n)
+    | FRValueDistOrNumber(FRValueDist(n)) => IEvDistribution(n)
+    | FRValueDist(dist) => IEvDistribution(dist)
+    | FRValueArray(elements) => IEvArray(elements->E.A2.fmap(matchReverse))
    | FRValueRecord(frValueRecord) => {
-        let record =
-          frValueRecord->E.A2.fmap(((name, value)) => (name, matchReverse(value)))->E.Dict.fromArray
-        EvRecord(record)
+        let map =
+          frValueRecord
+          ->E.A2.fmap(((name, value)) => (name, matchReverse(value)))
+          ->Belt.Map.String.fromArray
+        IEvRecord(map)
      }
    | FRValueDict(frValueRecord) => {
-        let record =
+        let map =
          frValueRecord
          ->Js.Dict.entries
          ->E.A2.fmap(((name, value)) => (name, matchReverse(value)))
-          ->E.Dict.fromArray
-        EvRecord(record)
+          ->Belt.Map.String.fromArray
+        IEvRecord(map)
      }
-    | FRValueLambda(l) => EvLambda(l)
-    | FRValueString(string) => EvString(string)
-    | FRValueVariant(string) => EvString(string)
+    | FRValueLambda(l) => IEvLambda(l)
+    | FRValueString(string) => IEvString(string)
+    | FRValueVariant(string) => IEvString(string)
    | FRValueAny(f) => matchReverse(f)
    }

-  let matchWithExpressionValueArray = (inputs: array<t>, args: array<expressionValue>): option<
-    array<frValue>,
-  > => {
+  let matchWithExpressionValueArray = (
+    inputs: array<t>,
+    args: array<internalExpressionValue>,
+  ): option<array<frValue>> => {
    let isSameLength = E.A.length(inputs) == E.A.length(args)
    if !isSameLength {
      None
@ -204,14 +207,14 @@ module Matcher = {
  }

  module FnDefinition = {
-    let matchAssumingSameName = (f: fnDefinition, args: array<expressionValue>) => {
+    let matchAssumingSameName = (f: fnDefinition, args: array<internalExpressionValue>) => {
      switch FRType.matchWithExpressionValueArray(f.inputs, args) {
      | Some(_) => MatchSimple.FullMatch
      | None => MatchSimple.SameNameDifferentArguments
      }
    }

-    let match = (f: fnDefinition, fnName: string, args: array<expressionValue>) => {
+    let match = (f: fnDefinition, fnName: string, args: array<internalExpressionValue>) => {
      if f.name !== fnName {
        MatchSimple.DifferentName
      } else {
@ -224,7 +227,7 @@ module Matcher = {
    type definitionId = int
    type match = Match.t<array<definitionId>, definitionId>

-    let match = (f: function, fnName: string, args: array<expressionValue>): match => {
+    let match = (f: function, fnName: string, args: array<internalExpressionValue>): match => {
      let matchedDefinition = () =>
        E.A.getIndexBy(f.definitions, r =>
          MatchSimple.isFullMatch(FnDefinition.match(r, fnName, args))
@ -258,7 +261,7 @@ module Matcher = {
  }

  module Registry = {
-    let _findExactMatches = (r: registry, fnName: string, args: array<expressionValue>) => {
+    let _findExactMatches = (r: registry, fnName: string, args: array<internalExpressionValue>) => {
      let functionMatchPairs = r->E.A2.fmap(l => (l, Function.match(l, fnName, args)))
      let fullMatch = functionMatchPairs->E.A.getBy(((_, match)) => Match.isFullMatch(match))
      fullMatch->E.O.bind(((fn, match)) =>
@ -269,7 +272,7 @@ module Matcher = {
      )
    }

-    let _findNameMatches = (r: registry, fnName: string, args: array<expressionValue>) => {
+    let _findNameMatches = (r: registry, fnName: string, args: array<internalExpressionValue>) => {
      let functionMatchPairs = r->E.A2.fmap(l => (l, Function.match(l, fnName, args)))
      let getNameMatches =
        functionMatchPairs
@ -288,7 +291,7 @@ module Matcher = {
      E.A.toNoneIfEmpty(matches)
    }

-    let findMatches = (r: registry, fnName: string, args: array<expressionValue>) => {
+    let findMatches = (r: registry, fnName: string, args: array<internalExpressionValue>) => {
      switch _findExactMatches(r, fnName, args) {
      | Some(r) => Match.FullMatch(r)
      | None =>
@ -316,7 +319,7 @@ module FnDefinition = {
    t.name ++ `(${inputs})`
  }

-  let run = (t: t, args: array<expressionValue>, env: DistributionOperation.env) => {
+  let run = (t: t, args: array<internalExpressionValue>, env: DistributionOperation.env) => {
    let argValues = FRType.matchWithExpressionValueArray(t.inputs, args)
    switch argValues {
    | Some(values) => t.run(values, env)
@ -370,7 +373,7 @@ module Registry = {
  let matchAndRun = (
    ~registry: registry,
    ~fnName: string,
-    ~args: array<expressionValue>,
+    ~args: array<internalExpressionValue>,
    ~env: DistributionOperation.env,
  ) => {
    let matchToDef = m => Matcher.Registry.matchToDef(registry, m)
--- a/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Helpers.res
+++ b/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Helpers.res
@ -4,11 +4,11 @@ let impossibleError = "Wrong inputs / Logically impossible"

 module Wrappers = {
  let symbolic = r => DistributionTypes.Symbolic(r)
-  let evDistribution = r => ReducerInterface_ExpressionValue.EvDistribution(r)
-  let evNumber = r => ReducerInterface_ExpressionValue.EvNumber(r)
-  let evArray = r => ReducerInterface_ExpressionValue.EvArray(r)
-  let evRecord = r => ReducerInterface_ExpressionValue.EvRecord(r)
-  let evString = r => ReducerInterface_ExpressionValue.EvString(r)
+  let evDistribution = r => ReducerInterface_InternalExpressionValue.IEvDistribution(r)
+  let evNumber = r => ReducerInterface_InternalExpressionValue.IEvNumber(r)
+  let evArray = r => ReducerInterface_InternalExpressionValue.IEvArray(r)
+  let evRecord = r => ReducerInterface_InternalExpressionValue.IEvRecord(r)
+  let evString = r => ReducerInterface_InternalExpressionValue.IEvString(r)
  let symbolicEvDistribution = r => r->DistributionTypes.Symbolic->evDistribution
 }

--- a/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Library.res
+++ b/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Library.res
@ -9,7 +9,7 @@ module Declaration = {
    ("inputs", FRTypeArray(FRTypeRecord([("min", FRTypeNumber), ("max", FRTypeNumber)]))),
  ])

-  let fromExpressionValue = (e: frValue): result<expressionValue, string> => {
+  let fromExpressionValue = (e: frValue): result<internalExpressionValue, string> => {
    switch FunctionRegistry_Helpers.Prepare.ToValueArray.Record.twoArgs([e]) {
    | Ok([FRValueLambda(lambda), FRValueArray(inputs)]) => {
        open FunctionRegistry_Helpers.Prepare
@ -20,7 +20,7 @@ module Declaration = {
        inputs
        ->E.A2.fmap(getMinMax)
        ->E.A.R.firstErrorOrOpen
-        ->E.R2.fmap(args => ReducerInterface_ExpressionValue.EvDeclaration(
+        ->E.R2.fmap(args => ReducerInterface_InternalExpressionValue.IEvDeclaration(
          Declaration.make(lambda, args),
        ))
      }
@ -43,7 +43,9 @@ let inputsTodist = (inputs: array<FunctionRegistry_Core.frValue>, makeDist) => {
  let expressionValue =
    xyCoords
    ->E.R.bind(r => r->XYShape.T.makeFromZipped->E.R2.errMap(XYShape.Error.toString))
-    ->E.R2.fmap(r => ReducerInterface_ExpressionValue.EvDistribution(PointSet(makeDist(r))))
+    ->E.R2.fmap(r => ReducerInterface_InternalExpressionValue.IEvDistribution(
+      PointSet(makeDist(r)),
+    ))
  expressionValue
 }

@ -369,7 +371,7 @@ to(5,10)
                E.Dict.concat(d1, d2) |> Js.Dict.map((. r) =>
                  FunctionRegistry_Core.FRType.matchReverse(r)
                )
-              newDict->Wrappers.evRecord->Ok
+              newDict->Js.Dict.entries->Belt.Map.String.fromArray->Wrappers.evRecord->Ok
            }
          | _ => Error(impossibleError)
          }
@ -390,6 +392,7 @@ to(5,10)
        ->Prepare.ToTypedArray.dicts
        ->E.R2.fmap(E.Dict.concatMany)
        ->E.R2.fmap(Js.Dict.map((. r) => FunctionRegistry_Core.FRType.matchReverse(r)))
+        ->E.R2.fmap(r => r->Js.Dict.entries->Belt.Map.String.fromArray)
        ->E.R2.fmap(Wrappers.evRecord)
      ),
    ],
@ -462,7 +465,7 @@ to(5,10)
            }
          })
          ->E.A.R.firstErrorOrOpen
-          ->E.R2.fmap(Js.Dict.fromArray)
+          ->E.R2.fmap(Belt.Map.String.fromArray)
          ->E.R2.fmap(Wrappers.evRecord)
        inputs->getOrError(0)->E.R.bind(Prepare.ToValueArray.Array.arrayOfArrays)
          |> E.R2.bind(convertInternalItems)
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer.res
@ -1,13 +1,14 @@
 module ErrorValue = Reducer_ErrorValue
 module Expression = Reducer_Expression
-module ExpressionValue = ReducerInterface_ExpressionValue
+module ExternalExpressionValue = ReducerInterface_ExternalExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Lambda = Reducer_Expression_Lambda

-type environment = ReducerInterface_ExpressionValue.environment
+type environment = ReducerInterface_InternalExpressionValue.environment
 type errorValue = Reducer_ErrorValue.errorValue
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
-type lambdaValue = ExpressionValue.lambdaValue
+type expressionValue = ExternalExpressionValue.t
+type externalBindings = ReducerInterface_ExternalExpressionValue.externalBindings
+type lambdaValue = ExternalExpressionValue.lambdaValue

 let evaluate = Expression.evaluate
 let evaluateUsingOptions = Expression.evaluateUsingOptions
@ -15,13 +16,20 @@ let evaluatePartialUsingExternalBindings = Expression.evaluatePartialUsingExtern
 let parse = Expression.parse

 let foreignFunctionInterface = (
-  lambdaValue: lambdaValue,
+  lambdaValue: ExternalExpressionValue.lambdaValue,
  argArray: array<expressionValue>,
-  environment: ExpressionValue.environment,
+  environment: ExternalExpressionValue.environment,
 ) => {
-  Lambda.foreignFunctionInterface(lambdaValue, argArray, environment, Expression.reduceExpression)
+  let internallambdaValue = InternalExpressionValue.lambdaValueToInternal(lambdaValue)
+  let internalArgArray = argArray->Js.Array2.map(InternalExpressionValue.toInternal)
+  Lambda.foreignFunctionInterface(
+    internallambdaValue,
+    internalArgArray,
+    environment,
+    Expression.reduceExpression,
+  )->Belt.Result.map(InternalExpressionValue.toExternal)
 }

-let defaultEnvironment = ExpressionValue.defaultEnvironment
+let defaultEnvironment = ExternalExpressionValue.defaultEnvironment

 let defaultExternalBindings = ReducerInterface_StdLib.externalStdLib
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer.resi
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer.resi
@ -2,27 +2,29 @@ module ErrorValue = Reducer_ErrorValue
 module Expression = Reducer_Expression

@genType
-type environment = ReducerInterface_ExpressionValue.environment
+type environment = ReducerInterface_ExternalExpressionValue.environment
@genType
 type errorValue = Reducer_ErrorValue.errorValue
@genType
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
+type expressionValue = ReducerInterface_ExternalExpressionValue.t
@genType
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
+type externalBindings = ReducerInterface_ExternalExpressionValue.externalBindings
@genType
-type lambdaValue = ReducerInterface_ExpressionValue.lambdaValue
+type lambdaValue = ReducerInterface_ExternalExpressionValue.lambdaValue

@genType
 let evaluateUsingOptions: (
-  ~environment: option<QuriSquiggleLang.ReducerInterface_ExpressionValue.environment>,
-  ~externalBindings: option<QuriSquiggleLang.ReducerInterface_ExpressionValue.externalBindings>,
+  ~environment: option<QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.environment>,
+  ~externalBindings: option<
+    QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.externalBindings,
+  >,
  string,
 ) => result<expressionValue, errorValue>
@genType
 let evaluatePartialUsingExternalBindings: (
  string,
-  QuriSquiggleLang.ReducerInterface_ExpressionValue.externalBindings,
-  QuriSquiggleLang.ReducerInterface_ExpressionValue.environment,
+  QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.externalBindings,
+  QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.environment,
 ) => result<externalBindings, errorValue>
@genType
 let evaluate: string => result<expressionValue, errorValue>
@ -31,9 +33,9 @@ let parse: string => result<Expression.expression, errorValue>

@genType
 let foreignFunctionInterface: (
-  QuriSquiggleLang.ReducerInterface_ExpressionValue.lambdaValue,
-  array<QuriSquiggleLang.ReducerInterface_ExpressionValue.expressionValue>,
-  QuriSquiggleLang.ReducerInterface_ExpressionValue.environment,
+  QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.lambdaValue,
+  array<QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.t>,
+  QuriSquiggleLang.ReducerInterface_ExternalExpressionValue.environment,
 ) => result<expressionValue, errorValue>

@genType
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Bindings.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Bindings.res
@ -1,12 +0,0 @@
-include Reducer_Category_Module // Bindings inherit from Module
-
-open ReducerInterface_ExpressionValue
-
-let emptyBindings = emptyModule
-
-let toExpressionValue = (container: t): expressionValue => EvRecord(toRecord(container))
-let fromExpressionValue = (aValue: expressionValue): t =>
-  switch aValue {
-  | EvRecord(r) => fromRecord(r)
-  | _ => emptyBindings
-  }
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module.res
@ -1,33 +1,112 @@
 module ExpressionT = Reducer_Expression_T
-open ReducerInterface_ExpressionValue
+open ReducerInterface_InternalExpressionValue
 let expressionValueToString = toString

-type t = ExpressionT.bindings
+type t = ReducerInterface_InternalExpressionValue.nameSpace

 let typeAliasesKey = "_typeAliases_"
 let typeReferencesKey = "_typeReferences_"

-let emptyModule: t = Belt.Map.String.empty
+let getType = (nameSpace: t, id: string) => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.get(container, typeAliasesKey)->Belt.Option.flatMap(aliases =>
+    switch aliases {
+    | IEvRecord(r) => Belt.Map.String.get(r, id)
+    | _ => None
+    }
+  )
+}

-let cloneRecord = (r: record): record => r->Js.Dict.entries->Js.Dict.fromArray
-let fromRecord = (r: record): t => Js.Dict.entries(r)->Belt.Map.String.fromArray
-let toRecord = (container: t): record => Belt.Map.String.toArray(container)->Js.Dict.fromArray
+let getTypeOf = (nameSpace: t, id: string) => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.get(container, typeReferencesKey)->Belt.Option.flatMap(defs =>
+    switch defs {
+    | IEvRecord(r) => Belt.Map.String.get(r, id)
+    | _ => None
+    }
+  )
+}

