diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_Helpers.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_Helpers.res
index 8e3d8257..4fb1382f 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_Helpers.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_Helpers.res
@@ -2,7 +2,7 @@
 module ErrorValue = Reducer_ErrorValue
 module ExternalExpressionValue = ReducerInterface.ExternalExpressionValue
 module InternalExpressionValue = ReducerInterface.InternalExpressionValue
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 
 let removeDefaultsInternal = (iev: InternalExpressionValue.t) => {
   switch iev {
diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_TestMacroHelpers.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_TestMacroHelpers.res
index 103609a2..2337dde2 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_TestMacroHelpers.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_TestMacroHelpers.res
@@ -8,7 +8,7 @@ module InternalExpressionValue = ReducerInterface.InternalExpressionValue
 module ExpressionWithContext = Reducer_ExpressionWithContext
 module Macro = Reducer_Expression_Macro
 module T = Reducer_Expression_T
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 
 let testMacro_ = (
   tester,
diff --git 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
deleted file mode 100644
index 957b0dc8..00000000
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module_TypeChecker.res
+++ /dev/null
@@ -1,16 +0,0 @@
-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
-  }
-}
diff --git 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
index 6012d179..484b0acb 100644
--- 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
@@ -3,8 +3,9 @@ module ExpressionT = Reducer_Expression_T
 module ExternalLibrary = ReducerInterface.ExternalLibrary
 module Lambda = Reducer_Expression_Lambda
 module MathJs = Reducer_MathJs
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 module Result = Belt.Result
+module TypeBuilder = Reducer_Type_TypeBuilder
 open ReducerInterface_InternalExpressionValue
 open Reducer_ErrorValue
 
@@ -166,59 +167,6 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
     )
   }
 
-  let typeModifier_memberOf = (aType, anArray) => {
-    let newRecord = Belt.Map.String.fromArray([
-      ("typeTag", IEvString("typeIdentifier")),
-      ("typeIdentifier", aType),
-    ])
-    newRecord->Belt.Map.String.set("memberOf", anArray)->IEvRecord->Ok
-  }
-  let typeModifier_memberOf_update = (aRecord, anArray) => {
-    aRecord->Belt.Map.String.set("memberOf", anArray)->IEvRecord->Ok
-  }
-
-  let typeModifier_min = (aType, value) => {
-    let newRecord = Belt.Map.String.fromArray([
-      ("typeTag", IEvString("typeIdentifier")),
-      ("typeIdentifier", aType),
-    ])
-    newRecord->Belt.Map.String.set("min", value)->IEvRecord->Ok
-  }
-  let typeModifier_min_update = (aRecord, value) => {
-    aRecord->Belt.Map.String.set("min", value)->IEvRecord->Ok
-  }
-
-  let typeModifier_max = (aType, value) => {
-    let newRecord = Belt.Map.String.fromArray([
-      ("typeTag", IEvString("typeIdentifier")),
-      ("typeIdentifier", aType),
-    ])
-    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 =>
-    aRecord->Belt.Map.String.set("opaque", IEvBool(true))->IEvRecord->Ok
-
-  let typeOr = evArray => {
-    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 = Belt.Map.String.fromArray([
-      ("typeTag", IEvString("typeFunction")),
-      ("inputs", IEvArray(inputs)),
-      ("output", output),
-    ])
-    newRecord->IEvRecord->Ok
-  }
-
   switch call {
   | ("$_atIndex_$", [IEvArray(aValueArray), IEvNumber(fIndex)]) => arrayAtIndex(aValueArray, fIndex)
   | ("$_atIndex_$", [IEvModule(dict), IEvString(sIndex)]) => moduleAtIndex(dict, sIndex)
@@ -233,20 +181,24 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
   | ("$_setTypeOfBindings_$", [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))
+    TypeBuilder.typeModifier_memberOf(IEvTypeIdentifier(typeIdentifier), IEvArray(arr))
+  | ("$_typeModifier_memberOf_$", [IEvType(typeRecord), IEvArray(arr)]) =>
+    TypeBuilder.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)
+    TypeBuilder.typeModifier_min(IEvTypeIdentifier(typeIdentifier), value)
+  | ("$_typeModifier_min_$", [IEvType(typeRecord), value]) =>
+    TypeBuilder.typeModifier_min_update(typeRecord, value)
   | ("$_typeModifier_max_$", [IEvTypeIdentifier(typeIdentifier), value]) =>
-    typeModifier_max(IEvTypeIdentifier(typeIdentifier), value)
-  | ("$_typeModifier_max_$", [IEvRecord(typeRecord), value]) =>
-    typeModifier_max_update(typeRecord, value)
-  | ("$_typeModifier_opaque_$", [IEvRecord(typeRecord)]) => typeModifier_opaque_update(typeRecord)
-  | ("$_typeOr_$", [IEvArray(arr)]) => typeOr(IEvArray(arr))
-  | ("$_typeFunction_$", [IEvArray(arr)]) => typeFunction(arr)
+    TypeBuilder.typeModifier_max(IEvTypeIdentifier(typeIdentifier), value)
+  | ("$_typeModifier_max_$", [IEvType(typeRecord), value]) =>
+    TypeBuilder.typeModifier_max_update(typeRecord, value)
+  | ("$_typeModifier_opaque_$", [IEvType(typeRecord)]) =>
+    TypeBuilder.typeModifier_opaque_update(typeRecord)
+  | ("$_typeOr_$", [IEvArray(arr)]) => TypeBuilder.typeOr(IEvArray(arr))
+  | ("$_typeFunction_$", [IEvArray(arr)]) => TypeBuilder.typeFunction(arr)
+  | ("$_typeTuple_$", [IEvArray(elems)]) => TypeBuilder.typeTuple(elems)
+  | ("$_typeArray_$", [elem]) => TypeBuilder.typeArray(elem)
+  | ("$_typeRecord_$", [IEvArray(arrayOfPairs)]) => TypeBuilder.typeRecord(arrayOfPairs)
   | ("concat", [IEvArray(aValueArray), IEvArray(bValueArray)]) =>
     doAddArray(aValueArray, bValueArray)
   | ("concat", [IEvString(aValueString), IEvString(bValueString)]) =>
diff --git 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
index 1467f815..e0e6902e 100644
--- 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
@@ -9,7 +9,7 @@ module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
 module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module ExpressionWithContext = Reducer_ExpressionWithContext
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 module Result = Belt.Result
 open Reducer_Expression_ExpressionBuilder
 
