diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_defineFFI_test.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_defineFFI_test.res
new file mode 100644
index 00000000..1cc238a9
--- /dev/null
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_defineFFI_test.res
@@ -0,0 +1,94 @@
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+module ExpressionT = Reducer_Expression_T
+module Module = Reducer_Module
+module Bindings = Reducer_Module
+module ErrorValue = Reducer_ErrorValue
+
+open Jest
+open Expect
+
+// ----------------------
+// --- Start of Module File
+// ----------------------
+
+module FooImplementation = {
+  let fooNumber = 0.0
+  let fooString = "Foo String"
+  let fooBool = true
+  let makeFoo = (a: string, b: string, _environment): string => `I am ${a}-foo and I am ${b}-foo`
+  let makeBar = (a: float, b: float, _environment): string =>
+    `I am ${a->Js.Float.toString}-bar and I am ${b->Js.Float.toString}-bar`
+}
+
+module FooFFI = {
+  let makeFoo: ExpressionT.optionFfiFn = (args: array<InternalExpressionValue.t>, environment) => {
+    switch args {
+    | [IEvString(a), IEvString(b)] => FooImplementation.makeFoo(a, b, environment)->IEvString->Some
+    | _ => None
+    }
+  }
+  let makeBar: ExpressionT.optionFfiFn = (args: array<InternalExpressionValue.t>, environment) =>
+    switch args {
+    | [IEvNumber(a), IEvNumber(b)] => FooImplementation.makeBar(a, b, environment)->IEvString->Some
+    | _ => None
+    }
+}
+
+let fooModule: Module.t =
+  Module.emptyStdLib
+  ->Module.defineNumber("fooNumber", FooImplementation.fooNumber)
+  ->Module.defineString("fooString", FooImplementation.fooString)
+  ->Module.defineBool("fooBool", FooImplementation.fooBool)
+  ->Module.defineFunction("makeFoo", FooFFI.makeFoo)
+  ->Module.defineFunction("makeBar", FooFFI.makeBar)
+
+let makeBindings = (prevBindings: Bindings.t): Bindings.t =>
+  prevBindings->Module.defineModule("Foo", fooModule)
+
+// ----------------------
+// --- End of Module File
+// ----------------------
+
+let stdLibWithFoo = Bindings.emptyBindings->makeBindings
+let evalWithFoo = sourceCode =>
+  Reducer_Expression.parse(sourceCode)->Belt.Result.flatMap(expr =>
+    Reducer_Expression.reduceExpression(
+      expr,
+      stdLibWithFoo,
+      InternalExpressionValue.defaultEnvironment,
+    )
+  )
+let evalToStringResultWithFoo = sourceCode =>
+  evalWithFoo(sourceCode)->InternalExpressionValue.toStringResult
+
+describe("Module", () => {
+  test("fooNumber", () => {
+    let result = evalToStringResultWithFoo("Foo.fooNumber")
+    expect(result)->toEqual("Ok(0)")
+  })
+  test("fooString", () => {
+    let result = evalToStringResultWithFoo("Foo.fooString")
+    expect(result)->toEqual("Ok('Foo String')")
+  })
+  test("fooBool", () => {
+    let result = evalToStringResultWithFoo("Foo.fooBool")
+    expect(result)->toEqual("Ok(true)")
+  })
+  test("fooBool", () => {
+    let result = evalToStringResultWithFoo("Foo.fooBool")
+    expect(result)->toEqual("Ok(true)")
+  })
+  test("makeFoo", () => {
+    let result = evalToStringResultWithFoo("Foo.makeFoo('a', 'b')")
+    expect(result)->toEqual("Ok('I am a-foo and I am b-foo')")
+  })
+  test("makeFoo wrong arguments", () => {
+    let result = evalToStringResultWithFoo("Foo.makeFoo(1, 2)")
+    // Notice the error with types
+    expect(result)->toEqual("Error(Function not found: makeFoo(Number,Number))")
+  })
+  test("makeBar", () => {
+    let result = evalToStringResultWithFoo("Foo.makeBar(1, 2)")
+    expect(result)->toEqual("Ok('I am 1-bar and I am 2-bar')")
+  })
+})
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 717b4b1d..bf52b398 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
@@ -19,12 +19,19 @@ let checkArity = (
   lambdaValue: ExpressionValue.lambdaValue,
   args: list<internalExpressionValue>,
 ) => {
-  let argsLength = Belt.List.length(args)
-  let parametersLength = Js.Array2.length(lambdaValue.parameters)
-  if argsLength !== parametersLength {
-    ErrorValue.REArityError(None, parametersLength, argsLength)->Error
-  } else {
-    args->Ok
+  let reallyCheck = {
+    let argsLength = Belt.List.length(args)
+    let parametersLength = Js.Array2.length(lambdaValue.parameters)
+    if argsLength !== parametersLength {
+      ErrorValue.REArityError(None, parametersLength, argsLength)->Error
+    } else {
+      args->Ok
+    }
+  }
+  let exprOrFFI = castInternalCodeToExpression(lambdaValue.body)
+  switch exprOrFFI {
+  | NotFFI(_) => reallyCheck
+  | FFI(_) => args->Ok
   }
 }
 