-let toExpressionValue = (container: t): expressionValue => EvModule(toRecord(container))
-let fromExpressionValue = (aValue: expressionValue): t =>
+let getWithDefault = (nameSpace: t, id: string, default) => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.getWithDefault(container, id, default)
+}
+
+let get = (nameSpace: t, id: string) => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.get(container, id)
+}
+
+let emptyMap: map = Belt.Map.String.empty
+
+let setTypeAlias = (nameSpace: t, id: string, value): t => {
+  let NameSpace(container) = nameSpace
+  let rValue = Belt.Map.String.getWithDefault(container, typeAliasesKey, IEvRecord(emptyMap))
+  let r = switch rValue {
+  | IEvRecord(r) => r
+  | _ => emptyMap
+  }
+  let r2 = Belt.Map.String.set(r, id, value)->IEvRecord
+  Belt.Map.String.set(container, typeAliasesKey, r2)->NameSpace
+}
+
+let setTypeOf = (nameSpace: t, id: string, value): t => {
+  let NameSpace(container) = nameSpace
+  let rValue = Belt.Map.String.getWithDefault(container, typeReferencesKey, IEvRecord(emptyMap))
+  let r = switch rValue {
+  | IEvRecord(r) => r
+  | _ => emptyMap
+  }
+  let r2 = Belt.Map.String.set(r, id, value)->IEvRecord
+  Belt.Map.String.set(container, typeReferencesKey, r2)->NameSpace
+}
+
+let set = (nameSpace: t, id: string, value): t => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.set(container, id, value)->NameSpace
+}
+
+let emptyModule: t = NameSpace(Belt.Map.String.empty)
+
+let fromTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceFromTypeScriptBindings
+let toTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceToTypeScriptBindings
+
+let toExpressionValue = (nameSpace: t): internalExpressionValue => IEvModule(nameSpace)
+let fromExpressionValue = (aValue: internalExpressionValue): t =>
  switch aValue {
-  | EvModule(r) => fromRecord(r)
+  | IEvModule(nameSpace) => nameSpace
  | _ => emptyModule
  }

-let toString = (container: t): string => container->toRecord->EvRecord->expressionValueToString
+let fromArray = a => Belt.Map.String.fromArray(a)->NameSpace
+
+let merge = (nameSpace: t, other: t): t => {
+  let NameSpace(container) = nameSpace
+  let NameSpace(otherContainer) = other
+  otherContainer
+  ->Belt.Map.String.reduce(container, (container, key, value) =>
+    Belt.Map.String.set(container, key, value)
+  )
+  ->NameSpace
+}
+
+let removeOther = (nameSpace: t, other: t): t => {
+  let NameSpace(container) = nameSpace
+  let NameSpace(otherContainer) = other
+  let keys = Belt.Map.String.keysToArray(otherContainer)
+  Belt.Map.String.keep(container, (key, _value) => {
+    let removeThis = Js.Array2.includes(keys, key)
+    !removeThis
+  })->NameSpace
+}

 // -- Module definition
-let define = (container: t, identifier: string, ev: expressionValue): t =>
-  Belt.Map.String.set(container, identifier, ev) // TODO build lambda for polymorphic functions here
+let define = (nameSpace: t, identifier: string, ev: internalExpressionValue): t => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.set(container, identifier, ev)->NameSpace // TODO build lambda for polymorphic functions here
+}
+let defineNumber = (nameSpace: t, identifier: string, value: float): t =>
+  nameSpace->define(identifier, IEvNumber(value))

-let defineNumber = (container: t, identifier: string, value: float): t =>
-  container->define(identifier, EvNumber(value))
-
-let defineModule = (container: t, identifier: string, value: t): t =>
-  container->define(identifier, toExpressionValue(value))
+let defineModule = (nameSpace: t, identifier: string, value: t): t =>
+  nameSpace->define(identifier, toExpressionValue(value))
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module_TypeChecker.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module_TypeChecker.res
@ -0,0 +1,16 @@
+module ExpressionValue = ReducerInterface_InternalExpressionValue
+module ExpressionT = Reducer_Expression_T
+open ExpressionValue
+
+let isOfResolvedType = (aType, aValue) => {
+  let caseTypeIdentifier = (aTypeIdentifier0, aValue) => {
+    let valueType = aValue->valueToValueType->valueTypeToString->Js.String2.toLowerCase
+    let aTypeIdentifier = aTypeIdentifier0->Js.String2.toLowerCase
+    aTypeIdentifier === valueType
+  }
+
+  switch aType {
+  | IEvTypeIdentifier(aTypeIdentifier) => caseTypeIdentifier(aTypeIdentifier, aValue)
+  | _ => false
+  }
+}
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res
@ -1,10 +1,11 @@
-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module ExpressionT = Reducer_Expression_T
 module ExternalLibrary = ReducerInterface.ExternalLibrary
 module Lambda = Reducer_Expression_Lambda
 module MathJs = Reducer_MathJs
+module Module = Reducer_Category_Module
 module Result = Belt.Result
-open ReducerInterface.ExpressionValue
+open ReducerInterface_InternalExpressionValue
 open Reducer_ErrorValue

 /*
@ -31,23 +32,29 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
  let constructRecord = arrayOfPairs => {
    Belt.Array.map(arrayOfPairs, pairValue =>
      switch pairValue {
-      | EvArray([EvString(key), valueValue]) => (key, valueValue)
-      | _ => ("wrong key type", pairValue->toStringWithType->EvString)
+      | IEvArray([IEvString(key), valueValue]) => (key, valueValue)
+      | _ => ("wrong key type", pairValue->toStringWithType->IEvString)
      }
    )
-    ->Js.Dict.fromArray
-    ->EvRecord
+    ->Belt.Map.String.fromArray
+    ->IEvRecord
    ->Ok
  }

-  let arrayAtIndex = (aValueArray: array<expressionValue>, fIndex: float) =>
+  let arrayAtIndex = (aValueArray: array<internalExpressionValue>, fIndex: float) =>
    switch Belt.Array.get(aValueArray, Belt.Int.fromFloat(fIndex)) {
    | Some(value) => value->Ok
    | None => REArrayIndexNotFound("Array index not found", Belt.Int.fromFloat(fIndex))->Error
    }

-  let recordAtIndex = (dict: Js.Dict.t<expressionValue>, sIndex) =>
-    switch Js.Dict.get(dict, sIndex) {
+  let moduleAtIndex = (nameSpace: nameSpace, sIndex) =>
+    switch Module.get(nameSpace, sIndex) {
+    | Some(value) => value->Ok
+    | None => RERecordPropertyNotFound("Module property not found", sIndex)->Error
+    }
+
+  let recordAtIndex = (dict: Belt.Map.String.t<internalExpressionValue>, sIndex) =>
+    switch Belt.Map.String.get(dict, sIndex) {
    | Some(value) => value->Ok
    | None => RERecordPropertyNotFound("Record property not found", sIndex)->Error
    }
@ -55,70 +62,37 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
  let doAddArray = (originalA, b) => {
    let a = originalA->Js.Array2.copy
    let _ = Js.Array2.pushMany(a, b)
-    a->EvArray->Ok
+    a->IEvArray->Ok
  }
  let doAddString = (a, b) => {
    let answer = Js.String2.concat(a, b)
-    answer->EvString->Ok
+    answer->IEvString->Ok
  }

-  let inspect = (value: expressionValue) => {
+  let inspect = (value: internalExpressionValue) => {
    Js.log(value->toString)
    value->Ok
  }

-  let inspectLabel = (value: expressionValue, label: string) => {
+  let inspectLabel = (value: internalExpressionValue, label: string) => {
    Js.log(`${label}: ${value->toString}`)
    value->Ok
  }

-  let doSetBindings = (
-    externalBindings: externalBindings,
-    symbol: string,
-    value: expressionValue,
-  ) => {
-    Bindings.fromExternalBindings(externalBindings)
-    ->Belt.Map.String.set(symbol, value)
-    ->Bindings.toExternalBindings
-    ->EvRecord
-    ->Ok
-  }
-
-  let doSetBindingsInNamespace = (
-    externalBindings: externalBindings,
-    symbol: string,
-    value: expressionValue,
-    namespace: string,
-  ) => {
-    let bindings = Bindings.fromExternalBindings(externalBindings)
-    let evAliases = bindings->Belt.Map.String.getWithDefault(namespace, EvRecord(Js.Dict.empty()))
-    let newEvAliases = switch evAliases {
-    | EvRecord(dict) => {
-        Js.Dict.set(dict, symbol, value)
-        dict->EvRecord
-      }
-    | _ => Js.Dict.empty()->EvRecord
-    }
-    bindings
-    ->Belt.Map.String.set(namespace, newEvAliases)
-    ->Bindings.toExternalBindings
-    ->EvRecord
-    ->Ok
+  let doSetBindings = (bindings: nameSpace, symbol: string, value: internalExpressionValue) => {
+    Module.set(bindings, symbol, value)->IEvModule->Ok
  }

  let doSetTypeAliasBindings = (
-    externalBindings: externalBindings,
+    bindings: nameSpace,
    symbol: string,
-    value: expressionValue,
-  ) => doSetBindingsInNamespace(externalBindings, symbol, value, Bindings.typeAliasesKey)
+    value: internalExpressionValue,
+  ) => Module.setTypeAlias(bindings, symbol, value)->IEvModule->Ok

-  let doSetTypeOfBindings = (
-    externalBindings: externalBindings,
-    symbol: string,
-    value: expressionValue,
-  ) => doSetBindingsInNamespace(externalBindings, symbol, value, Bindings.typeReferencesKey)
+  let doSetTypeOfBindings = (bindings: nameSpace, symbol: string, value: internalExpressionValue) =>
+    Module.setTypeOf(bindings, symbol, value)->IEvModule->Ok

-  let doExportBindings = (externalBindings: externalBindings) => EvRecord(externalBindings)->Ok
+  let doExportBindings = (bindings: nameSpace) => bindings->Module.toExpressionValue->Ok

  let doKeepArray = (aValueArray, aLambdaValue) => {
    let rMappedList = aValueArray->Belt.Array.reduceReverse(Ok(list{}), (rAcc, elem) =>
@ -126,13 +100,13 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
        let rNewElem = Lambda.doLambdaCall(aLambdaValue, list{elem}, environment, reducer)
        rNewElem->Result.map(newElem =>
          switch newElem {
-          | EvBool(true) => list{elem, ...acc}
+          | IEvBool(true) => list{elem, ...acc}
          | _ => acc
          }
        )
      })
    )
-    rMappedList->Result.map(mappedList => mappedList->Belt.List.toArray->EvArray)
+    rMappedList->Result.map(mappedList => mappedList->Belt.List.toArray->IEvArray)
  }

  let doMapArray = (aValueArray, aLambdaValue) => {
@ -142,35 +116,36 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
        rNewElem->Result.map(newElem => list{newElem, ...acc})
      })
    )
-    rMappedList->Result.map(mappedList => mappedList->Belt.List.toArray->EvArray)
+    rMappedList->Result.map(mappedList => mappedList->Belt.List.toArray->IEvArray)
  }

  module SampleMap = {
    type t = SampleSetDist.t
    let doLambdaCall = (aLambdaValue, list) =>
      switch Lambda.doLambdaCall(aLambdaValue, list, environment, reducer) {
-      | Ok(EvNumber(f)) => Ok(f)
+      | Ok(IEvNumber(f)) => Ok(f)
      | _ => Error(Operation.SampleMapNeedsNtoNFunction)
      }

    let toType = r =>
      switch r {
-      | Ok(r) => Ok(EvDistribution(SampleSet(r)))
+      | Ok(r) => Ok(IEvDistribution(SampleSet(r)))
      | Error(r) => Error(REDistributionError(SampleSetError(r)))
      }

    let map1 = (sampleSetDist: t, aLambdaValue) => {
-      let fn = r => doLambdaCall(aLambdaValue, list{EvNumber(r)})
+      let fn = r => doLambdaCall(aLambdaValue, list{IEvNumber(r)})
      toType(SampleSetDist.samplesMap(~fn, sampleSetDist))
    }

    let map2 = (t1: t, t2: t, aLambdaValue) => {
-      let fn = (a, b) => doLambdaCall(aLambdaValue, list{EvNumber(a), EvNumber(b)})
+      let fn = (a, b) => doLambdaCall(aLambdaValue, list{IEvNumber(a), IEvNumber(b)})
      SampleSetDist.map2(~fn, ~t1, ~t2)->toType
    }

    let map3 = (t1: t, t2: t, t3: t, aLambdaValue) => {
-      let fn = (a, b, c) => doLambdaCall(aLambdaValue, list{EvNumber(a), EvNumber(b), EvNumber(c)})
+      let fn = (a, b, c) =>
+        doLambdaCall(aLambdaValue, list{IEvNumber(a), IEvNumber(b), IEvNumber(c)})
      SampleSetDist.map3(~fn, ~t1, ~t2, ~t3)->toType
    }
  }
@ -192,132 +167,128 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
  }

  let typeModifier_memberOf = (aType, anArray) => {
-    let newRecord = Js.Dict.fromArray([
-      ("typeTag", EvString("typeIdentifier")),
+    let newRecord = Belt.Map.String.fromArray([
+      ("typeTag", IEvString("typeIdentifier")),
      ("typeIdentifier", aType),
    ])
-    newRecord->Js.Dict.set("memberOf", anArray)
-    newRecord->EvRecord->Ok
+    newRecord->Belt.Map.String.set("memberOf", anArray)->IEvRecord->Ok
  }
  let typeModifier_memberOf_update = (aRecord, anArray) => {
-    let newRecord = aRecord->Js.Dict.entries->Js.Dict.fromArray
-    newRecord->Js.Dict.set("memberOf", anArray)
-    newRecord->EvRecord->Ok
+    aRecord->Belt.Map.String.set("memberOf", anArray)->IEvRecord->Ok
  }

  let typeModifier_min = (aType, value) => {
-    let newRecord = Js.Dict.fromArray([
-      ("typeTag", EvString("typeIdentifier")),
+    let newRecord = Belt.Map.String.fromArray([
+      ("typeTag", IEvString("typeIdentifier")),
      ("typeIdentifier", aType),
    ])
-    newRecord->Js.Dict.set("min", value)
-    newRecord->EvRecord->Ok
+    newRecord->Belt.Map.String.set("min", value)->IEvRecord->Ok
  }
  let typeModifier_min_update = (aRecord, value) => {
-    let newRecord = aRecord->Js.Dict.entries->Js.Dict.fromArray
-    newRecord->Js.Dict.set("min", value)
-    newRecord->EvRecord->Ok
+    aRecord->Belt.Map.String.set("min", value)->IEvRecord->Ok
  }

  let typeModifier_max = (aType, value) => {
-    let newRecord = Js.Dict.fromArray([
-      ("typeTag", EvString("typeIdentifier")),
+    let newRecord = Belt.Map.String.fromArray([
+      ("typeTag", IEvString("typeIdentifier")),
      ("typeIdentifier", aType),
    ])
-    newRecord->Js.Dict.set("max", value)
-    newRecord->EvRecord->Ok
-  }
-  let typeModifier_max_update = (aRecord, value) => {
-    let newRecord = aRecord->Js.Dict.entries->Js.Dict.fromArray
-    newRecord->Js.Dict.set("max", value)
-    newRecord->EvRecord->Ok
+    newRecord->Belt.Map.String.set("max", value)->IEvRecord->Ok
  }
+  let typeModifier_max_update = (aRecord, value) =>
+    aRecord->Belt.Map.String.set("max", value)->IEvRecord->Ok