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Exception.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Exception.res
new file mode 100644
index 00000000..d7ca335c
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Exception.res
@@ -0,0 +1,3 @@
+// There are switch stament cases in the code which are impossible to reach by design.
+// ImpossibleException is a sign of programming error.
+exception ImpossibleException
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res
index e7c82e77..9df4ff28 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_Expression.res
@@ -6,7 +6,7 @@ module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Lambda = Reducer_Expression_Lambda
 module Macro = Reducer_Expression_Macro
 module MathJs = Reducer_MathJs
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 module Result = Belt.Result
 module T = Reducer_Expression_T
 
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res
index 1f6b18b7..7ab1ba05 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Expression/Reducer_ExpressionWithContext.res
@@ -3,7 +3,7 @@ module ErrorValue = Reducer_ErrorValue
 module ExpressionT = Reducer_Expression_T
 module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Result = Belt.Result
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 
 type bindings = ExpressionT.bindings
 type context = bindings
diff --git 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
index e96ae4a1..6e1ef774 100644
--- 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
@@ -2,7 +2,7 @@ module ErrorValue = Reducer_ErrorValue
 module ExpressionT = Reducer_Expression_T
 module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module Result = Belt.Result
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 
 type errorValue = Reducer_ErrorValue.errorValue
 type expression = ExpressionT.expression
diff --git 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
index 9fd4a34a..590cc99f 100644
--- 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
@@ -2,7 +2,7 @@ module BBindingsReplacer = Reducer_Expression_BindingsReplacer
 module BErrorValue = Reducer_ErrorValue
 module BExpressionT = Reducer_Expression_T
 module BInternalExpressionValue = ReducerInterface_InternalExpressionValue
-module BModule = Reducer_Category_Module
+module BModule = Reducer_Module
 
 type errorValue = BErrorValue.errorValue
 type expression = BExpressionT.expression
@@ -10,8 +10,6 @@ type expressionOrFFI = BExpressionT.expressionOrFFI
 type ffiFn = BExpressionT.ffiFn
 type internalCode = ReducerInterface_InternalExpressionValue.internalCode
 
-external castExpressionToInternalCode: expressionOrFFI => internalCode = "%identity"
-
 let eArray = anArray => anArray->BInternalExpressionValue.IEvArray->BExpressionT.EValue
 
 let eArrayString = anArray => anArray->BInternalExpressionValue.IEvArrayString->BExpressionT.EValue
@@ -37,17 +35,12 @@ let eLambda = (
   BInternalExpressionValue.IEvLambda({
     parameters: parameters,
     context: context,
-    body: NotFFI(expr)->castExpressionToInternalCode,
+    body: NotFFI(expr)->BModule.castExpressionToInternalCode,
   })->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 eLambdaFFI = (ffiFn: ffiFn) => {
+  ffiFn->BModule.eLambdaFFIValue->BExpressionT.EValue
 }
 
 let eNumber = aNumber => aNumber->BInternalExpressionValue.IEvNumber->BExpressionT.EValue
@@ -81,6 +74,9 @@ let eBindExpression = (bindingExpr: expression, expression: expression): express
 let eBindExpressionDefault = (expression: expression): expression =>
   eFunction("$$_bindExpression_$$", list{expression})
 