diff --git 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
index 38910da2..dee70c88 100644
--- 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
@@ -71,6 +71,8 @@ type ffiFn = (
   environment,
 ) => result<internalExpressionValue, Reducer_ErrorValue.errorValue>
 
+type optionFfiFn = (array<internalExpressionValue>, environment) => option<internalExpressionValue>
+
 type expressionOrFFI =
   | NotFFI(expression)
   | FFI(ffiFn)
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
index 9f016f66..62f9d8c7 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Module/Reducer_Module.res
@@ -1,4 +1,6 @@
 module ExpressionT = Reducer_Expression_T
+module InternalExpressionValue = ReducerInterface_InternalExpressionValue
+open Reducer_ErrorValue
 open ReducerInterface_InternalExpressionValue
 
 let expressionValueToString = toString
@@ -68,6 +70,7 @@ let set = (nameSpace: t, id: string, value): t => {
 }
 
 let emptyModule: t = NameSpace(emptyMap)
+let emptyBindings = emptyModule
 
 let fromTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceFromTypeScriptBindings
 let toTypeScriptBindings = ReducerInterface_InternalExpressionValue.nameSpaceToTypeScriptBindings
@@ -110,16 +113,50 @@ let eLambdaFFIValue = (ffiFn: ExpressionT.ffiFn) => {
   })
 }
 
+let functionNotFoundError = (call: functionCall) =>
+  REFunctionNotFound(call->functionCallToCallSignature->functionCallSignatureToString)->Error
+
+let functionNotFoundErrorFFIFn = (functionName: string): ExpressionT.ffiFn => {
+  (args: array<internalExpressionValue>, _environment: environment): result<
+    internalExpressionValue,
+    errorValue,
+  > => {
+    let call = (functionName, args)
+    functionNotFoundError(call)
+  }
+}
+
+let convertOptionToFfiFn = (
+  myFunctionName: string,
+  myFunction: ExpressionT.optionFfiFn,
+): ExpressionT.ffiFn => {
+  (args: array<InternalExpressionValue.t>, environment) => {
+    myFunction(args, environment)
+    ->Belt.Option.map(v => v->Ok)
+    ->Belt.Option.getWithDefault(functionNotFoundErrorFFIFn(myFunctionName)(args, environment))
+  }
+}
+
 // -- Module definition
 let define = (nameSpace: t, identifier: string, ev: internalExpressionValue): t => {
   let NameSpace(container) = nameSpace
   Belt.Map.String.set(container, identifier, ev)->NameSpace
 }
+
 let defineNumber = (nameSpace: t, identifier: string, value: float): t =>
   nameSpace->define(identifier, IEvNumber(value))
 
+let defineString = (nameSpace: t, identifier: string, value: string): t =>
+  nameSpace->define(identifier, IEvString(value))
+
+let defineBool = (nameSpace: t, identifier: string, value: bool): t =>
+  nameSpace->define(identifier, IEvBool(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)
+let defineFunction = (nameSpace: t, identifier: string, value: ExpressionT.optionFfiFn): t => {
+  nameSpace->define(identifier, convertOptionToFfiFn(identifier, value)->eLambdaFFIValue)
+}
+
+let emptyStdLib: t = emptyModule->defineBool("stdlib", true)
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
index d14f5729..7c3c5e54 100644
--- 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
@@ -1,25 +1,11 @@
 module ExpressionBuilder = Reducer_Expression_ExpressionBuilder
 module ExpressionT = Reducer_Expression_T
-open Reducer_ErrorValue
-open ReducerInterface_InternalExpressionValue
+// 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))
+  ExpressionBuilder.eLambdaFFI(Reducer_Module.functionNotFoundErrorFFIFn(functionName))
 }
 
 let addGuard = (