-  let typeModifier_opaque_update = aRecord => {
-    let newRecord = aRecord->Js.Dict.entries->Js.Dict.fromArray
-    newRecord->Js.Dict.set("opaque", EvBool(true))
-    newRecord->EvRecord->Ok
-  }
+  let typeModifier_opaque_update = aRecord =>
+    aRecord->Belt.Map.String.set("opaque", IEvBool(true))->IEvRecord->Ok

  let typeOr = evArray => {
-    let newRecord = Js.Dict.fromArray([("typeTag", EvString("typeOr")), ("typeOr", evArray)])
-    newRecord->EvRecord->Ok
+    let newRecord = Belt.Map.String.fromArray([
+      ("typeTag", IEvString("typeOr")),
+      ("typeOr", evArray),
+    ])
+    newRecord->IEvRecord->Ok
  }
  let typeFunction = anArray => {
    let output = Belt.Array.getUnsafe(anArray, Js.Array2.length(anArray) - 1)
    let inputs = Js.Array2.slice(anArray, ~start=0, ~end_=-1)
-    let newRecord = Js.Dict.fromArray([
-      ("typeTag", EvString("typeFunction")),
-      ("inputs", EvArray(inputs)),
+    let newRecord = Belt.Map.String.fromArray([
+      ("typeTag", IEvString("typeFunction")),
+      ("inputs", IEvArray(inputs)),
      ("output", output),
    ])
-    newRecord->EvRecord->Ok
+    newRecord->IEvRecord->Ok
  }

  switch call {
-  | ("$_atIndex_$", [EvArray(aValueArray), EvNumber(fIndex)]) => arrayAtIndex(aValueArray, fIndex)
-  | ("$_atIndex_$", [EvModule(dict), EvString(sIndex)]) => recordAtIndex(dict, sIndex)
-  | ("$_atIndex_$", [EvRecord(dict), EvString(sIndex)]) => recordAtIndex(dict, sIndex)
-  | ("$_constructArray_$", [EvArray(aValueArray)]) => EvArray(aValueArray)->Ok
-  | ("$_constructRecord_$", [EvArray(arrayOfPairs)]) => constructRecord(arrayOfPairs)
-  | ("$_exportBindings_$", [EvRecord(externalBindings)]) => doExportBindings(externalBindings)
-  | ("$_setBindings_$", [EvRecord(externalBindings), EvSymbol(symbol), value]) =>
-    doSetBindings(externalBindings, symbol, value)
-  | ("$_setTypeAliasBindings_$", [EvRecord(externalBindings), EvTypeIdentifier(symbol), value]) =>
-    doSetTypeAliasBindings(externalBindings, symbol, value)
-  | ("$_setTypeOfBindings_$", [EvRecord(externalBindings), EvSymbol(symbol), value]) =>
-    doSetTypeOfBindings(externalBindings, symbol, value)
-  | ("$_typeModifier_memberOf_$", [EvTypeIdentifier(typeIdentifier), EvArray(arr)]) =>
-    typeModifier_memberOf(EvTypeIdentifier(typeIdentifier), EvArray(arr))
-  | ("$_typeModifier_memberOf_$", [EvRecord(typeRecord), EvArray(arr)]) =>
-    typeModifier_memberOf_update(typeRecord, EvArray(arr))
-  | ("$_typeModifier_min_$", [EvTypeIdentifier(typeIdentifier), value]) =>
-    typeModifier_min(EvTypeIdentifier(typeIdentifier), value)
-  | ("$_typeModifier_min_$", [EvRecord(typeRecord), value]) =>
+  | ("$_atIndex_$", [IEvArray(aValueArray), IEvNumber(fIndex)]) => arrayAtIndex(aValueArray, fIndex)
+  | ("$_atIndex_$", [IEvModule(dict), IEvString(sIndex)]) => moduleAtIndex(dict, sIndex)
+  | ("$_atIndex_$", [IEvRecord(dict), IEvString(sIndex)]) => recordAtIndex(dict, sIndex)
+  | ("$_constructArray_$", [IEvArray(aValueArray)]) => IEvArray(aValueArray)->Ok
+  | ("$_constructRecord_$", [IEvArray(arrayOfPairs)]) => constructRecord(arrayOfPairs)
+  | ("$_exportBindings_$", [IEvModule(nameSpace)]) => doExportBindings(nameSpace)
+  | ("$_setBindings_$", [IEvModule(nameSpace), IEvSymbol(symbol), value]) =>
+    doSetBindings(nameSpace, symbol, value)
+  | ("$_setTypeAliasBindings_$", [IEvModule(nameSpace), IEvTypeIdentifier(symbol), value]) =>
+    doSetTypeAliasBindings(nameSpace, symbol, value)
+  | ("$_setTypeOfBindings_$", [IEvModule(nameSpace), IEvSymbol(symbol), value]) =>
+    doSetTypeOfBindings(nameSpace, symbol, value)
+  | ("$_typeModifier_memberOf_$", [IEvTypeIdentifier(typeIdentifier), IEvArray(arr)]) =>
+    typeModifier_memberOf(IEvTypeIdentifier(typeIdentifier), IEvArray(arr))
+  | ("$_typeModifier_memberOf_$", [IEvRecord(typeRecord), IEvArray(arr)]) =>
+    typeModifier_memberOf_update(typeRecord, IEvArray(arr))
+  | ("$_typeModifier_min_$", [IEvTypeIdentifier(typeIdentifier), value]) =>
+    typeModifier_min(IEvTypeIdentifier(typeIdentifier), value)
+  | ("$_typeModifier_min_$", [IEvRecord(typeRecord), value]) =>
    typeModifier_min_update(typeRecord, value)
-  | ("$_typeModifier_max_$", [EvTypeIdentifier(typeIdentifier), value]) =>
-    typeModifier_max(EvTypeIdentifier(typeIdentifier), value)
-  | ("$_typeModifier_max_$", [EvRecord(typeRecord), value]) =>
+  | ("$_typeModifier_max_$", [IEvTypeIdentifier(typeIdentifier), value]) =>
+    typeModifier_max(IEvTypeIdentifier(typeIdentifier), value)
+  | ("$_typeModifier_max_$", [IEvRecord(typeRecord), value]) =>
    typeModifier_max_update(typeRecord, value)
-  | ("$_typeModifier_opaque_$", [EvRecord(typeRecord)]) => typeModifier_opaque_update(typeRecord)
-  | ("$_typeOr_$", [EvArray(arr)]) => typeOr(EvArray(arr))
-  | ("$_typeFunction_$", [EvArray(arr)]) => typeFunction(arr)
-  | ("concat", [EvArray(aValueArray), EvArray(bValueArray)]) => doAddArray(aValueArray, bValueArray)
-  | ("concat", [EvString(aValueString), EvString(bValueString)]) =>
+  | ("$_typeModifier_opaque_$", [IEvRecord(typeRecord)]) => typeModifier_opaque_update(typeRecord)
+  | ("$_typeOr_$", [IEvArray(arr)]) => typeOr(IEvArray(arr))
+  | ("$_typeFunction_$", [IEvArray(arr)]) => typeFunction(arr)
+  | ("concat", [IEvArray(aValueArray), IEvArray(bValueArray)]) =>
+    doAddArray(aValueArray, bValueArray)
+  | ("concat", [IEvString(aValueString), IEvString(bValueString)]) =>
    doAddString(aValueString, bValueString)
-  | ("inspect", [value, EvString(label)]) => inspectLabel(value, label)
+  | ("inspect", [value, IEvString(label)]) => inspectLabel(value, label)
  | ("inspect", [value]) => inspect(value)
-  | ("filter", [EvArray(aValueArray), EvLambda(aLambdaValue)]) =>
+  | ("filter", [IEvArray(aValueArray), IEvLambda(aLambdaValue)]) =>
    doKeepArray(aValueArray, aLambdaValue)
-  | ("map", [EvArray(aValueArray), EvLambda(aLambdaValue)]) => doMapArray(aValueArray, aLambdaValue)
-  | ("mapSamples", [EvDistribution(SampleSet(dist)), EvLambda(aLambdaValue)]) =>
+  | ("map", [IEvArray(aValueArray), IEvLambda(aLambdaValue)]) =>
+    doMapArray(aValueArray, aLambdaValue)
+  | ("mapSamples", [IEvDistribution(SampleSet(dist)), IEvLambda(aLambdaValue)]) =>
    SampleMap.map1(dist, aLambdaValue)
  | (
      "mapSamples2",
-      [EvDistribution(SampleSet(dist1)), EvDistribution(SampleSet(dist2)), EvLambda(aLambdaValue)],
+      [
+        IEvDistribution(SampleSet(dist1)),
+        IEvDistribution(SampleSet(dist2)),
+        IEvLambda(aLambdaValue),
+      ],
    ) =>
    SampleMap.map2(dist1, dist2, aLambdaValue)
  | (
      "mapSamples3",
      [
-        EvDistribution(SampleSet(dist1)),
-        EvDistribution(SampleSet(dist2)),
-        EvDistribution(SampleSet(dist3)),
-        EvLambda(aLambdaValue),
+        IEvDistribution(SampleSet(dist1)),
+        IEvDistribution(SampleSet(dist2)),
+        IEvDistribution(SampleSet(dist3)),
+        IEvLambda(aLambdaValue),
      ],
    ) =>
    SampleMap.map3(dist1, dist2, dist3, aLambdaValue)
-  | ("reduce", [EvArray(aValueArray), initialValue, EvLambda(aLambdaValue)]) =>
+  | ("reduce", [IEvArray(aValueArray), initialValue, IEvLambda(aLambdaValue)]) =>
    doReduceArray(aValueArray, initialValue, aLambdaValue)
-  | ("reduceReverse", [EvArray(aValueArray), initialValue, EvLambda(aLambdaValue)]) =>
+  | ("reduceReverse", [IEvArray(aValueArray), initialValue, IEvLambda(aLambdaValue)]) =>
    doReduceReverseArray(aValueArray, initialValue, aLambdaValue)
-  | ("reverse", [EvArray(aValueArray)]) => aValueArray->Belt.Array.reverse->EvArray->Ok
-  | (_, [EvBool(_)])
-  | (_, [EvNumber(_)])
-  | (_, [EvString(_)])
-  | (_, [EvBool(_), EvBool(_)])
-  | (_, [EvNumber(_), EvNumber(_)])
-  | (_, [EvString(_), EvString(_)]) =>
+  | ("reverse", [IEvArray(aValueArray)]) => aValueArray->Belt.Array.reverse->IEvArray->Ok
+  | (_, [IEvBool(_)])
+  | (_, [IEvNumber(_)])
+  | (_, [IEvString(_)])
+  | (_, [IEvBool(_), IEvBool(_)])
+  | (_, [IEvNumber(_), IEvNumber(_)])
+  | (_, [IEvString(_), IEvString(_)]) =>
    callMathJs(call)
  | call =>
    Error(REFunctionNotFound(call->functionCallToCallSignature->functionCallSignatureToString)) // Report full type signature as error
@ -328,7 +299,7 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
  Reducer uses Result monad while reducing expressions
 */
 let dispatch = (call: functionCall, environment, reducer: ExpressionT.reducerFn): result<
-  expressionValue,
+  internalExpressionValue,
  errorValue,
 > =>
  try {
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltInMacros.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltInMacros.res
@ -3,19 +3,19 @@
  they take expressions as parameters and return a new expression.
  Macros are used to define language building blocks. They are like Lisp macros.
 */
-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module ErrorValue = Reducer_ErrorValue
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module ExpressionWithContext = Reducer_ExpressionWithContext
+module Module = Reducer_Category_Module
 module Result = Belt.Result
 open Reducer_Expression_ExpressionBuilder

-type environment = ExpressionValue.environment
+type environment = InternalExpressionValue.environment
 type errorValue = ErrorValue.errorValue
 type expression = ExpressionT.expression
-type expressionValue = ExpressionValue.expressionValue
 type expressionWithContext = ExpressionWithContext.expressionWithContext

 let dispatchMacroCall = (
@ -27,13 +27,13 @@ let dispatchMacroCall = (
  let useExpressionToSetBindings = (bindingExpr: expression, environment, statement, newCode) => {
    let rExternalBindingsValue = reduceExpression(bindingExpr, bindings, environment)

-    rExternalBindingsValue->Result.flatMap(externalBindingsValue => {
-      let newBindings = Bindings.fromValue(externalBindingsValue)
+    rExternalBindingsValue->Result.flatMap(nameSpaceValue => {
+      let newBindings = Module.fromExpressionValue(nameSpaceValue)

-      let rNewStatement = Bindings.replaceSymbols(newBindings, statement)
+      let rNewStatement = BindingsReplacer.replaceSymbols(newBindings, statement)
      rNewStatement->Result.map(boundStatement =>
        ExpressionWithContext.withContext(
-          newCode(newBindings->Bindings.toExternalBindings->eRecord, boundStatement),
+          newCode(newBindings->eModule, boundStatement),
          newBindings,
        )
      )
@ -51,7 +51,7 @@ let dispatchMacroCall = (
  let doBindStatement = (bindingExpr: expression, statement: expression, environment) => {
    let defaultStatement = ErrorValue.REAssignmentExpected->Error
    switch statement {
-    | ExpressionT.EList(list{ExpressionT.EValue(EvCall(callName)), symbolExpr, statement}) => {
+    | ExpressionT.EList(list{ExpressionT.EValue(IEvCall(callName)), symbolExpr, statement}) => {
        let setBindingsFn = correspondingSetBindingsFn(callName)
        if setBindingsFn !== "" {
          useExpressionToSetBindings(bindingExpr, environment, statement, (
@ -77,7 +77,7 @@ let dispatchMacroCall = (
      ) => boundStatement)

    switch statement {
-    | ExpressionT.EList(list{ExpressionT.EValue(EvCall(callName)), symbolExpr, statement}) => {
+    | ExpressionT.EList(list{ExpressionT.EValue(IEvCall(callName)), symbolExpr, statement}) => {
        let setBindingsFn = correspondingSetBindingsFn(callName)
        if setBindingsFn !== "" {
          useExpressionToSetBindings(bindingExpr, environment, statement, (
@ -123,10 +123,7 @@ let dispatchMacroCall = (
    bindings: ExpressionT.bindings,
    parameters: array<string>,
    lambdaDefinition: ExpressionT.expression,
-  ) =>
-    ExpressionWithContext.noContext(
-      eLambda(parameters, bindings->Bindings.toExternalBindings, lambdaDefinition),
-    )->Ok
+  ) => ExpressionWithContext.noContext(eLambda(parameters, bindings, lambdaDefinition))->Ok

  let doTernary = (
    condition: expression,
@ -139,11 +136,11 @@ let dispatchMacroCall = (
    let rCondition = reduceExpression(blockCondition, bindings, environment)
    rCondition->Result.flatMap(conditionValue =>
      switch conditionValue {
-      | ExpressionValue.EvBool(false) => {
+      | InternalExpressionValue.IEvBool(false) => {
          let ifFalseBlock = eBlock(list{ifFalse})
          ExpressionWithContext.withContext(ifFalseBlock, bindings)->Ok
        }
-      | ExpressionValue.EvBool(true) => {
+      | InternalExpressionValue.IEvBool(true) => {
          let ifTrueBlock = eBlock(list{ifTrue})
          ExpressionWithContext.withContext(ifTrueBlock, bindings)->Ok
        }
@ -158,32 +155,32 @@ let dispatchMacroCall = (
  > =>
    switch aList {
    | list{
-        ExpressionT.EValue(EvCall("$$_bindStatement_$$")),
+        ExpressionT.EValue(IEvCall("$$_bindStatement_$$")),
        bindingExpr: ExpressionT.expression,
        statement,
      } =>
      doBindStatement(bindingExpr, statement, environment)
-    | list{ExpressionT.EValue(EvCall("$$_bindStatement_$$")), statement} =>
+    | list{ExpressionT.EValue(IEvCall("$$_bindStatement_$$")), statement} =>
      // bindings of the context are used when there is no binding expression
-      doBindStatement(eRecord(Bindings.toExternalBindings(bindings)), statement, environment)
+      doBindStatement(eModule(bindings), statement, environment)
    | list{
-        ExpressionT.EValue(EvCall("$$_bindExpression_$$")),
+        ExpressionT.EValue(IEvCall("$$_bindExpression_$$")),
        bindingExpr: ExpressionT.expression,
        expression,
      } =>
      doBindExpression(bindingExpr, expression, environment)
-    | list{ExpressionT.EValue(EvCall("$$_bindExpression_$$")), expression} =>
+    | list{ExpressionT.EValue(IEvCall("$$_bindExpression_$$")), expression} =>
      // bindings of the context are used when there is no binding expression
-      doBindExpression(eRecord(Bindings.toExternalBindings(bindings)), expression, environment)
-    | list{ExpressionT.EValue(EvCall("$$_block_$$")), ...exprs} =>
+      doBindExpression(eModule(bindings), expression, environment)
+    | list{ExpressionT.EValue(IEvCall("$$_block_$$")), ...exprs} =>
      doBlock(exprs, bindings, environment)
    | list{
-        ExpressionT.EValue(EvCall("$$_lambda_$$")),
-        ExpressionT.EValue(EvArrayString(parameters)),
+        ExpressionT.EValue(IEvCall("$$_lambda_$$")),
+        ExpressionT.EValue(IEvArrayString(parameters)),
        lambdaDefinition,
      } =>
      doLambdaDefinition(bindings, parameters, lambdaDefinition)
-    | list{ExpressionT.EValue(EvCall("$$_ternary_$$")), condition, ifTrue, ifFalse} =>
+    | list{ExpressionT.EValue(IEvCall("$$_ternary_$$")), condition, ifTrue, ifFalse} =>
      doTernary(condition, ifTrue, ifFalse, bindings, environment)
    | _ => ExpressionWithContext.noContext(ExpressionT.EList(aList))->Ok
    }
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res
@ -1,20 +1,20 @@
-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module BuiltIn = Reducer_Dispatch_BuiltIn
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
-module ExpressionValue = ReducerInterface.ExpressionValue
 module Extra = Reducer_Extra
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Lambda = Reducer_Expression_Lambda
 module Macro = Reducer_Expression_Macro
 module MathJs = Reducer_MathJs
+module Module = Reducer_Category_Module
 module Result = Belt.Result
 module T = Reducer_Expression_T