+let eTernary = (truth: expression, trueCase: expression, falseCase: expression): expression =>
+  eFunction("$$_ternary_$$", list{truth, trueCase, falseCase})
+
 let eIdentifier = (name: string): expression =>
   name->BInternalExpressionValue.IEvSymbol->BExpressionT.EValue
 
diff --git 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
index 28bdbc02..717b4b1d 100644
--- 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
@@ -3,7 +3,7 @@ module ErrorValue = Reducer_ErrorValue
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
 module ExpressionValue = ReducerInterface_InternalExpressionValue
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 module Result = Belt.Result
 
 type environment = ReducerInterface_InternalExpressionValue.environment
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
similarity index 87%
rename from packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module.res
rename to packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
index 584f9089..9f016f66 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Category/Reducer_Category_Module.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
@@ -1,5 +1,6 @@
 module ExpressionT = Reducer_Expression_T
 open ReducerInterface_InternalExpressionValue
+
 let expressionValueToString = toString
 
 type t = ReducerInterface_InternalExpressionValue.nameSpace
@@ -66,7 +67,7 @@ let set = (nameSpace: t, id: string, value): t => {
   Belt.Map.String.set(container, id, value)->NameSpace
 }
 
-let emptyModule: t = NameSpace(Belt.Map.String.empty)
+let emptyModule: t = NameSpace(emptyMap)
 
 let fromTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceFromTypeScriptBindings
 let toTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceToTypeScriptBindings
@@ -100,13 +101,25 @@ let removeOther = (nameSpace: t, other: t): t => {
   })->NameSpace
 }
 
+external castExpressionToInternalCode: ExpressionT.expressionOrFFI => internalCode = "%identity"
+let eLambdaFFIValue = (ffiFn: ExpressionT.ffiFn) => {
+  IEvLambda({
+    parameters: [],
+    context: emptyModule,
+    body: FFI(ffiFn)->castExpressionToInternalCode,
+  })
+}
+
 // -- Module definition
 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
+  Belt.Map.String.set(container, identifier, ev)->NameSpace
 }
 let defineNumber = (nameSpace: t, identifier: string, value: float): t =>
   nameSpace->define(identifier, IEvNumber(value))
 
 let defineModule = (nameSpace: t, identifier: string, value: t): t =>
   nameSpace->define(identifier, toExpressionValue(value))
+
+let defineFFI = (nameSpace: t, identifier: string, value: ExpressionT.ffiFn): t =>
+  nameSpace->define(identifier, value->eLambdaFFIValue)
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module_Lambda.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module_Lambda.res
new file mode 100644
index 00000000..d14f5729
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module_Lambda.res
@@ -0,0 +1,29 @@
+module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
+module ExpressionT = Reducer_Expression_T
+open Reducer_ErrorValue
+open ReducerInterface_InternalExpressionValue
+
+type expression = ExpressionT.expression
+
+let defaultCase = (call: functionCall) =>
+  REFunctionNotFound(call->functionCallToCallSignature->functionCallSignatureToString)->Error
+
+let defaultCaseFFIFn = (functionName: string): ExpressionT.ffiFn => {
+  (args: array<internalExpressionValue>, _environment: environment): result<
+    internalExpressionValue,
+    errorValue,
+  > => {
+    let call = (functionName, args)
+    defaultCase(call)
+  }
+}
+
+let defaultCaseFFI = (functionName: string): expression => {
+  ExpressionBuilder.eLambdaFFI(defaultCaseFFIFn(functionName))
+}
+
+let addGuard = (
+  guard: expression,
+  expression: expression,
+  previousExpression: expression,
+): expression => ExpressionBuilder.eTernary(guard, expression, previousExpression)
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_GeneratedParser.peggy b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_GeneratedParser.peggy
index f7489c26..226d2b6f 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_GeneratedParser.peggy
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_GeneratedParser.peggy
@@ -343,11 +343,18 @@ typeModifierExpression = head:basicType tail:(_ '<-' _nl @typeModifier)*
     / modifier:identifier _ noArguments
       { return {modifier: modifier, args: []}; }
   
-basicType = typeConstructor / typeArray / typeRecord / typeInParanthesis / typeIdentifier
+basicType = typeConstructor / typeArray / typeTuple / typeRecord / typeInParanthesis / typeIdentifier
 
 typeArray = '[' _nl elem:typeExpression _nl ']' 
   {return h.apply('$_typeArray_$', elem)}
 
+typeTuple = '[' _nl elems:array_typeTupleArguments _nl ']' 
+  { return h.apply('$_typeTuple_$', h.constructArray(elems))}
+
+  array_typeTupleArguments
+    = head:typeExpression tail:(_ ',' _nl @typeExpression)* 
+    { return [head, ...tail]; }
+
 typeRecord = '{' _nl elems:array_typeRecordArguments _nl '}' 
   { return h.apply('$_typeRecord_$', h.constructRecord(elems)); }  
 
@@ -375,9 +382,3 @@ typeOfStatement = identifier:identifier _ ':' _nl typeExpression:typeExpression
   { return h.makeFunctionCall('$_typeOf_$', [identifier, typeExpression])}
 
 typeInParanthesis = '(' _nl typeExpression:typeExpression _nl ')' {return typeExpression}
-
-// TODO: min max example
-// TODO: Example of foo = {a: 2, b: 5}; type fooKeys = string $ memberOf(foo->keys)
-// TODO: Example of memberOf( [1,2,3] )
-// TODO: Example of $ 
-// TODO: Cons(a, list) | EmptyList
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_Parse.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_Parse.res
index 1a37ccc0..67873c61 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_Parse.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_Parse.res
@@ -5,7 +5,11 @@ type node = {"type": string}
 
 @module("./Reducer_Peggy_GeneratedParser.js") external parse__: string => node = "parse"
 
-let syntaxErrorToLocation: Js.Exn.t => Reducer_ErrorValue.location = error => %raw(`error.location`)
+type withLocation = {"location": Reducer_ErrorValue.location}
+external castWithLocation: Js.Exn.t => withLocation = "%identity"
+
+let syntaxErrorToLocation = (error: Js.Exn.t): Reducer_ErrorValue.location =>
+  castWithLocation(error)["location"]
 