-type environment = ReducerInterface_ExpressionValue.environment
+type environment = InternalExpressionValue.environment
 type errorValue = Reducer_ErrorValue.errorValue
 type expression = T.expression
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
-type internalCode = ReducerInterface_ExpressionValue.internalCode
+type internalExpressionValue = InternalExpressionValue.t
+type externalExpressionValue = ReducerInterface_ExternalExpressionValue.t
 type t = expression

 /*
@ -27,7 +27,7 @@ let parse = (peggyCode: string): result<t, errorValue> =>
  Recursively evaluate/reduce the expression (Lisp AST)
 */
 let rec reduceExpression = (expression: t, bindings: T.bindings, environment: environment): result<
-  expressionValue,
+  internalExpressionValue,
  'e,
 > => {
  // Js.log(`reduce: ${T.toString(expression)} bindings: ${bindings->Bindings.toString}`)
@ -35,7 +35,7 @@ let rec reduceExpression = (expression: t, bindings: T.bindings, environment: en
  | T.EValue(value) => value->Ok
  | T.EList(list) =>
    switch list {
-    | list{EValue(EvCall(fName)), ..._args} =>
+    | list{EValue(IEvCall(fName)), ..._args} =>
      switch Macro.isMacroName(fName) {
      // A macro expands then reduces itself
      | true => Macro.doMacroCall(expression, bindings, environment, reduceExpression)
@ -50,11 +50,11 @@ and reduceExpressionList = (
  expressions: list<t>,
  bindings: T.bindings,
  environment: environment,
-): result<expressionValue, 'e> => {
-  let racc: result<list<expressionValue>, 'e> = expressions->Belt.List.reduceReverse(Ok(list{}), (
-    racc,
-    each: expression,
-  ) =>
+): result<internalExpressionValue, 'e> => {
+  let racc: result<
+    list<internalExpressionValue>,
+    'e,
+  > = expressions->Belt.List.reduceReverse(Ok(list{}), (racc, each: expression) =>
    racc->Result.flatMap(acc => {
      each
      ->reduceExpression(bindings, environment)
@ -69,12 +69,12 @@ and reduceExpressionList = (
 /*
    After reducing each level of expression(Lisp AST), we have a value list to evaluate
 */
-and reduceValueList = (valueList: list<expressionValue>, environment): result<
-  expressionValue,
+and reduceValueList = (valueList: list<internalExpressionValue>, environment): result<
+  internalExpressionValue,
  'e,
 > =>
  switch valueList {
-  | list{EvCall(fName), ...args} => {
+  | list{IEvCall(fName), ...args} => {
      let rCheckedArgs = switch fName {
      | "$_setBindings_$" | "$_setTypeOfBindings_$" | "$_setTypeAliasBindings_$" => args->Ok
      | _ => args->Lambda.checkIfReduced
@ -84,14 +84,14 @@ and reduceValueList = (valueList: list<expressionValue>, environment): result<
        (fName, checkedArgs->Belt.List.toArray)->BuiltIn.dispatch(environment, reduceExpression)
      )
    }
-  | list{EvLambda(_)} =>
+  | list{IEvLambda(_)} =>
    // TODO: remove on solving issue#558
    valueList
    ->Lambda.checkIfReduced
    ->Result.flatMap(reducedValueList =>
-      reducedValueList->Belt.List.toArray->ExpressionValue.EvArray->Ok
+      reducedValueList->Belt.List.toArray->InternalExpressionValue.IEvArray->Ok
    )
-  | list{EvLambda(lamdaCall), ...args} =>
+  | list{IEvLambda(lamdaCall), ...args} =>
    args
    ->Lambda.checkIfReduced
    ->Result.flatMap(checkedArgs =>
@ -102,60 +102,55 @@ and reduceValueList = (valueList: list<expressionValue>, environment): result<
    valueList
    ->Lambda.checkIfReduced
    ->Result.flatMap(reducedValueList =>
-      reducedValueList->Belt.List.toArray->ExpressionValue.EvArray->Ok
+      reducedValueList->Belt.List.toArray->InternalExpressionValue.IEvArray->Ok
    )
  }

 let evalUsingBindingsExpression_ = (aExpression, bindings, environment): result<
-  expressionValue,
+  internalExpressionValue,
  'e,
 > => reduceExpression(aExpression, bindings, environment)

 let evaluateUsingOptions = (
-  ~environment: option<ReducerInterface_ExpressionValue.environment>,
-  ~externalBindings: option<ReducerInterface_ExpressionValue.externalBindings>,
+  ~environment: option<ReducerInterface_ExternalExpressionValue.environment>,
+  ~externalBindings: option<ReducerInterface_ExternalExpressionValue.externalBindings>,
  code: string,
-): result<expressionValue, errorValue> => {
+): result<externalExpressionValue, errorValue> => {
  let anEnvironment = Belt.Option.getWithDefault(
    environment,
-    ReducerInterface_ExpressionValue.defaultEnvironment,
+    ReducerInterface_ExternalExpressionValue.defaultEnvironment,
  )

-  let anExternalBindings = switch externalBindings {
-  | Some(bindings) => {
-      let cloneLib = ReducerInterface_StdLib.externalStdLib->Reducer_Category_Bindings.cloneRecord
-      Js.Dict.entries(bindings)->Js.Array2.reduce((acc, (key, value)) => {
-        acc->Js.Dict.set(key, value)
-        acc
-      }, cloneLib)
-    }
-  | None => ReducerInterface_StdLib.externalStdLib
-  }
+  let mergedBindings: InternalExpressionValue.nameSpace = Module.merge(
+    ReducerInterface_StdLib.internalStdLib,
+    Belt.Option.map(externalBindings, Module.fromTypeScriptBindings)->Belt.Option.getWithDefault(
+      Module.emptyModule,
+    ),
+  )

-  let bindings = anExternalBindings->Bindings.fromExternalBindings
-
-  parse(code)->Result.flatMap(expr => evalUsingBindingsExpression_(expr, bindings, anEnvironment))
+  parse(code)
+  ->Result.flatMap(expr => evalUsingBindingsExpression_(expr, mergedBindings, anEnvironment))
+  ->Result.map(ReducerInterface_InternalExpressionValue.toExternal)
 }

 /*
-  Evaluates Squiggle code and bindings via Reducer and answers the result
+  IEvaluates Squiggle code and bindings via Reducer and answers the result
 */
-let evaluate = (code: string): result<expressionValue, errorValue> => {
+let evaluate = (code: string): result<externalExpressionValue, errorValue> => {
  evaluateUsingOptions(~environment=None, ~externalBindings=None, code)
 }
-let eval = evaluate
 let evaluatePartialUsingExternalBindings = (
  code: string,
-  externalBindings: ReducerInterface_ExpressionValue.externalBindings,
-  environment: ReducerInterface_ExpressionValue.environment,
-): result<externalBindings, errorValue> => {
+  externalBindings: ReducerInterface_ExternalExpressionValue.externalBindings,
+  environment: ReducerInterface_ExternalExpressionValue.environment,
+): result<ReducerInterface_ExternalExpressionValue.externalBindings, errorValue> => {
  let rAnswer = evaluateUsingOptions(
    ~environment=Some(environment),
    ~externalBindings=Some(externalBindings),
    code,
  )
  switch rAnswer {
-  | Ok(EvRecord(externalBindings)) => Ok(externalBindings)
+  | Ok(EvModule(externalBindings)) => Ok(externalBindings)
  | Ok(_) =>
    Error(Reducer_ErrorValue.RESyntaxError(`Partials must end with an assignment or record`, None))
  | Error(err) => err->Error
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res
@ -1,16 +1,16 @@
-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module ErrorValue = Reducer_ErrorValue
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Result = Belt.Result
+module Module = Reducer_Category_Module

 type bindings = ExpressionT.bindings
 type context = bindings
-type environment = ExpressionValue.environment
+type environment = InternalExpressionValue.environment
 type errorValue = Reducer_ErrorValue.errorValue
 type expression = ExpressionT.expression
-type expressionValue = ExpressionValue.expressionValue
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
+type internalExpressionValue = InternalExpressionValue.t
 type reducerFn = ExpressionT.reducerFn

 type expressionWithContext =
@ -22,7 +22,7 @@ let callReducer = (
  bindings: bindings,
  environment: environment,
  reducer: reducerFn,
-): result<expressionValue, errorValue> => {
+): result<internalExpressionValue, errorValue> => {
  switch expressionWithContext {
  | ExpressionNoContext(expr) =>
    // Js.log(`callReducer: bindings ${Bindings.toString(bindings)} expr ${ExpressionT.toString(expr)}`)
@ -40,7 +40,9 @@ let toString = expressionWithContext =>
  switch expressionWithContext {
  | ExpressionNoContext(expr) => ExpressionT.toString(expr)
  | ExpressionWithContext(expr, context) =>
-    `${ExpressionT.toString(expr)} context: ${Bindings.toString(context)}`
+    `${ExpressionT.toString(expr)} context: ${context
+      ->Module.toExpressionValue
+      ->InternalExpressionValue.toString}`
  }

 let toStringResult = rExpressionWithContext =>
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Bindings.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Bindings.res
@ -1,72 +0,0 @@
-module ErrorValue = Reducer_ErrorValue
-module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
-module Result = Belt.Result
-module Bindings = Reducer_Category_Bindings
-
-type errorValue = Reducer_ErrorValue.errorValue
-type expression = ExpressionT.expression
-type expressionValue = ExpressionValue.expressionValue
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
-
-let emptyBindings = Reducer_Category_Bindings.emptyBindings
-
-let typeAliasesKey = Bindings.typeAliasesKey
-let typeReferencesKey = Bindings.typeReferencesKey
-
-let toExternalBindings = (bindings: ExpressionT.bindings): externalBindings =>
-  Bindings.toRecord(bindings)
-
-let fromExternalBindings = (externalBindings: externalBindings): ExpressionT.bindings =>
-  Bindings.fromRecord(externalBindings)
-
-let fromValue = (aValue: expressionValue) => Bindings.fromExpressionValue(aValue)
-
-let isMacroName = (fName: string): bool => fName->Js.String2.startsWith("$$")
-
-let rec replaceSymbols = (bindings: ExpressionT.bindings, expression: expression): result<
-  expression,
-  errorValue,
-> =>
-  switch expression {
-  | ExpressionT.EValue(value) =>
-    replaceSymbolOnValue(bindings, value)->Result.map(evValue => evValue->ExpressionT.EValue)
-  | ExpressionT.EList(list) =>
-    switch list {
-    | list{EValue(EvCall(fName)), ..._args} =>
-      switch isMacroName(fName) {
-      // A macro reduces itself so we dont dive in it
-      | true => expression->Ok
-      | false => replaceSymbolsOnExpressionList(bindings, list)
-      }
-    | _ => replaceSymbolsOnExpressionList(bindings, list)
-    }
-  }
-
-and replaceSymbolsOnExpressionList = (bindings, list) => {
-  let racc = list->Belt.List.reduceReverse(Ok(list{}), (racc, each: expression) =>
-    racc->Result.flatMap(acc => {
-      replaceSymbols(bindings, each)->Result.flatMap(newNode => {
-        acc->Belt.List.add(newNode)->Ok
-      })
-    })
-  )
-  racc->Result.map(acc => acc->ExpressionT.EList)
-}
-and replaceSymbolOnValue = (bindings, evValue: expressionValue) =>
-  switch evValue {
-  | EvSymbol(symbol) => Belt.Map.String.getWithDefault(bindings, symbol, evValue)->Ok
-  | EvCall(symbol) => Belt.Map.String.getWithDefault(bindings, symbol, evValue)->checkIfCallable
-  | _ => evValue->Ok
-  }
-and checkIfCallable = (evValue: expressionValue) =>
-  switch evValue {
-  | EvCall(_) | EvLambda(_) => evValue->Ok
-  | _ => ErrorValue.RENotAFunction(ExpressionValue.toString(evValue))->Error
-  }
-
-let toString = (bindings: ExpressionT.bindings) =>
-  bindings->toExternalBindings->ExpressionValue.EvRecord->ExpressionValue.toString
-
-let externalBindingsToString = (externalBindings: externalBindings) =>
-  externalBindings->ExpressionValue.EvRecord->ExpressionValue.toString
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_BindingsReplacer.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_BindingsReplacer.res
@ -0,0 +1,53 @@
+module ErrorValue = Reducer_ErrorValue
+module ExpressionT = Reducer_Expression_T
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+module Result = Belt.Result
+module Module = Reducer_Category_Module
+
+type errorValue = Reducer_ErrorValue.errorValue
+type expression = ExpressionT.expression
+type internalExpressionValue = InternalExpressionValue.t
+type externalBindings = ReducerInterface_ExternalExpressionValue.externalBindings
+
+let isMacroName = (fName: string): bool => fName->Js.String2.startsWith("$$")
+
+let rec replaceSymbols = (bindings: ExpressionT.bindings, expression: expression): result<
+  expression,
+  errorValue,
+> =>
+  switch expression {
+  | ExpressionT.EValue(value) =>
+    replaceSymbolOnValue(bindings, value)->Result.map(evValue => evValue->ExpressionT.EValue)
+  | ExpressionT.EList(list) =>
+    switch list {
+    | list{EValue(IEvCall(fName)), ..._args} =>
+      switch isMacroName(fName) {
+      // A macro reduces itself so we dont dive in it
+      | true => expression->Ok
+      | false => replaceSymbolsOnExpressionList(bindings, list)
+      }
+    | _ => replaceSymbolsOnExpressionList(bindings, list)
+    }
+  }
+
+and replaceSymbolsOnExpressionList = (bindings, list) => {
+  let racc = list->Belt.List.reduceReverse(Ok(list{}), (racc, each: expression) =>
+    racc->Result.flatMap(acc => {
+      replaceSymbols(bindings, each)->Result.flatMap(newNode => {
+        acc->Belt.List.add(newNode)->Ok
+      })
+    })
+  )
+  racc->Result.map(acc => acc->ExpressionT.EList)
+}
+and replaceSymbolOnValue = (bindings, evValue: internalExpressionValue) =>
+  switch evValue {
+  | IEvSymbol(symbol) => Module.getWithDefault(bindings, symbol, evValue)->Ok
+  | IEvCall(symbol) => Module.getWithDefault(bindings, symbol, evValue)->checkIfCallable
+  | _ => evValue->Ok
+  }
+and checkIfCallable = (evValue: internalExpressionValue) =>
+  switch evValue {
+  | IEvCall(_) | IEvLambda(_) => evValue->Ok
+  | _ => ErrorValue.RENotAFunction(InternalExpressionValue.toString(evValue))->Error
+  }
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_ExpressionBuilder.res
@ -1,24 +1,28 @@
-module BBindings = Reducer_Expression_Bindings
+module BBindingsReplacer = Reducer_Expression_BindingsReplacer
 module BErrorValue = Reducer_ErrorValue
 module BExpressionT = Reducer_Expression_T
-module BExpressionValue = ReducerInterface.ExpressionValue
+module BInternalExpressionValue = ReducerInterface_InternalExpressionValue
+module BModule = Reducer_Category_Module

 type errorValue = BErrorValue.errorValue
 type expression = BExpressionT.expression
-type internalCode = ReducerInterface_ExpressionValue.internalCode
+type expressionOrFFI = BExpressionT.expressionOrFFI
+type ffiFn = BExpressionT.ffiFn
+type internalCode = ReducerInterface_InternalExpressionValue.internalCode