 @genType
 let parse = (expr: string): result<node, errorValue> =>
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_T.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_T.res
new file mode 100644
index 00000000..9b847ca7
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_T.res
@@ -0,0 +1,82 @@
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+open InternalExpressionValue
+
+type rec iType =
+  | ItTypeIdentifier(string)
+  | ItModifiedType({modifiedType: iType})
+  | ItTypeOr({typeOr: array<iType>})
+  | ItTypeFunction({inputs: array<iType>, output: iType})
+  | ItTypeArray({element: iType})
+  | ItTypeTuple({elements: array<iType>})
+  | ItTypeRecord({properties: Belt.Map.String.t<iType>})
+
+let rec fromTypeMap = typeMap => {
+  let default = IEvString("")
+  let evTypeTag: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "typeTag",
+    default,
+  )
+  let evTypeIdentifier: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "typeIdentifier",
+    default,
+  )
+  let evTypeOr: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "typeOr",
+    default,
+  )
+  let evInputs: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "inputs",
+    default,
+  )
+  let evOutput: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "output",
+    default,
+  )
+  let evElement: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "element",
+    default,
+  )
+  let evElements: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "elements",
+    default,
+  )
+  let evProperties: InternalExpressionValue.t = Belt.Map.String.getWithDefault(
+    typeMap,
+    "properties",
+    default,
+  )
+  //TODO: map type modifiers
+  switch evTypeTag {
+  | IEvString("typeIdentifier") => ItModifiedType({modifiedType: fromIEvValue(evTypeIdentifier)})
+  | IEvString("typeOr") => ItTypeOr({typeOr: fromIEvArray(evTypeOr)})
+  | IEvString("typeFunction") =>
+    ItTypeFunction({inputs: fromIEvArray(evInputs), output: fromIEvValue(evOutput)})
+  | IEvString("typeArray") => ItTypeArray({element: fromIEvValue(evElement)})
+  | IEvString("typeTuple") => ItTypeTuple({elements: fromIEvArray(evElements)})
+  | IEvString("typeRecord") => ItTypeRecord({properties: fromIEvRecord(evProperties)})
+  | _ => raise(Reducer_Exception.ImpossibleException)
+  }
+}
+and fromIEvValue = (ievValue: InternalExpressionValue.t) =>
+  switch ievValue {
+  | IEvTypeIdentifier(typeIdentifier) => ItTypeIdentifier({typeIdentifier})
+  | IEvType(typeMap) => fromTypeMap(typeMap)
+  | _ => raise(Reducer_Exception.ImpossibleException)
+  }
+and fromIEvArray = (ievArray: InternalExpressionValue.t) =>
+  switch ievArray {
+  | IEvArray(array) => array->Belt.Array.map(fromIEvValue)
+  | _ => raise(Reducer_Exception.ImpossibleException)
+  }
+and fromIEvRecord = (ievRecord: InternalExpressionValue.t) =>
+  switch ievRecord {
+  | IEvRecord(record) => record->Belt.Map.String.map(fromIEvValue)
+  | _ => raise(Reducer_Exception.ImpossibleException)
+  }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeBuilder.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeBuilder.res
new file mode 100644
index 00000000..e51f901a
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeBuilder.res
@@ -0,0 +1,88 @@
+open ReducerInterface_InternalExpressionValue
+
+let typeModifier_memberOf = (aType, anArray) => {
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeIdentifier")),
+    ("typeIdentifier", aType),
+  ])
+  newRecord->Belt.Map.String.set("memberOf", anArray)->IEvType->Ok
+}
+
+let typeModifier_memberOf_update = (aRecord, anArray) => {
+  aRecord->Belt.Map.String.set("memberOf", anArray)->IEvType->Ok
+}
+
+let typeModifier_min = (aType, value) => {
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeIdentifier")),
+    ("typeIdentifier", aType),
+  ])
+  newRecord->Belt.Map.String.set("min", value)->IEvType->Ok
+}
+
+let typeModifier_min_update = (aRecord, value) => {
+  aRecord->Belt.Map.String.set("min", value)->IEvType->Ok
+}
+
+let typeModifier_max = (aType, value) => {
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeIdentifier")),
+    ("typeIdentifier", aType),
+  ])
+  newRecord->Belt.Map.String.set("max", value)->IEvType->Ok
+}
+
+let typeModifier_max_update = (aRecord, value) =>
+  aRecord->Belt.Map.String.set("max", value)->IEvType->Ok
+
+let typeModifier_opaque_update = aRecord =>
+  aRecord->Belt.Map.String.set("opaque", IEvBool(true))->IEvType->Ok
+
+let typeOr = evArray => {
+  let newRecord = Belt.Map.String.fromArray([("typeTag", IEvString("typeOr")), ("typeOr", evArray)])
+  newRecord->IEvType->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 = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeFunction")),
+    ("inputs", IEvArray(inputs)),
+    ("output", output),
+  ])
+  newRecord->IEvType->Ok
+}
+
+let typeArray = element => {
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeTuple")),
+    ("element", element),
+  ])
+  newRecord->IEvType->Ok
+}
+
+let typeTuple = anArray => {
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeArray")),
+    ("elements", IEvArray(anArray)),
+  ])
+  newRecord->IEvType->Ok
+}
+
+let typeRecord = arrayOfPairs => {
+  let newProperties =
+    Belt.Array.map(arrayOfPairs, pairValue =>
+      switch pairValue {
+      | IEvArray([IEvString(key), valueValue]) => (key, valueValue)
+      | _ => ("wrong key type", pairValue->toStringWithType->IEvString)
+      }
+    )
+    ->Belt.Map.String.fromArray
+    ->IEvRecord
+  let newRecord = Belt.Map.String.fromArray([
+    ("typeTag", IEvString("typeRecord")),
+    ("properties", newProperties),
+  ])
+  newRecord->IEvType->Ok
+}
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeChecker.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeChecker.res
new file mode 100644
index 00000000..b76ba357
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Type/Reducer_Type_TypeChecker.res
@@ -0,0 +1,81 @@
+//TODO: Work in progress. Code is commented to make an a release of other features
+
+module ExpressionT = Reducer_Expression_T
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+module T = Reducer_Type_T
+module TypeBuilder = Reducer_Type_TypeBuilder
+open InternalExpressionValue
+
+type typeErrorValue =
+  | TypeError(T.iType, InternalExpressionValue.t)
+  | TypeErrorWithPosition(T.iType, InternalExpressionValue.t, int)
+  | TypeErrorWithProperty(T.iType, InternalExpressionValue.t, string)
+
+let rec isOfResolvedIType = (anIType: T.iType, aValue): result<bool, typeErrorValue> => {
+  let caseTypeIdentifier = (anUpperTypeName, aValue) => {
+    let aTypeName = anUpperTypeName->Js.String2.toLowerCase
+    let valueTypeName = aValue->valueToValueType->valueTypeToString->Js.String2.toLowerCase
+    switch aTypeName === valueTypeName {
+    | true => Ok(true)
+    | false => TypeError(anIType, aValue)->Error
+    }
+  }
+
+  let _caseRecord = (anIType, evValue, propertyMap, map) => {
+    Belt.Map.String.reduce(propertyMap, Ok(true), (acc, property, propertyType) => {
+      Belt.Result.flatMap(acc, _ =>
+        switch Belt.Map.String.get(map, property) {
+        | Some(propertyValue) => isOfResolvedIType(propertyType, propertyValue)
+        | None => TypeErrorWithProperty(anIType, evValue, property)->Error
+        }
+      )
+    })
+  }
+  let _caseArray = (anIType, evValue, elementType, anArray) => {
+    Belt.Array.reduceWithIndex(anArray, Ok(true), (acc, element, index) => {
+      switch isOfResolvedIType(elementType, element) {
+      | Ok(_) => acc
+      | Error(_) => TypeErrorWithPosition(anIType, evValue, index)->Error
+      }
+    })
+  }
+
+  switch anIType {
+  | ItTypeIdentifier(name) => caseTypeIdentifier(name, aValue)
+  // | ItModifiedType({modifiedType: anIType}) => raise(Reducer_Exception.ImpossibleException)
+  // | ItTypeOr({typeOr: anITypeArray}) => raise(Reducer_Exception.ImpossibleException)
+  // | ItTypeFunction({inputs: anITypeArray, output: anIType}) =>
+  //   raise(Reducer_Exception.ImpossibleException)
+  // | ItTypeArray({element: anIType}) => raise(Reducer_Exception.ImpossibleException)
+  // | ItTypeTuple({elements: anITypeArray}) => raise(Reducer_Exception.ImpossibleException)
+  // | ItTypeRecord({properties: anITypeMap}) => raise(Reducer_Exception.ImpossibleException)
+  | _ => raise(Reducer_Exception.ImpossibleException)
+  }
+}
+
+let isOfResolvedType = (aType: InternalExpressionValue.t, aValue): result<bool, typeErrorValue> =>
+  aType->T.fromIEvValue->isOfResolvedIType(aValue)
+
+// let checkArguments = (
+//   evFunctionType: InternalExpressionValue.t,
+//   args: array<InternalExpressionValue.t>,
+// ) => {
+//   let functionType = switch evFunctionType {
+//   | IEvRecord(functionType) => functionType
+//   | _ => raise(Reducer_Exception.ImpossibleException)
+//   }
+//   let evInputs = functionType->Belt.Map.String.getWithDefault("inputs", []->IEvArray)
+//   let inputs = switch evInputs {
+//   | IEvArray(inputs) => inputs
+//   | _ => raise(Reducer_Exception.ImpossibleException)
+//   }
+//   let rTupleType = TypeBuilder.typeTuple(inputs)
+//   Belt.Result.flatMap(rTupleType, tuppleType => isOfResolvedType(tuppleType, args->IEvArray))
+// }
+
+// let compileTypeExpression = (typeExpression: string, bindings: ExpressionT.bindings, reducerFn: ExpressionT.reducerFn) => {
+//     statement = `type compiled=${typeExpression}`
+
+// }
+
+//TODO: asGuard
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res
index bd5d8a35..9bd356d4 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExternalExpressionValue.res
@@ -24,6 +24,7 @@ type rec externalExpressionValue =
   | EvDeclaration(lambdaDeclaration)
   | EvTypeIdentifier(string)
   | EvModule(record)
+  | EvType(record)
 and record = Js.Dict.t<externalExpressionValue>
 and externalBindings = record
 and lambdaValue = {
@@ -50,17 +51,18 @@ let rec toString = aValue =>
     }
   | EvBool(aBool) => Js.String.make(aBool)
   | EvCall(fName) => `:${fName}`
+  | EvDate(date) => DateTime.Date.toString(date)
+  | EvDeclaration(d) => Declaration.toString(d, r => toString(EvLambda(r)))
+  | EvDistribution(dist) => GenericDist.toString(dist)
   | EvLambda(lambdaValue) => `lambda(${Js.Array2.toString(lambdaValue.parameters)}=>internal code)`
+  | EvModule(m) => `@${m->toStringRecord}`
   | EvNumber(aNumber) => Js.String.make(aNumber)
+  | EvRecord(aRecord) => aRecord->toStringRecord
   | EvString(aString) => `'${aString}'`
   | EvSymbol(aString) => `:${aString}`
-  | EvRecord(aRecord) => aRecord->toStringRecord
-  | EvDistribution(dist) => GenericDist.toString(dist)
-  | EvDate(date) => DateTime.Date.toString(date)
   | EvTimeDuration(t) => DateTime.Duration.toString(t)
-  | EvDeclaration(d) => Declaration.toString(d, r => toString(EvLambda(r)))
+  | EvType(t) => `type${t->toStringRecord}`
   | EvTypeIdentifier(id) => `#${id}`
-  | EvModule(m) => `@${m->toStringRecord}`
   }
 and toStringRecord = aRecord => {
   let pairs =
@@ -88,72 +90,3 @@ type environment = DistributionOperation.env
 
 @genType
 let defaultEnvironment: environment = DistributionOperation.defaultEnv
-
-type expressionValueType =
-  | EvtArray
-  | EvtArrayString
-  | EvtBool
-  | EvtCall
-  | EvtDistribution
-  | EvtLambda
-  | EvtNumber
-  | EvtRecord
-  | EvtString
-  | EvtSymbol
-  | EvtDate
-  | EvtTimeDuration
-  | EvtDeclaration
-  | EvtTypeIdentifier
-  | EvtModule
-
-type functionCallSignature = CallSignature(string, array<expressionValueType>)
-type functionDefinitionSignature =
-  FunctionDefinitionSignature(functionCallSignature, expressionValueType)
-
-let valueToValueType = value =>
-  switch value {
-  | EvArray(_) => EvtArray
-  | EvArrayString(_) => EvtArrayString
-  | EvBool(_) => EvtBool
-  | EvCall(_) => EvtCall
-  | EvDistribution(_) => EvtDistribution
-  | EvLambda(_) => EvtLambda
-  | EvNumber(_) => EvtNumber
-  | EvRecord(_) => EvtRecord
-  | EvString(_) => EvtString
-  | EvSymbol(_) => EvtSymbol
-  | EvDate(_) => EvtDate
-  | EvTimeDuration(_) => EvtTimeDuration
-  | EvDeclaration(_) => EvtDeclaration
-  | EvTypeIdentifier(_) => EvtTypeIdentifier
-  | EvModule(_) => EvtModule
-  }
-
-let functionCallToCallSignature = (functionCall: functionCall): functionCallSignature => {
-  let (fn, args) = functionCall
-  CallSignature(fn, args->Js.Array2.map(valueToValueType))
-}
-
-let valueTypeToString = (valueType: expressionValueType): 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})`
-}
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
index 30de55dd..a6766071 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
@@ -7,8 +7,8 @@ type environment = ExternalExpressionValue.environment
 let defaultEnvironment = ExternalExpressionValue.defaultEnvironment
 