-external castExpressionToInternalCode: expression => internalCode = "%identity"
+external castExpressionToInternalCode: expressionOrFFI => internalCode = "%identity"

-let eArray = anArray => anArray->BExpressionValue.EvArray->BExpressionT.EValue
+let eArray = anArray => anArray->BInternalExpressionValue.IEvArray->BExpressionT.EValue

-let eArrayString = anArray => anArray->BExpressionValue.EvArrayString->BExpressionT.EValue
+let eArrayString = anArray => anArray->BInternalExpressionValue.IEvArrayString->BExpressionT.EValue

-let eBindings = (anArray: array<(string, BExpressionValue.expressionValue)>) =>
-  anArray->Js.Dict.fromArray->BExpressionValue.EvRecord->BExpressionT.EValue
+let eBindings = (anArray: array<(string, BInternalExpressionValue.t)>) =>
+  anArray->BModule.fromArray->BModule.toExpressionValue->BExpressionT.EValue

-let eBool = aBool => aBool->BExpressionValue.EvBool->BExpressionT.EValue
+let eBool = aBool => aBool->BInternalExpressionValue.IEvBool->BExpressionT.EValue

-let eCall = (name: string): expression => name->BExpressionValue.EvCall->BExpressionT.EValue
+let eCall = (name: string): expression =>
+  name->BInternalExpressionValue.IEvCall->BExpressionT.EValue

 let eFunction = (fName: string, lispArgs: list<expression>): expression => {
  let fn = fName->eCall
@ -27,29 +31,41 @@ let eFunction = (fName: string, lispArgs: list<expression>): expression => {

 let eLambda = (
  parameters: array<string>,
-  context: BExpressionValue.externalBindings,
+  context: BInternalExpressionValue.nameSpace,
  expr: expression,
 ) => {
-  // Js.log(`eLambda context ${BBindings.externalBindingsToString(context)}`)
-  BExpressionValue.EvLambda({
+  BInternalExpressionValue.IEvLambda({
    parameters: parameters,
    context: context,
-    body: expr->castExpressionToInternalCode,
+    body: NotFFI(expr)->castExpressionToInternalCode,
  })->BExpressionT.EValue
 }

-let eNumber = aNumber => aNumber->BExpressionValue.EvNumber->BExpressionT.EValue
+let eLambdaFFI = (parameters: array<string>, ffiFn: ffiFn) => {
+  let context = BModule.emptyModule
+  BInternalExpressionValue.IEvLambda({
+    parameters: parameters,
+    context: context,
+    body: FFI(ffiFn)->castExpressionToInternalCode,
+  })->BExpressionT.EValue
+}

-let eRecord = aRecord => aRecord->BExpressionValue.EvRecord->BExpressionT.EValue
+let eNumber = aNumber => aNumber->BInternalExpressionValue.IEvNumber->BExpressionT.EValue

-let eString = aString => aString->BExpressionValue.EvString->BExpressionT.EValue
+let eRecord = aMap => aMap->BInternalExpressionValue.IEvRecord->BExpressionT.EValue

-let eSymbol = (name: string): expression => name->BExpressionValue.EvSymbol->BExpressionT.EValue
+let eString = aString => aString->BInternalExpressionValue.IEvString->BExpressionT.EValue
+
+let eSymbol = (name: string): expression =>
+  name->BInternalExpressionValue.IEvSymbol->BExpressionT.EValue

 let eList = (list: list<expression>): expression => list->BExpressionT.EList

 let eBlock = (exprs: list<expression>): expression => eFunction("$$_block_$$", exprs)

+let eModule = (nameSpace: BInternalExpressionValue.nameSpace): expression =>
+  nameSpace->BInternalExpressionValue.IEvModule->BExpressionT.EValue
+
 let eLetStatement = (symbol: string, valueExpression: expression): expression =>
  eFunction("$_let_$", list{eSymbol(symbol), valueExpression})

@ -65,7 +81,8 @@ let eBindExpression = (bindingExpr: expression, expression: expression): express
 let eBindExpressionDefault = (expression: expression): expression =>
  eFunction("$$_bindExpression_$$", list{expression})

-let eIdentifier = (name: string): expression => name->BExpressionValue.EvSymbol->BExpressionT.EValue
+let eIdentifier = (name: string): expression =>
+  name->BInternalExpressionValue.IEvSymbol->BExpressionT.EValue

 let eTypeIdentifier = (name: string): expression =>
-  name->BExpressionValue.EvTypeIdentifier->BExpressionT.EValue
+  name->BInternalExpressionValue.IEvTypeIdentifier->BExpressionT.EValue
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Lambda.res
@ -1,19 +1,24 @@
-module Bindings = Reducer_Expression_Bindings
+module BindingsReplacer = Reducer_Expression_BindingsReplacer
 module ErrorValue = Reducer_ErrorValue
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+module ExpressionValue = ReducerInterface_InternalExpressionValue
+module Module = Reducer_Category_Module
 module Result = Belt.Result

-type environment = ReducerInterface_ExpressionValue.environment
+type environment = ReducerInterface_InternalExpressionValue.environment
 type expression = ExpressionT.expression
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
-type externalBindings = ReducerInterface_ExpressionValue.externalBindings
-type internalCode = ReducerInterface_ExpressionValue.internalCode
+type expressionOrFFI = ExpressionT.expressionOrFFI
+type internalExpressionValue = ReducerInterface_InternalExpressionValue.t
+// type tmpExternalBindings = ReducerInterface_InternalExpressionValue.tmpExternalBindings
+type internalCode = ReducerInterface_InternalExpressionValue.internalCode

-external castInternalCodeToExpression: internalCode => expression = "%identity"
+external castInternalCodeToExpression: internalCode => expressionOrFFI = "%identity"

-let checkArity = (lambdaValue: ExpressionValue.lambdaValue, args: list<expressionValue>) => {
+let checkArity = (
+  lambdaValue: ExpressionValue.lambdaValue,
+  args: list<internalExpressionValue>,
+) => {
  let argsLength = Belt.List.length(args)
  let parametersLength = Js.Array2.length(lambdaValue.parameters)
  if argsLength !== parametersLength {
@ -23,34 +28,44 @@ let checkArity = (lambdaValue: ExpressionValue.lambdaValue, args: list<expressio
  }
 }

-let checkIfReduced = (args: list<expressionValue>) =>
+let checkIfReduced = (args: list<internalExpressionValue>) =>
  args->Belt.List.reduceReverse(Ok(list{}), (rAcc, arg) =>
    rAcc->Result.flatMap(acc =>
      switch arg {
-      | EvSymbol(symbol) => ErrorValue.RESymbolNotFound(symbol)->Error
+      | IEvSymbol(symbol) => ErrorValue.RESymbolNotFound(symbol)->Error
      | _ => list{arg, ...acc}->Ok
      }
    )
  )

+let caseNotFFI = (lambdaValue: ExpressionValue.lambdaValue, expr, args, environment, reducer) => {
+  let parameterList = lambdaValue.parameters->Belt.List.fromArray
+  let zippedParameterList = parameterList->Belt.List.zip(args)
+  let bindings = Belt.List.reduce(zippedParameterList, lambdaValue.context, (
+    acc,
+    (variable, variableValue),
+  ) => acc->Module.set(variable, variableValue))
+  let newExpression = ExpressionBuilder.eBlock(list{expr})
+  reducer(newExpression, bindings, environment)
+}
+
+let caseFFI = (ffiFn: ExpressionT.ffiFn, args, environment) => {
+  ffiFn(args->Belt.List.toArray, environment)
+}
+
 let applyParametersToLambda = (
  lambdaValue: ExpressionValue.lambdaValue,
  args,
  environment,
  reducer: ExpressionT.reducerFn,
-): result<expressionValue, 'e> => {
+): result<internalExpressionValue, 'e> => {
  checkArity(lambdaValue, args)->Result.flatMap(args =>
    checkIfReduced(args)->Result.flatMap(args => {
-      let expr = castInternalCodeToExpression(lambdaValue.body)
-      let parameterList = lambdaValue.parameters->Belt.List.fromArray
-      let zippedParameterList = parameterList->Belt.List.zip(args)
-      let bindings = Belt.List.reduce(
-        zippedParameterList,
-        lambdaValue.context->Bindings.fromExternalBindings,
-        (acc, (variable, variableValue)) => acc->Belt.Map.String.set(variable, variableValue),
-      )
-      let newExpression = ExpressionBuilder.eBlock(list{expr})
-      reducer(newExpression, bindings, environment)
+      let exprOrFFI = castInternalCodeToExpression(lambdaValue.body)
+      switch exprOrFFI {
+      | NotFFI(expr) => caseNotFFI(lambdaValue, expr, args, environment, reducer)
+      | FFI(ffiFn) => caseFFI(ffiFn, args, environment)
+      }
    })
  )
 }
@ -60,10 +75,10 @@ let doLambdaCall = (lambdaValue: ExpressionValue.lambdaValue, args, environment,

 let foreignFunctionInterface = (
  lambdaValue: ExpressionValue.lambdaValue,
-  argArray: array<expressionValue>,
+  argArray: array<internalExpressionValue>,
  environment: ExpressionValue.environment,
  reducer: ExpressionT.reducerFn,
-): result<expressionValue, 'e> => {
+): result<internalExpressionValue, 'e> => {
  let args = argArray->Belt.List.fromArray
  applyParametersToLambda(lambdaValue, args, environment, reducer)
 }
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Macro.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_Macro.res
@ -1,11 +1,11 @@
 module ExpressionT = Reducer_Expression_T
-module ExpressionValue = ReducerInterface.ExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module ExpressionWithContext = Reducer_ExpressionWithContext
 module Result = Belt.Result

-type environment = ExpressionValue.environment
+type environment = InternalExpressionValue.environment
 type expression = ExpressionT.expression
-type expressionValue = ExpressionValue.expressionValue
+type internalExpressionValue = InternalExpressionValue.t
 type expressionWithContext = ExpressionWithContext.expressionWithContext

 let expandMacroCall = (
@ -26,7 +26,7 @@ let doMacroCall = (
  bindings: ExpressionT.bindings,
  environment: environment,
  reduceExpression: ExpressionT.reducerFn,
-): result<expressionValue, 'e> =>
+): result<internalExpressionValue, 'e> =>
  expandMacroCall(
    macroExpression,
    bindings,
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_T.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression_T.res
@ -7,28 +7,28 @@
  The act of defining the semantics of a functional language is to write it in terms of Lisp AST.
 */
 module Extra = Reducer_Extra
-module ExpressionValue = ReducerInterface.ExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue

-type expressionValue = ExpressionValue.expressionValue
-type environment = ExpressionValue.environment
+type internalExpressionValue = InternalExpressionValue.t
+type environment = ReducerInterface_InternalExpressionValue.environment

 type rec expression =
  | EList(list<expression>) // A list to map-reduce
-  | EValue(expressionValue) // Irreducible built-in value. Reducer should not know the internals. External libraries are responsible
-and bindings = Belt.Map.String.t<expressionValue>
+  | EValue(internalExpressionValue) // Irreducible built-in value. Reducer should not know the internals. External libraries are responsible
+and bindings = InternalExpressionValue.nameSpace

 type reducerFn = (
  expression,
  bindings,
  environment,
-) => result<expressionValue, Reducer_ErrorValue.errorValue>
+) => result<internalExpressionValue, Reducer_ErrorValue.errorValue>

 /*
  Converts the expression to String
 */
 let rec toString = expression =>
  switch expression {
-  | EList(list{EValue(EvCall("$$_block_$$")), ...statements}) =>
+  | EList(list{EValue(IEvCall("$$_block_$$")), ...statements}) =>
    `{${Belt.List.map(statements, aValue => toString(aValue))
      ->Extra.List.interperse("; ")
      ->Belt.List.toArray
@ -38,7 +38,7 @@ let rec toString = expression =>
      ->Extra.List.interperse(" ")
      ->Belt.List.toArray
      ->Js.String.concatMany("")})`
-  | EValue(aValue) => ExpressionValue.toString(aValue)
+  | EValue(aValue) => InternalExpressionValue.toString(aValue)
  }

 let toStringResult = codeResult =>
@ -65,3 +65,12 @@ let inspectResult = (r: result<expression, Reducer_ErrorValue.errorValue>): resu
  Js.log(toStringResult(r))
  r
 }
+
+type ffiFn = (
+  array<internalExpressionValue>,
+  environment,
+) => result<internalExpressionValue, Reducer_ErrorValue.errorValue>
+
+type expressionOrFFI =
+  | NotFFI(expression)
+  | FFI(ffiFn)
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Js/Reducer_Js_Gate.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Js/Reducer_Js_Gate.res
@ -1,4 +1,4 @@
-open ReducerInterface.ExpressionValue
+open ReducerInterface_InternalExpressionValue
 open Reducer_ErrorValue

 external castBool: unit => bool = "%identity"
@ -8,10 +8,10 @@ external castString: unit => string = "%identity"
 /*
  As JavaScript returns us any type, we need to type check and cast type propertype before using it
 */
-let jsToEv = (jsValue): result<expressionValue, errorValue> =>
+let jsToIEv = (jsValue): result<internalExpressionValue, errorValue> =>
  switch Js.typeof(jsValue) {
-  | "boolean" => jsValue->castBool->EvBool->Ok
-  | "number" => jsValue->castNumber->EvNumber->Ok
-  | "string" => jsValue->castString->EvString->Ok
+  | "boolean" => jsValue->castBool->IEvBool->Ok
+  | "number" => jsValue->castNumber->IEvNumber->Ok
+  | "string" => jsValue->castString->IEvString->Ok
  | other => RETodo(`Unhandled MathJs literal type: ${Js.String.make(other)}`)->Error
  }
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_MathJs/Reducer_MathJs_Eval.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_MathJs/Reducer_MathJs_Eval.res
@ -1,5 +1,5 @@
 module JavaScript = Reducer_Js
-open ReducerInterface.ExpressionValue
+open ReducerInterface_InternalExpressionValue
 open Reducer_ErrorValue

@module("mathjs") external dummy_: string => unit = "evaluate"
@ -17,10 +17,10 @@ let eval__: string => 'a = %raw(`function (expr) { return {value: Mathjs.evaluat
 /*
  Call MathJs evaluate and return as a variant
 */
-let eval = (expr: string): result<expressionValue, errorValue> => {
+let eval = (expr: string): result<internalExpressionValue, errorValue> => {
  try {
    let answer = eval__(expr)
-    answer["value"]->JavaScript.Gate.jsToEv
+    answer["value"]->JavaScript.Gate.jsToIEv
  } catch {
  | Js.Exn.Error(obj) => REJavaScriptExn(Js.Exn.message(obj), Js.Exn.name(obj))->Error
  }
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface.res
@ -1,2 +1,4 @@
-module ExpressionValue = ReducerInterface_ExpressionValue
+module ExternalExpressionValue = ReducerInterface_ExternalExpressionValue
 module ExternalLibrary = ReducerInterface_ExternalLibrary
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+module StdLib = ReducerInterface_StdLib
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Date.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Date.res
@ -1,27 +1,27 @@
-module EV = ReducerInterface_ExpressionValue
-type expressionValue = EV.expressionValue
+module IEV = ReducerInterface_InternalExpressionValue
+type internalExpressionValue = IEV.t

-let dispatch = (call: EV.functionCall, _: DistributionOperation.env): option<
-  result<expressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
+let dispatch = (call: IEV.functionCall, _: DistributionOperation.env): option<
+  result<internalExpressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
 > => {
  switch call {
-  | ("toString", [EvDate(t)]) => EV.EvString(DateTime.Date.toString(t))->Ok->Some
-  | ("makeDateFromYear", [EvNumber(year)]) =>
+  | ("toString", [IEvDate(t)]) => IEV.IEvString(DateTime.Date.toString(t))->Ok->Some
+  | ("makeDateFromYear", [IEvNumber(year)]) =>
    switch DateTime.Date.makeFromYear(year) {
-    | Ok(t) => EV.EvDate(t)->Ok->Some
+    | Ok(t) => IEV.IEvDate(t)->Ok->Some
    | Error(e) => Reducer_ErrorValue.RETodo(e)->Error->Some
    }
-  | ("dateFromNumber", [EvNumber(f)]) => EV.EvDate(DateTime.Date.fromFloat(f))->Ok->Some
-  | ("toNumber", [EvDate(f)]) => EV.EvNumber(DateTime.Date.toFloat(f))->Ok->Some
-  | ("subtract", [EvDate(d1), EvDate(d2)]) =>
+  | ("dateFromNumber", [IEvNumber(f)]) => IEV.IEvDate(DateTime.Date.fromFloat(f))->Ok->Some
+  | ("toNumber", [IEvDate(f)]) => IEV.IEvNumber(DateTime.Date.toFloat(f))->Ok->Some
+  | ("subtract", [IEvDate(d1), IEvDate(d2)]) =>
    switch DateTime.Date.subtract(d1, d2) {
-    | Ok(d) => EV.EvTimeDuration(d)->Ok
+    | Ok(d) => IEV.IEvTimeDuration(d)->Ok
    | Error(e) => Error(RETodo(e))
    }->Some
-  | ("subtract", [EvDate(d1), EvTimeDuration(d2)]) =>
-    EV.EvDate(DateTime.Date.subtractDuration(d1, d2))->Ok->Some
-  | ("add", [EvDate(d1), EvTimeDuration(d2)]) =>
-    EV.EvDate(DateTime.Date.addDuration(d1, d2))->Ok->Some
+  | ("subtract", [IEvDate(d1), IEvTimeDuration(d2)]) =>
+    IEV.IEvDate(DateTime.Date.subtractDuration(d1, d2))->Ok->Some
+  | ("add", [IEvDate(d1), IEvTimeDuration(d2)]) =>
+    IEV.IEvDate(DateTime.Date.addDuration(d1, d2))->Ok->Some
  | _ => None
  }
 }
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Duration.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Duration.res
@ -1,33 +1,36 @@
-module EV = ReducerInterface_ExpressionValue
-type expressionValue = EV.expressionValue
+module IEV = ReducerInterface_InternalExpressionValue
+type internalExpressionValue = IEV.t

-let dispatch = (call: EV.functionCall, _: DistributionOperation.env): option<
-  result<expressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
+let dispatch = (call: IEV.functionCall, _: DistributionOperation.env): option<
+  result<internalExpressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
 > => {
  switch call {
-  | ("toString", [EvTimeDuration(t)]) => EV.EvString(DateTime.Duration.toString(t))->Ok->Some
-  | ("minutes", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromMinutes(f))->Ok->Some
-  | ("fromUnit_minutes", [EvNumber(f)]) =>
-    EV.EvTimeDuration(DateTime.Duration.fromMinutes(f))->Ok->Some
-  | ("hours", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromHours(f))->Ok->Some
-  | ("fromUnit_hours", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromHours(f))->Ok->Some
-  | ("days", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromDays(f))->Ok->Some
-  | ("fromUnit_days", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromDays(f))->Ok->Some
-  | ("years", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromYears(f))->Ok->Some
-  | ("fromUnit_years", [EvNumber(f)]) => EV.EvTimeDuration(DateTime.Duration.fromYears(f))->Ok->Some
-  | ("toHours", [EvTimeDuration(f)]) => EV.EvNumber(DateTime.Duration.toHours(f))->Ok->Some
-  | ("toMinutes", [EvTimeDuration(f)]) => EV.EvNumber(DateTime.Duration.toMinutes(f))->Ok->Some
-  | ("toDays", [EvTimeDuration(f)]) => EV.EvNumber(DateTime.Duration.toDays(f))->Ok->Some
-  | ("toYears", [EvTimeDuration(f)]) => EV.EvNumber(DateTime.Duration.toYears(f))->Ok->Some
-  | ("add", [EvTimeDuration(d1), EvTimeDuration(d2)]) =>
-    EV.EvTimeDuration(DateTime.Duration.add(d1, d2))->Ok->Some
-  | ("subtract", [EvTimeDuration(d1), EvTimeDuration(d2)]) =>
-    EV.EvTimeDuration(DateTime.Duration.subtract(d1, d2))->Ok->Some
-  | ("multiply", [EvTimeDuration(d1), EvNumber(d2)]) =>
-    EV.EvTimeDuration(DateTime.Duration.multiply(d1, d2))->Ok->Some
-  | ("divide", [EvTimeDuration(d1), EvNumber(d2)]) =>
-    EV.EvTimeDuration(DateTime.Duration.divide(d1, d2))->Ok->Some
-  | ("divide", [EvTimeDuration(d1), EvTimeDuration(d2)]) => EV.EvNumber(d1 /. d2)->Ok->Some
+  | ("toString", [IEvTimeDuration(t)]) => IEV.IEvString(DateTime.Duration.toString(t))->Ok->Some
+  | ("minutes", [IEvNumber(f)]) => IEV.IEvTimeDuration(DateTime.Duration.fromMinutes(f))->Ok->Some
+  | ("fromUnit_minutes", [IEvNumber(f)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.fromMinutes(f))->Ok->Some
+  | ("hours", [IEvNumber(f)]) => IEV.IEvTimeDuration(DateTime.Duration.fromHours(f))->Ok->Some
+  | ("fromUnit_hours", [IEvNumber(f)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.fromHours(f))->Ok->Some
+  | ("days", [IEvNumber(f)]) => IEV.IEvTimeDuration(DateTime.Duration.fromDays(f))->Ok->Some
+  | ("fromUnit_days", [IEvNumber(f)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.fromDays(f))->Ok->Some
+  | ("years", [IEvNumber(f)]) => IEV.IEvTimeDuration(DateTime.Duration.fromYears(f))->Ok->Some
+  | ("fromUnit_years", [IEvNumber(f)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.fromYears(f))->Ok->Some
+  | ("toHours", [IEvTimeDuration(f)]) => IEV.IEvNumber(DateTime.Duration.toHours(f))->Ok->Some
+  | ("toMinutes", [IEvTimeDuration(f)]) => IEV.IEvNumber(DateTime.Duration.toMinutes(f))->Ok->Some
+  | ("toDays", [IEvTimeDuration(f)]) => IEV.IEvNumber(DateTime.Duration.toDays(f))->Ok->Some
+  | ("toYears", [IEvTimeDuration(f)]) => IEV.IEvNumber(DateTime.Duration.toYears(f))->Ok->Some
+  | ("add", [IEvTimeDuration(d1), IEvTimeDuration(d2)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.add(d1, d2))->Ok->Some
+  | ("subtract", [IEvTimeDuration(d1), IEvTimeDuration(d2)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.subtract(d1, d2))->Ok->Some
+  | ("multiply", [IEvTimeDuration(d1), IEvNumber(d2)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.multiply(d1, d2))->Ok->Some
+  | ("divide", [IEvTimeDuration(d1), IEvNumber(d2)]) =>
+    IEV.IEvTimeDuration(DateTime.Duration.divide(d1, d2))->Ok->Some
+  | ("divide", [IEvTimeDuration(d1), IEvTimeDuration(d2)]) => IEV.IEvNumber(d1 /. d2)->Ok->Some
  | _ => None
  }
 }
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res
@ -8,8 +8,8 @@ module ErrorValue = Reducer_ErrorValue
 type internalCode = Object

@genType
-type rec expressionValue =
-  | EvArray(array<expressionValue>)
+type rec externalExpressionValue =
+  | EvArray(array<externalExpressionValue>)
  | EvArrayString(array<string>)
  | EvBool(bool)
  | EvCall(string) // External function call
@ -24,7 +24,7 @@ type rec expressionValue =
  | EvDeclaration(lambdaDeclaration)
  | EvTypeIdentifier(string)
  | EvModule(record)
-and record = Js.Dict.t<expressionValue>
+and record = Js.Dict.t<externalExpressionValue>
 and externalBindings = record
 and lambdaValue = {
  parameters: array<string>,
@ -33,7 +33,10 @@ and lambdaValue = {
 }
 and lambdaDeclaration = Declaration.declaration<lambdaValue>

-type functionCall = (string, array<expressionValue>)
+@genType
+type t = externalExpressionValue
+
+type functionCall = (string, array<externalExpressionValue>)

 let rec toString = aValue =>
  switch aValue {
@ -68,26 +71,7 @@ and toStringRecord = aRecord => {
  `{${pairs}}`
 }

-let toStringWithType = aValue =>
-  switch aValue {
-  | EvArray(_) => `Array::${toString(aValue)}`
-  | EvArrayString(_) => `ArrayString::${toString(aValue)}`
-  | EvBool(_) => `Bool::${toString(aValue)}`
-  | EvCall(_) => `Call::${toString(aValue)}`
-  | EvDistribution(_) => `Distribution::${toString(aValue)}`
-  | EvLambda(_) => `Lambda::${toString(aValue)}`
-  | EvNumber(_) => `Number::${toString(aValue)}`
-  | EvRecord(_) => `Record::${toString(aValue)}`
-  | EvString(_) => `String::${toString(aValue)}`
-  | EvSymbol(_) => `Symbol::${toString(aValue)}`
-  | EvDate(_) => `Date::${toString(aValue)}`
-  | EvTimeDuration(_) => `Date::${toString(aValue)}`
-  | EvDeclaration(_) => `Declaration::${toString(aValue)}`
-  | EvTypeIdentifier(_) => `TypeIdentifier::${toString(aValue)}`
-  | EvModule(_) => `Module::${toString(aValue)}`
-  }
-
-let argsToString = (args: array<expressionValue>): string => {
+let argsToString = (args: array<externalExpressionValue>): string => {
  args->Js.Array2.map(arg => arg->toString)->Js.Array2.toString
 }

@ -99,18 +83,6 @@ let toStringResult = x =>
  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
  }

-let toStringResultOkless = (codeResult: result<expressionValue, ErrorValue.errorValue>): string =>
-  switch codeResult {
-  | Ok(a) => toString(a)
-  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
-  }
-
-let toStringResultRecord = x =>
-  switch x {
-  | Ok(a) => `Ok(${toStringRecord(a)})`
-  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
-  }
-
@genType
 type environment = DistributionOperation.env

@ -141,14 +113,14 @@ type functionDefinitionSignature =
 let valueToValueType = value =>
  switch value {
  | EvArray(_) => EvtArray
-  | EvArrayString(_) => EvtArray
+  | EvArrayString(_) => EvtArrayString
  | EvBool(_) => EvtBool
  | EvCall(_) => EvtCall
  | EvDistribution(_) => EvtDistribution
  | EvLambda(_) => EvtLambda
  | EvNumber(_) => EvtNumber
  | EvRecord(_) => EvtRecord
-  | EvString(_) => EvtArray
+  | EvString(_) => EvtString
  | EvSymbol(_) => EvtSymbol
  | EvDate(_) => EvtDate
  | EvTimeDuration(_) => EvtTimeDuration
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalLibrary.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalLibrary.res
@ -1,6 +1,6 @@
-module ExpressionValue = ReducerInterface_ExpressionValue
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue

-type expressionValue = ExpressionValue.expressionValue
+type internalExpressionValue = InternalExpressionValue.t

 // module Sample = {
 //   // In real life real libraries should be somewhere else
@ -17,14 +17,14 @@ type expressionValue = ExpressionValue.expressionValue
 // I expect that it's important to build this first, so it doesn't get recalculated for each tryRegistry() call.
 let registry = FunctionRegistry_Library.registry

-let tryRegistry = ((fnName, args): ExpressionValue.functionCall, env) => {
+let tryRegistry = ((fnName, args): InternalExpressionValue.functionCall, env) => {
  FunctionRegistry_Core.Registry.matchAndRun(~registry, ~fnName, ~args, ~env)->E.O2.fmap(
    E.R2.errMap(_, s => Reducer_ErrorValue.RETodo(s)),
  )
 }

-let dispatch = (call: ExpressionValue.functionCall, environment, chain): result<
-  expressionValue,
+let dispatch = (call: InternalExpressionValue.functionCall, environment, chain): result<
+  internalExpressionValue,
  'e,
 > => {
  E.A.O.firstSomeFn([
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_GenericDistribution.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_GenericDistribution.res
@ -1,5 +1,5 @@
-module ExpressionValue = ReducerInterface_ExpressionValue
-type expressionValue = ExpressionValue.expressionValue
+module IEV = ReducerInterface_InternalExpressionValue
+type internalExpressionValue = IEV.t

 module Helpers = {
  let arithmeticMap = r =>
@ -18,14 +18,14 @@ module Helpers = {
    | _ => #Multiply
    }

-  let catchAndConvertTwoArgsToDists = (args: array<expressionValue>): option<(
+  let catchAndConvertTwoArgsToDists = (args: array<internalExpressionValue>): option<(
    DistributionTypes.genericDist,
    DistributionTypes.genericDist,
  )> =>
    switch args {
-    | [EvDistribution(a), EvDistribution(b)] => Some((a, b))
-    | [EvNumber(a), EvDistribution(b)] => Some((GenericDist.fromFloat(a), b))
-    | [EvDistribution(a), EvNumber(b)] => Some((a, GenericDist.fromFloat(b)))
+    | [IEvDistribution(a), IEvDistribution(b)] => Some((a, b))
+    | [IEvNumber(a), IEvDistribution(b)] => Some((GenericDist.fromFloat(a), b))
+    | [IEvDistribution(a), IEvNumber(b)] => Some((a, GenericDist.fromFloat(b)))
    | _ => None
    }

@ -80,23 +80,28 @@ module Helpers = {
    )->DistributionOperation.run(~env)
  }

-  let parseNumber = (args: expressionValue): Belt.Result.t<float, string> =>
+  let parseNumber = (args: internalExpressionValue): Belt.Result.t<float, string> =>
    switch args {
-    | EvNumber(x) => Ok(x)
+    | IEvNumber(x) => Ok(x)
    | _ => Error("Not a number")
    }

-  let parseNumberArray = (ags: array<expressionValue>): Belt.Result.t<array<float>, string> =>
-    E.A.fmap(parseNumber, ags) |> E.A.R.firstErrorOrOpen
+  let parseNumberArray = (ags: array<internalExpressionValue>): Belt.Result.t<
+    array<float>,
+    string,
+  > => E.A.fmap(parseNumber, ags) |> E.A.R.firstErrorOrOpen

-  let parseDist = (args: expressionValue): Belt.Result.t<DistributionTypes.genericDist, string> =>
+  let parseDist = (args: internalExpressionValue): Belt.Result.t<
+    DistributionTypes.genericDist,
+    string,
+  > =>
    switch args {
-    | EvDistribution(x) => Ok(x)
-    | EvNumber(x) => Ok(GenericDist.fromFloat(x))
+    | IEvDistribution(x) => Ok(x)
+    | IEvNumber(x) => Ok(GenericDist.fromFloat(x))
    | _ => Error("Not a distribution")
    }