 type rec t =
-  | IEvArray(array<t>) // FIXME: Convert
-  | IEvArrayString(array<string>) // FIXME: Convert
+  | IEvArray(array<t>) // FIXME: Convert to MapInt
+  | IEvArrayString(array<string>)
   | IEvBool(bool)
   | IEvCall(string) // External function call
   | IEvDate(Js.Date.t)
@@ -21,6 +21,7 @@ type rec t =
   | IEvString(string)
   | IEvSymbol(string)
   | IEvTimeDuration(float)
+  | IEvType(map)
   | IEvTypeIdentifier(string)
 and map = Belt.Map.String.t<t>
 and nameSpace = NameSpace(Belt.Map.String.t<t>)
@@ -56,6 +57,7 @@ let rec toString = aValue =>
   | IEvRecord(aMap) => aMap->toStringMap
   | IEvString(aString) => `'${aString}'`
   | IEvSymbol(aString) => `:${aString}`
+  | IEvType(aMap) => aMap->toStringMap
   | IEvTimeDuration(t) => DateTime.Duration.toString(t)
   | IEvTypeIdentifier(id) => `#${id}`
   }
@@ -78,17 +80,18 @@ let toStringWithType = aValue =>
   | IEvArrayString(_) => `ArrayString::${toString(aValue)}`
   | IEvBool(_) => `Bool::${toString(aValue)}`
   | IEvCall(_) => `Call::${toString(aValue)}`
+  | IEvDate(_) => `Date::${toString(aValue)}`
+  | IEvDeclaration(_) => `Declaration::${toString(aValue)}`
   | IEvDistribution(_) => `Distribution::${toString(aValue)}`
   | IEvLambda(_) => `Lambda::${toString(aValue)}`
+  | IEvModule(_) => `Module::${toString(aValue)}`
   | IEvNumber(_) => `Number::${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)}`
+  | IEvType(_) => `Type::${toString(aValue)}`
   | IEvTypeIdentifier(_) => `TypeIdentifier::${toString(aValue)}`
-  | IEvModule(_) => `Module::${toString(aValue)}`
   }
 
 let argsToString = (args: array<t>): string => {
@@ -120,17 +123,18 @@ type internalExpressionValueType =
   | EvtArrayString
   | EvtBool
   | EvtCall
+  | EvtDate
+  | EvtDeclaration
   | EvtDistribution
   | EvtLambda
+  | EvtModule
   | EvtNumber
   | EvtRecord
   | EvtString
   | EvtSymbol
-  | EvtDate
   | EvtTimeDuration
-  | EvtDeclaration
+  | EvtType
   | EvtTypeIdentifier
-  | EvtModule
 
 type functionCallSignature = CallSignature(string, array<internalExpressionValueType>)
 type functionDefinitionSignature =
@@ -142,17 +146,18 @@ let valueToValueType = value =>
   | IEvArrayString(_) => EvtArrayString
   | IEvBool(_) => EvtBool
   | IEvCall(_) => EvtCall
+  | IEvDate(_) => EvtDate
+  | IEvDeclaration(_) => EvtDeclaration
   | IEvDistribution(_) => EvtDistribution
   | IEvLambda(_) => EvtLambda
+  | IEvModule(_) => EvtModule
   | IEvNumber(_) => EvtNumber
   | IEvRecord(_) => EvtRecord
   | IEvString(_) => EvtString
   | IEvSymbol(_) => EvtSymbol
-  | IEvDate(_) => EvtDate
   | IEvTimeDuration(_) => EvtTimeDuration
-  | IEvDeclaration(_) => EvtDeclaration
+  | IEvType(_) => EvtType
   | IEvTypeIdentifier(_) => EvtTypeIdentifier
-  | IEvModule(_) => EvtModule
   }
 
 let functionCallToCallSignature = (functionCall: functionCall): functionCallSignature => {
@@ -166,17 +171,18 @@ let valueTypeToString = (valueType: internalExpressionValueType): string =>
   | EvtArrayString => `ArrayString`
   | EvtBool => `Bool`
   | EvtCall => `Call`
+  | EvtDate => `Date`
+  | EvtDeclaration => `Declaration`
   | EvtDistribution => `Distribution`
   | EvtLambda => `Lambda`
+  | EvtModule => `Module`
   | EvtNumber => `Number`
   | EvtRecord => `Record`
   | EvtString => `String`
   | EvtSymbol => `Symbol`
-  | EvtDate => `Date`
   | EvtTimeDuration => `Duration`
-  | EvtDeclaration => `Declaration`
+  | EvtType => `Type`
   | EvtTypeIdentifier => `TypeIdentifier`
-  | EvtModule => `Module`
   }
 
 let functionCallSignatureToString = (functionCallSignature: functionCallSignature): string => {
@@ -190,6 +196,11 @@ let rec toExternal = (iev: t): ExternalExpressionValue.t => {
   | IEvArrayString(v) => EvArrayString(v)
   | IEvBool(v) => EvBool(v)
   | IEvCall(v) => EvCall(v)
+  | IEvDeclaration(v) => {
+      let fn = lambdaValueToExternal(v.fn)
+      let args = v.args
+      EvDeclaration({fn: fn, args: args})
+    }
   | IEvDistribution(v) => EvDistribution(v)
   | IEvLambda(v) => EvLambda(lambdaValueToExternal(v))
   | IEvNumber(v) => EvNumber(v)
@@ -198,11 +209,7 @@ let rec toExternal = (iev: t): ExternalExpressionValue.t => {
   | 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})
-    }
+  | IEvType(v) => v->mapToExternal->EvType
   | IEvTypeIdentifier(v) => EvTypeIdentifier(v)
   | IEvModule(v) => v->nameSpaceToTypeScriptBindings->EvModule
   }
@@ -228,21 +235,22 @@ let rec toInternal = (ev: ExternalExpressionValue.t): t => {
   | 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)
+  | EvDistribution(v) => IEvDistribution(v)
+  | EvLambda(v) => IEvLambda(lambdaValueToInternal(v))
   | EvModule(v) => v->nameSpaceFromTypeScriptBindings->IEvModule
+  | EvNumber(v) => IEvNumber(v)
+  | EvRecord(v) => v->recordToInternal->IEvRecord
+  | EvString(v) => IEvString(v)
+  | EvSymbol(v) => IEvSymbol(v)
+  | EvTimeDuration(v) => IEvTimeDuration(v)
+  | EvType(v) => v->recordToInternal->IEvType
+  | EvTypeIdentifier(v) => IEvTypeIdentifier(v)
   }
 }
 and recordToInternal = v =>
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res
index b714790a..c08b7cb1 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_StdLib.res
@@ -1,4 +1,4 @@
-module Module = Reducer_Category_Module
+module Module = Reducer_Module
 
 let internalStdLib = Module.emptyModule->SquiggleLibrary_Math.makeBindings
 
diff --git a/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res b/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
index ba049bea..d85927a1 100644
--- a/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
+++ b/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
@@ -1,5 +1,5 @@
-module Bindings = Reducer_Category_Module
-module Module = Reducer_Category_Module
+module Bindings = Reducer_Module
+module Module = Reducer_Module
 
 let availableNumbers: array<(string, float)> = [
   ("pi", Js.Math._PI),