-  let parseDistributionArray = (ags: array<expressionValue>): Belt.Result.t<
+  let parseDistributionArray = (ags: array<internalExpressionValue>): Belt.Result.t<
    array<DistributionTypes.genericDist>,
    string,
  > => E.A.fmap(parseDist, ags) |> E.A.R.firstErrorOrOpen
@ -122,18 +127,18 @@ module Helpers = {
  }

  let mixture = (
-    args: array<expressionValue>,
+    args: array<internalExpressionValue>,
    ~env: DistributionOperation.env,
  ): DistributionOperation.outputType => {
    let error = (err: string): DistributionOperation.outputType =>
      err->DistributionTypes.ArgumentError->GenDistError
    switch args {
-    | [EvArray(distributions)] =>
+    | [IEvArray(distributions)] =>
      switch parseDistributionArray(distributions) {
      | Ok(distrs) => mixtureWithDefaultWeights(distrs, ~env)
      | Error(err) => error(err)
      }
-    | [EvArray(distributions), EvArray(weights)] =>
+    | [IEvArray(distributions), IEvArray(weights)] =>
      switch (parseDistributionArray(distributions), parseNumberArray(weights)) {
      | (Ok(distrs), Ok(wghts)) => mixtureWithGivenWeights(distrs, wghts, ~env)
      | (Error(err), Ok(_)) => error(err)
@ -142,7 +147,7 @@ module Helpers = {
      }
    | _ =>
      switch E.A.last(args) {
-      | Some(EvArray(b)) => {
+      | Some(IEvArray(b)) => {
          let weights = parseNumberArray(b)
          let distributions = parseDistributionArray(
            Belt.Array.slice(args, ~offset=0, ~len=E.A.length(args) - 1),
@ -152,8 +157,8 @@ module Helpers = {
          | Error(err) => error(err)
          }
        }
-      | Some(EvNumber(_))
-      | Some(EvDistribution(_)) =>
+      | Some(IEvNumber(_))
+      | Some(IEvDistribution(_)) =>
        switch parseDistributionArray(args) {
        | Ok(distributions) => mixtureWithDefaultWeights(distributions, ~env)
        | Error(err) => error(err)
@ -194,20 +199,19 @@ module SymbolicConstructors = {
    }
 }

-let dispatchToGenericOutput = (
-  call: ExpressionValue.functionCall,
-  env: DistributionOperation.env,
-): option<DistributionOperation.outputType> => {
+let dispatchToGenericOutput = (call: IEV.functionCall, env: DistributionOperation.env): option<
+  DistributionOperation.outputType,
+> => {
  let (fnName, args) = call
  switch (fnName, args) {
-  | ("triangular" as fnName, [EvNumber(f1), EvNumber(f2), EvNumber(f3)]) =>
+  | ("triangular" as fnName, [IEvNumber(f1), IEvNumber(f2), IEvNumber(f3)]) =>
    SymbolicConstructors.threeFloat(fnName)
    ->E.R.bind(r => r(f1, f2, f3))
    ->SymbolicConstructors.symbolicResultToOutput
-  | ("sample", [EvDistribution(dist)]) => Helpers.toFloatFn(#Sample, dist, ~env)
-  | ("sampleN", [EvDistribution(dist), EvNumber(n)]) =>
+  | ("sample", [IEvDistribution(dist)]) => Helpers.toFloatFn(#Sample, dist, ~env)
+  | ("sampleN", [IEvDistribution(dist), IEvNumber(n)]) =>
    Some(FloatArray(GenericDist.sampleN(dist, Belt.Int.fromFloat(n))))
-  | (("mean" | "stdev" | "variance" | "min" | "max" | "mode") as op, [EvDistribution(dist)]) => {
+  | (("mean" | "stdev" | "variance" | "min" | "max" | "mode") as op, [IEvDistribution(dist)]) => {
      let fn = switch op {
      | "mean" => #Mean
      | "stdev" => #Stdev
@ -219,13 +223,13 @@ let dispatchToGenericOutput = (
      }
      Helpers.toFloatFn(fn, dist, ~env)
    }
-  | ("integralSum", [EvDistribution(dist)]) => Helpers.toFloatFn(#IntegralSum, dist, ~env)
-  | ("toString", [EvDistribution(dist)]) => Helpers.toStringFn(ToString, dist, ~env)
-  | ("sparkline", [EvDistribution(dist)]) =>
+  | ("integralSum", [IEvDistribution(dist)]) => Helpers.toFloatFn(#IntegralSum, dist, ~env)
+  | ("toString", [IEvDistribution(dist)]) => Helpers.toStringFn(ToString, dist, ~env)
+  | ("sparkline", [IEvDistribution(dist)]) =>
    Helpers.toStringFn(ToSparkline(MagicNumbers.Environment.sparklineLength), dist, ~env)
-  | ("sparkline", [EvDistribution(dist), EvNumber(n)]) =>
+  | ("sparkline", [IEvDistribution(dist), IEvNumber(n)]) =>
    Helpers.toStringFn(ToSparkline(Belt.Float.toInt(n)), dist, ~env)
-  | ("exp", [EvDistribution(a)]) =>
+  | ("exp", [IEvDistribution(a)]) =>
    // https://mathjs.org/docs/reference/functions/exp.html
    Helpers.twoDiststoDistFn(
      Algebraic(AsDefault),
@ -234,55 +238,62 @@ let dispatchToGenericOutput = (
      a,
      ~env,
    )->Some
-  | ("normalize", [EvDistribution(dist)]) => Helpers.toDistFn(Normalize, dist, ~env)
-  | ("klDivergence", [EvDistribution(prediction), EvDistribution(answer)]) =>
+  | ("normalize", [IEvDistribution(dist)]) => Helpers.toDistFn(Normalize, dist, ~env)
+  | ("klDivergence", [IEvDistribution(prediction), IEvDistribution(answer)]) =>
    Some(DistributionOperation.run(FromDist(ToScore(KLDivergence(answer)), prediction), ~env))
-  | ("klDivergence", [EvDistribution(prediction), EvDistribution(answer), EvDistribution(prior)]) =>
+  | (
+      "klDivergence",
+      [IEvDistribution(prediction), IEvDistribution(answer), IEvDistribution(prior)],
+    ) =>
    Helpers.klDivergenceWithPrior(prediction, answer, prior, env)
  | (
    "logScoreWithPointAnswer",
-    [EvDistribution(prediction), EvNumber(answer), EvDistribution(prior)],
+    [IEvDistribution(prediction), IEvNumber(answer), IEvDistribution(prior)],
  )
  | (
    "logScoreWithPointAnswer",
-    [EvDistribution(prediction), EvDistribution(Symbolic(#Float(answer))), EvDistribution(prior)],
+    [
+      IEvDistribution(prediction),
+      IEvDistribution(Symbolic(#Float(answer))),
+      IEvDistribution(prior),
+    ],
  ) =>
    DistributionOperation.run(
      FromDist(ToScore(LogScore(answer, prior->Some)), prediction),
      ~env,
    )->Some
-  | ("logScoreWithPointAnswer", [EvDistribution(prediction), EvNumber(answer)])
+  | ("logScoreWithPointAnswer", [IEvDistribution(prediction), IEvNumber(answer)])
  | (
    "logScoreWithPointAnswer",
-    [EvDistribution(prediction), EvDistribution(Symbolic(#Float(answer)))],
+    [IEvDistribution(prediction), IEvDistribution(Symbolic(#Float(answer)))],
  ) =>
    DistributionOperation.run(FromDist(ToScore(LogScore(answer, None)), prediction), ~env)->Some
-  | ("isNormalized", [EvDistribution(dist)]) => Helpers.toBoolFn(IsNormalized, dist, ~env)
-  | ("toPointSet", [EvDistribution(dist)]) => Helpers.toDistFn(ToPointSet, dist, ~env)
-  | ("scaleLog", [EvDistribution(dist)]) =>
+  | ("isNormalized", [IEvDistribution(dist)]) => Helpers.toBoolFn(IsNormalized, dist, ~env)
+  | ("toPointSet", [IEvDistribution(dist)]) => Helpers.toDistFn(ToPointSet, dist, ~env)
+  | ("scaleLog", [IEvDistribution(dist)]) =>
    Helpers.toDistFn(Scale(#Logarithm, MagicNumbers.Math.e), dist, ~env)
-  | ("scaleLog10", [EvDistribution(dist)]) => Helpers.toDistFn(Scale(#Logarithm, 10.0), dist, ~env)
-  | ("scaleLog", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("scaleLog10", [IEvDistribution(dist)]) => Helpers.toDistFn(Scale(#Logarithm, 10.0), dist, ~env)
+  | ("scaleLog", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(Scale(#Logarithm, float), dist, ~env)
-  | ("scaleLogWithThreshold", [EvDistribution(dist), EvNumber(base), EvNumber(eps)]) =>
+  | ("scaleLogWithThreshold", [IEvDistribution(dist), IEvNumber(base), IEvNumber(eps)]) =>
    Helpers.toDistFn(Scale(#LogarithmWithThreshold(eps), base), dist, ~env)
-  | ("scaleMultiply", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("scaleMultiply", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(Scale(#Multiply, float), dist, ~env)
-  | ("scalePow", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("scalePow", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(Scale(#Power, float), dist, ~env)
-  | ("scaleExp", [EvDistribution(dist)]) =>
+  | ("scaleExp", [IEvDistribution(dist)]) =>
    Helpers.toDistFn(Scale(#Power, MagicNumbers.Math.e), dist, ~env)
-  | ("cdf", [EvDistribution(dist), EvNumber(float)]) => Helpers.toFloatFn(#Cdf(float), dist, ~env)
-  | ("pdf", [EvDistribution(dist), EvNumber(float)]) => Helpers.toFloatFn(#Pdf(float), dist, ~env)
-  | ("inv", [EvDistribution(dist), EvNumber(float)]) => Helpers.toFloatFn(#Inv(float), dist, ~env)
-  | ("quantile", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("cdf", [IEvDistribution(dist), IEvNumber(float)]) => Helpers.toFloatFn(#Cdf(float), dist, ~env)
+  | ("pdf", [IEvDistribution(dist), IEvNumber(float)]) => Helpers.toFloatFn(#Pdf(float), dist, ~env)
+  | ("inv", [IEvDistribution(dist), IEvNumber(float)]) => Helpers.toFloatFn(#Inv(float), dist, ~env)
+  | ("quantile", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toFloatFn(#Inv(float), dist, ~env)
-  | ("toSampleSet", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("toSampleSet", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(ToSampleSet(Belt.Int.fromFloat(float)), dist, ~env)
-  | ("toSampleSet", [EvDistribution(dist)]) =>
+  | ("toSampleSet", [IEvDistribution(dist)]) =>
    Helpers.toDistFn(ToSampleSet(env.sampleCount), dist, ~env)
-  | ("toList", [EvDistribution(SampleSet(dist))]) => Some(FloatArray(SampleSetDist.T.get(dist)))
-  | ("fromSamples", [EvArray(inputArray)]) => {
+  | ("toList", [IEvDistribution(SampleSet(dist))]) => Some(FloatArray(SampleSetDist.T.get(dist)))
+  | ("fromSamples", [IEvArray(inputArray)]) => {
      let _wrapInputErrors = x => SampleSetDist.NonNumericInput(x)
      let parsedArray = Helpers.parseNumberArray(inputArray)->E.R2.errMap(_wrapInputErrors)
      switch parsedArray {
@ -290,15 +301,15 @@ let dispatchToGenericOutput = (
      | Error(e) => GenDistError(SampleSetError(e))
      }->Some
    }
-  | ("inspect", [EvDistribution(dist)]) => Helpers.toDistFn(Inspect, dist, ~env)
-  | ("truncateLeft", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("inspect", [IEvDistribution(dist)]) => Helpers.toDistFn(Inspect, dist, ~env)
+  | ("truncateLeft", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(Truncate(Some(float), None), dist, ~env)
-  | ("truncateRight", [EvDistribution(dist), EvNumber(float)]) =>
+  | ("truncateRight", [IEvDistribution(dist), IEvNumber(float)]) =>
    Helpers.toDistFn(Truncate(None, Some(float)), dist, ~env)
-  | ("truncate", [EvDistribution(dist), EvNumber(float1), EvNumber(float2)]) =>
+  | ("truncate", [IEvDistribution(dist), IEvNumber(float1), IEvNumber(float2)]) =>
    Helpers.toDistFn(Truncate(Some(float1), Some(float2)), dist, ~env)
  | ("mx" | "mixture", args) => Helpers.mixture(args, ~env)->Some
-  | ("log", [EvDistribution(a)]) =>
+  | ("log", [IEvDistribution(a)]) =>
    Helpers.twoDiststoDistFn(
      Algebraic(AsDefault),
      "log",
@ -306,7 +317,7 @@ let dispatchToGenericOutput = (
      GenericDist.fromFloat(MagicNumbers.Math.e),
      ~env,
    )->Some
-  | ("log10", [EvDistribution(a)]) =>
+  | ("log10", [IEvDistribution(a)]) =>
    Helpers.twoDiststoDistFn(
      Algebraic(AsDefault),
      "log",
@ -314,7 +325,7 @@ let dispatchToGenericOutput = (
      GenericDist.fromFloat(10.0),
      ~env,
    )->Some
-  | ("unaryMinus", [EvDistribution(a)]) =>
+  | ("unaryMinus", [IEvDistribution(a)]) =>
    Helpers.twoDiststoDistFn(
      Algebraic(AsDefault),
      "multiply",
@ -337,7 +348,7 @@ let dispatchToGenericOutput = (
    Helpers.catchAndConvertTwoArgsToDists(args)->E.O2.fmap(((fst, snd)) =>
      Helpers.twoDiststoDistFn(Pointwise, arithmetic, fst, snd, ~env)
    )
-  | ("dotExp", [EvDistribution(a)]) =>
+  | ("dotExp", [IEvDistribution(a)]) =>
    Helpers.twoDiststoDistFn(
      Pointwise,
      "dotPow",
@ -350,17 +361,18 @@ let dispatchToGenericOutput = (
 }

 let genericOutputToReducerValue = (o: DistributionOperation.outputType): result<
-  expressionValue,
+  internalExpressionValue,
  Reducer_ErrorValue.errorValue,
 > =>
  switch o {
-  | Dist(d) => Ok(ReducerInterface_ExpressionValue.EvDistribution(d))
-  | Float(d) => Ok(EvNumber(d))
-  | String(d) => Ok(EvString(d))
-  | Bool(d) => Ok(EvBool(d))
-  | FloatArray(d) => Ok(EvArray(d |> E.A.fmap(r => ReducerInterface_ExpressionValue.EvNumber(r))))
+  | Dist(d) => Ok(ReducerInterface_InternalExpressionValue.IEvDistribution(d))
+  | Float(d) => Ok(IEvNumber(d))
+  | String(d) => Ok(IEvString(d))
+  | Bool(d) => Ok(IEvBool(d))
+  | FloatArray(d) =>
+    Ok(IEvArray(d |> E.A.fmap(r => ReducerInterface_InternalExpressionValue.IEvNumber(r))))
  | GenDistError(err) => Error(REDistributionError(err))
  }

-let dispatch = (call: ExpressionValue.functionCall, environment) =>
+let dispatch = (call: IEV.functionCall, environment) =>
  dispatchToGenericOutput(call, environment)->E.O2.fmap(genericOutputToReducerValue)
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_GenericDistribution.resi
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_GenericDistribution.resi
@ -1,4 +1,4 @@
 let dispatch: (
-  ReducerInterface_ExpressionValue.functionCall,
-  ReducerInterface_ExpressionValue.environment,
-) => option<result<ReducerInterface_ExpressionValue.expressionValue, Reducer_ErrorValue.errorValue>>
+  ReducerInterface_InternalExpressionValue.functionCall,
+  ReducerInterface_ExternalExpressionValue.environment,
+) => option<result<ReducerInterface_InternalExpressionValue.t, Reducer_ErrorValue.errorValue>>
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
@ -0,0 +1,259 @@
+module ErrorValue = Reducer_ErrorValue
+module ExternalExpressionValue = ReducerInterface_ExternalExpressionValue
+module Extra_Array = Reducer_Extra_Array
+type internalCode = ExternalExpressionValue.internalCode
+type environment = ExternalExpressionValue.environment
+
+let defaultEnvironment = ExternalExpressionValue.defaultEnvironment
+
+type rec t =
+  | IEvArray(array<t>) // FIXME: Convert
+  | IEvArrayString(array<string>) // FIXME: Convert
+  | IEvBool(bool)
+  | IEvCall(string) // External function call
+  | IEvDate(Js.Date.t)
+  | IEvDeclaration(lambdaDeclaration)
+  | IEvDistribution(DistributionTypes.genericDist)
+  | IEvLambda(lambdaValue)
+  | IEvModule(nameSpace)
+  | IEvNumber(float)
+  | IEvRecord(map)
+  | IEvString(string)
+  | IEvSymbol(string)
+  | IEvTimeDuration(float)
+  | IEvTypeIdentifier(string)
+and map = Belt.Map.String.t<t>
+and nameSpace = NameSpace(Belt.Map.String.t<t>)
+and lambdaValue = {
+  parameters: array<string>,
+  context: nameSpace,
+  body: internalCode,
+}
+and lambdaDeclaration = Declaration.declaration<lambdaValue>
+
+type internalExpressionValue = t
+
+type functionCall = (string, array<t>)
+
+let rec toString = aValue =>
+  switch aValue {
+  | IEvArray(anArray) => {
+      let args = anArray->Js.Array2.map(each => toString(each))->Js.Array2.toString
+      `[${args}]`
+    }
+  | IEvArrayString(anArray) => {
+      let args = anArray->Js.Array2.toString
+      `[${args}]`
+    }
+  | IEvBool(aBool) => Js.String.make(aBool)
+  | IEvCall(fName) => `:${fName}`
+  | IEvDate(date) => DateTime.Date.toString(date)
+  | IEvDeclaration(d) => Declaration.toString(d, r => toString(IEvLambda(r)))
+  | IEvDistribution(dist) => GenericDist.toString(dist)
+  | IEvLambda(lambdaValue) => `lambda(${Js.Array2.toString(lambdaValue.parameters)}=>internal code)`
+  | IEvModule(m) => `@${m->toStringNameSpace}`
+  | IEvNumber(aNumber) => Js.String.make(aNumber)
+  | IEvRecord(aMap) => aMap->toStringMap
+  | IEvString(aString) => `'${aString}'`
+  | IEvSymbol(aString) => `:${aString}`
+  | IEvTimeDuration(t) => DateTime.Duration.toString(t)
+  | IEvTypeIdentifier(id) => `#${id}`
+  }
+and toStringMap = aMap => {
+  let pairs =
+    aMap
+    ->Belt.Map.String.toArray
+    ->Js.Array2.map(((eachKey, eachValue)) => `${eachKey}: ${toString(eachValue)}`)
+    ->Js.Array2.toString
+  `{${pairs}}`
+}
+and toStringNameSpace = nameSpace => {
+  let NameSpace(container) = nameSpace
+  container->toStringMap
+}
+
+let toStringWithType = aValue =>
+  switch aValue {
+  | IEvArray(_) => `Array::${toString(aValue)}`
+  | IEvArrayString(_) => `ArrayString::${toString(aValue)}`
+  | IEvBool(_) => `Bool::${toString(aValue)}`
+  | IEvCall(_) => `Call::${toString(aValue)}`
+  | IEvDistribution(_) => `Distribution::${toString(aValue)}`
+  | IEvLambda(_) => `Lambda::${toString(aValue)}`
+  | IEvNumber(_) => `Number::${toString(aValue)}`
+  | IEvRecord(_) => `Record::${toString(aValue)}`
+  | IEvString(_) => `String::${toString(aValue)}`
+  | IEvSymbol(_) => `Symbol::${toString(aValue)}`
+  | IEvDate(_) => `Date::${toString(aValue)}`
+  | IEvTimeDuration(_) => `Date::${toString(aValue)}`
+  | IEvDeclaration(_) => `Declaration::${toString(aValue)}`
+  | IEvTypeIdentifier(_) => `TypeIdentifier::${toString(aValue)}`
+  | IEvModule(_) => `Module::${toString(aValue)}`
+  }
+
+let argsToString = (args: array<t>): string => {
+  args->Js.Array2.map(arg => arg->toString)->Js.Array2.toString
+}
+
+let toStringFunctionCall = ((fn, args)): string => `${fn}(${argsToString(args)})`
+
+let toStringResult = x =>
+  switch x {
+  | Ok(a) => `Ok(${toString(a)})`
+  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
+  }
+
+let toStringResultOkless = (codeResult: result<t, ErrorValue.errorValue>): string =>
+  switch codeResult {
+  | Ok(a) => toString(a)
+  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
+  }
+
+let toStringResultRecord = x =>
+  switch x {
+  | Ok(a) => `Ok(${ExternalExpressionValue.toStringRecord(a)})`
+  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
+  }
+
+type internalExpressionValueType =
+  | EvtArray
+  | EvtArrayString
+  | EvtBool
+  | EvtCall
+  | EvtDistribution
+  | EvtLambda
+  | EvtNumber
+  | EvtRecord
+  | EvtString
+  | EvtSymbol
+  | EvtDate
+  | EvtTimeDuration
+  | EvtDeclaration
+  | EvtTypeIdentifier
+  | EvtModule
+
+type functionCallSignature = CallSignature(string, array<internalExpressionValueType>)
+type functionDefinitionSignature =
+  FunctionDefinitionSignature(functionCallSignature, internalExpressionValueType)
+
+let valueToValueType = value =>
+  switch value {
+  | IEvArray(_) => EvtArray
+  | IEvArrayString(_) => EvtArrayString
+  | IEvBool(_) => EvtBool
+  | IEvCall(_) => EvtCall
+  | IEvDistribution(_) => EvtDistribution
+  | IEvLambda(_) => EvtLambda
+  | IEvNumber(_) => EvtNumber
+  | IEvRecord(_) => EvtRecord
+  | IEvString(_) => EvtString
+  | IEvSymbol(_) => EvtSymbol
+  | IEvDate(_) => EvtDate
+  | IEvTimeDuration(_) => EvtTimeDuration
+  | IEvDeclaration(_) => EvtDeclaration
+  | IEvTypeIdentifier(_) => EvtTypeIdentifier
+  | IEvModule(_) => EvtModule
+  }
+
+let functionCallToCallSignature = (functionCall: functionCall): functionCallSignature => {
+  let (fn, args) = functionCall
+  CallSignature(fn, args->Js.Array2.map(valueToValueType))
+}
+
+let valueTypeToString = (valueType: internalExpressionValueType): string =>
+  switch valueType {
+  | EvtArray => `Array`
+  | EvtArrayString => `ArrayString`
+  | EvtBool => `Bool`
+  | EvtCall => `Call`
+  | EvtDistribution => `Distribution`
+  | EvtLambda => `Lambda`
+  | EvtNumber => `Number`
+  | EvtRecord => `Record`
+  | EvtString => `String`
+  | EvtSymbol => `Symbol`
+  | EvtDate => `Date`
+  | EvtTimeDuration => `Duration`
+  | EvtDeclaration => `Declaration`
+  | EvtTypeIdentifier => `TypeIdentifier`
+  | EvtModule => `Module`
+  }
+
+let functionCallSignatureToString = (functionCallSignature: functionCallSignature): string => {
+  let CallSignature(fn, args) = functionCallSignature
+  `${fn}(${args->Js.Array2.map(valueTypeToString)->Js.Array2.toString})`
+}
+
+let rec toExternal = (iev: t): ExternalExpressionValue.t => {
+  switch iev {
+  | IEvArray(v) => v->Belt.Array.map(e => toExternal(e))->EvArray
+  | IEvArrayString(v) => EvArrayString(v)
+  | IEvBool(v) => EvBool(v)
+  | IEvCall(v) => EvCall(v)
+  | IEvDistribution(v) => EvDistribution(v)
+  | IEvLambda(v) => EvLambda(lambdaValueToExternal(v))
+  | IEvNumber(v) => EvNumber(v)
+  | IEvRecord(v) => v->mapToExternal->EvRecord
+  | IEvString(v) => EvString(v)
+  | IEvSymbol(v) => EvSymbol(v)
+  | IEvDate(v) => EvDate(v)
+  | IEvTimeDuration(v) => EvTimeDuration(v)
+  | IEvDeclaration(v) => {
+      let fn = lambdaValueToExternal(v.fn)
+      let args = v.args
+      EvDeclaration({fn: fn, args: args})
+    }
+  | IEvTypeIdentifier(v) => EvTypeIdentifier(v)
+  | IEvModule(v) => v->nameSpaceToTypeScriptBindings->EvModule
+  }
+}
+and mapToExternal = v =>
+  v->Belt.Map.String.map(e => toExternal(e))->Belt.Map.String.toArray->Js.Dict.fromArray
+and lambdaValueToExternal = v => {
+  let p = v.parameters
+  let c = v.context->nameSpaceToTypeScriptBindings
+  let b = v.body
+  {parameters: p, context: c, body: b}
+}
+and nameSpaceToTypeScriptBindings = (
+  nameSpace: nameSpace,
+): ReducerInterface_ExternalExpressionValue.externalBindings => {
+  let NameSpace(container) = nameSpace
+  Belt.Map.String.map(container, e => toExternal(e))->Belt.Map.String.toArray->Js.Dict.fromArray
+}
+
+let rec toInternal = (ev: ExternalExpressionValue.t): t => {
+  switch ev {
+  | EvArray(v) => v->Belt.Array.map(e => toInternal(e))->IEvArray
+  | EvArrayString(v) => IEvArrayString(v)
+  | EvBool(v) => IEvBool(v)
+  | EvCall(v) => IEvCall(v)
+  | EvDistribution(v) => IEvDistribution(v)
+  | EvLambda(v) => IEvLambda(lambdaValueToInternal(v))
+  | EvNumber(v) => IEvNumber(v)
+  | EvRecord(v) => v->recordToInternal->IEvRecord
+  | EvString(v) => IEvString(v)
+  | EvSymbol(v) => IEvSymbol(v)
+  | EvDate(v) => IEvDate(v)
+  | EvTimeDuration(v) => IEvTimeDuration(v)
+  | EvDeclaration(v) => {
+      let fn = lambdaValueToInternal(v.fn)
+      let args = v.args
+      IEvDeclaration({fn: fn, args: args})
+    }
+  | EvTypeIdentifier(v) => IEvTypeIdentifier(v)
+  | EvModule(v) => v->nameSpaceFromTypeScriptBindings->IEvModule
+  }
+}
+and recordToInternal = v =>
+  v->Js.Dict.entries->Belt.Map.String.fromArray->Belt.Map.String.map(e => toInternal(e))
+and lambdaValueToInternal = v => {
+  let p = v.parameters
+  let c = v.context->nameSpaceFromTypeScriptBindings
+  let b = v.body
+  {parameters: p, context: c, body: b}
+}
+and nameSpaceFromTypeScriptBindings = (
+  r: ReducerInterface_ExternalExpressionValue.externalBindings,
+): nameSpace =>
+  r->Js.Dict.entries->Belt.Map.String.fromArray->Belt.Map.String.map(e => toInternal(e))->NameSpace
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Number.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_Number.res
@ -1,5 +1,5 @@
-module EV = ReducerInterface_ExpressionValue
-type expressionValue = EV.expressionValue
+module IEV = ReducerInterface_InternalExpressionValue
+type internalExpressionValue = IEV.t

 module ScientificUnit = {
  let nameToMultiplier = str =>
@ -24,8 +24,8 @@ module ScientificUnit = {
  }
 }

-let dispatch = (call: EV.functionCall, _: DistributionOperation.env): option<
-  result<expressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
+let dispatch = (call: IEV.functionCall, _: DistributionOperation.env): option<
+  result<internalExpressionValue, QuriSquiggleLang.Reducer_ErrorValue.errorValue>,
 > => {
  switch call {
  | (
@ -37,9 +37,9 @@ let dispatch = (call: EV.functionCall, _: DistributionOperation.env): option<
      | "fromUnit_G"
      | "fromUnit_T"
      | "fromUnit_P") as op,
-      [EvNumber(f)],
+      [IEvNumber(f)],
    ) =>
-    op->ScientificUnit.getMultiplier->E.O2.fmap(multiplier => EV.EvNumber(f *. multiplier)->Ok)
+    op->ScientificUnit.getMultiplier->E.O2.fmap(multiplier => IEV.IEvNumber(f *. multiplier)->Ok)
  | _ => None
  }
 }
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res
@ -1,6 +1,6 @@
-module Bindings = Reducer_Category_Bindings
+module Module = Reducer_Category_Module

-let internalStdLib = Bindings.emptyBindings->SquiggleLibrary_Math.makeBindings
+let internalStdLib = Module.emptyModule->SquiggleLibrary_Math.makeBindings

@genType
-let externalStdLib = internalStdLib->Bindings.toRecord
+let externalStdLib = internalStdLib->Module.toTypeScriptBindings
--- a/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
+++ b/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
@ -1,4 +1,4 @@
-module Bindings = Reducer_Category_Bindings
+module Bindings = Reducer_Category_Module
 module Module = Reducer_Category_Module

 let availableNumbers: array<(string, float)> = [
@ -14,10 +14,10 @@ let availableNumbers: array<(string, float)> = [
  ("tau", 6.283185307179586),
 ]

-let mathBindings: Bindings.ExpressionT.bindings =
+let mathBindings: Bindings.t =
  availableNumbers
-  ->E.A2.fmap(((name, v)) => (name, ReducerInterface_ExpressionValue.EvNumber(v)))
-  ->Belt.Map.String.fromArray
+  ->E.A2.fmap(((name, v)) => (name, ReducerInterface_InternalExpressionValue.IEvNumber(v)))
+  ->Bindings.fromArray

 let makeBindings = (previousBindings: Bindings.t): Bindings.t =>
  previousBindings->Bindings.defineModule("Math", mathBindings)
--- a/packages/squiggle-lang/src/rescript/TypescriptInterface.res
+++ b/packages/squiggle-lang/src/rescript/TypescriptInterface.res
@ -50,10 +50,10 @@ let evaluatePartialUsingExternalBindings = Reducer.evaluatePartialUsingExternalB
 type externalBindings = Reducer.externalBindings

@genType
-type expressionValue = ReducerInterface_ExpressionValue.expressionValue
+type expressionValue = ReducerInterface_ExternalExpressionValue.t

@genType
-type recordEV = ReducerInterface_ExpressionValue.record
+type recordEV = ReducerInterface_ExternalExpressionValue.record

@genType
 type errorValue = Reducer_ErrorValue.errorValue
@ -77,19 +77,19 @@ let errorValueToString = Reducer_ErrorValue.errorToString
 let distributionErrorToString = DistributionTypes.Error.toString

@genType
-type lambdaValue = ReducerInterface_ExpressionValue.lambdaValue
+type lambdaValue = ReducerInterface_ExternalExpressionValue.lambdaValue

@genType
-type lambdaDeclaration = ReducerInterface_ExpressionValue.lambdaDeclaration
+type lambdaDeclaration = ReducerInterface_ExternalExpressionValue.lambdaDeclaration

@genType
 let defaultSamplingEnv = DistributionOperation.defaultEnv

@genType
-type environment = ReducerInterface_ExpressionValue.environment
+type environment = ReducerInterface_ExternalExpressionValue.environment

@genType
-let defaultEnvironment = ReducerInterface_ExpressionValue.defaultEnvironment
+let defaultEnvironment = ReducerInterface_ExternalExpressionValue.defaultEnvironment

@genType
 let foreignFunctionInterface = Reducer.foreignFunctionInterface