diff --git a/packages/squiggle-lang/__tests__/FunctionRegistry_test.res b/packages/squiggle-lang/__tests__/FunctionRegistry_test.res
new file mode 100644
index 00000000..e976f7b1
--- /dev/null
+++ b/packages/squiggle-lang/__tests__/FunctionRegistry_test.res
@@ -0,0 +1,82 @@
+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 = {
+  open FunctionRegistry_Core
+  open FunctionRegistry_Helpers
+
+  let library = [
+    Function.make(
+      ~name="add",
+      ~nameSpace="Foo",
+      ~definitions=[
+        FnDefinition.make(
+          ~requiresNamespace=false,
+          ~name="add",
+          ~inputs=[FRTypeNumber, FRTypeNumber],
+          ~run=(_, inputs, _) =>
+            switch inputs {
+            | [FRValueNumber(a), FRValueNumber(b)] => Ok(Wrappers.evNumber(a +. b))
+            | _ => Error("False")
+            },
+          (),
+        ),
+        FnDefinition.make(
+          ~requiresNamespace=true,
+          ~name="add",
+          ~inputs=[FRTypeNumber, FRTypeNumber, FRTypeNumber],
+          ~run=(_, inputs, _) =>
+            switch inputs {
+            | [FRValueNumber(a), FRValueNumber(b), FRValueNumber(c)] =>
+              Ok(Wrappers.evNumber(a +. b +. c))
+            | _ => Error("False")
+            },
+          (),
+        ),
+      ],
+      (),
+    ),
+  ]
+}
+
+let makeBindings = (previousBindings: Bindings.t): Bindings.t =>
+  previousBindings->FunctionRegistry_Core.Registry.makeModules(FooImplementation.library)
+
+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("add(1,2)", () => {
+    let result = evalToStringResultWithFoo("Foo.add(1,2)")
+    expect(result)->toEqual("Ok(3)")
+  })
+  test("add(1,2,3)", () => {
+    let result = evalToStringResultWithFoo("Foo.add(1,2,3)")
+    expect(result)->toEqual("Ok(6)")
+  })
+  test("add(1,2,3,5)", () => {
+    let result = evalToStringResultWithFoo("Foo.add(1,2,3,5)")
+    expect(result)->toEqual("Ok(6)")
+  })
+})
\ No newline at end of file
diff --git a/packages/squiggle-lang/package.json b/packages/squiggle-lang/package.json
index b2067205..ba6449ce 100644
--- a/packages/squiggle-lang/package.json
+++ b/packages/squiggle-lang/package.json
@@ -15,6 +15,7 @@
     "start": "rescript build -w -with-deps",
     "clean": "rescript clean && rm -rf dist",
     "test:reducer": "jest __tests__/Reducer*/",
+    "test:reducer2": "jest __tests__/FunctionRegistry_test",
     "benchmark": "ts-node benchmark/conversion_tests.ts",
     "test": "jest",
     "test:ts": "jest __tests__/TS/",
diff --git a/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res b/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res
index f18f493f..e0ef4441 100644
--- a/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res
+++ b/packages/squiggle-lang/src/rescript/FunctionRegistry/FunctionRegistry_Core.res
@@ -328,6 +328,14 @@ module FnDefinition = {
     t.name ++ `(${inputs})`
   }
 
+  let isMatch = (t: t, args: array<internalExpressionValue>) => {
+    let argValues = FRType.matchWithExpressionValueArray(t.inputs, args)
+    switch argValues {
+    | Some(values) => true
+    | None => false
+    }
+  }
+
   let run = (t: t, args: array<internalExpressionValue>, env: GenericDist.env) => {
     let argValues = FRType.matchWithExpressionValueArray(t.inputs, args)
     switch argValues {
@@ -387,6 +395,32 @@ module Function = {
   }
 }
 
+module NameSpace = {
+  type t = {name: string, functions: array<function>}
+  let definitions = (t: t) => t.functions->E.A2.fmap(f => f.definitions)->E.A.concatMany
+  let uniqueFnNames = (t: t) => definitions(t)->E.A2.fmap(r => r.name)->E.A.uniq
+  let nameToDefinitions = (t: t, name: string) => definitions(t)->E.A2.filter(r => r.name == name)
+
+  //todo: It could be good to set a warning if two definitions are both valid, but I don't expect this often.
+  let nameFfiFn = (t: t, name: string): Reducer_Expression_T.optionFfiFn => {
+    (args, environment) => {
+      let definitions =
+        nameToDefinitions(t, name)->E.A2.fmap((def, ()) =>
+          FnDefinition.isMatch(def, args)
+            ? FnDefinition.run(def, args, environment) |> E.R.toOption
+            : None
+        )
+      E.A.O.firstSomeFn(definitions)
+    }
+  }
+
+  let toModule = (t: t): Reducer_Module.t =>
+    E.A.reduce(uniqueFnNames(t), Reducer_Module.emptyStdLib, (acc, uniqueName) => {
+      let relevantDefinitions = nameFfiFn(t, uniqueName)
+      acc->Reducer_Module.defineFunction(uniqueName, relevantDefinitions)
+    })
+}
+
 module Registry = {
   let toJson = (r: registry) => r->E.A2.fmap(Function.toJson)
   let definitionsWithFunctions = (r: registry) =>
@@ -428,13 +462,11 @@ module Registry = {
   let makeModules = (prevBindings: Reducer_Module.t, t: registry): Reducer_Module.t => {
     let nameSpaces = allNamespaces(t)
     let nameSpaceBindings = nameSpaces->E.A2.fmap(nameSpace => {
-      let definitions =
-        t->definitionsWithFunctions->E.A2.filter(((_, fn)) => fn.nameSpace === nameSpace)
-
-      let newModule = E.A.reduce(definitions, Reducer_Module.emptyStdLib, (acc, (def, _)) => {
-        acc->Reducer_Module.defineFunction(def.name, FnDefinition.toFfiFn(def))
-      })
-      (nameSpace, newModule)
+      let foo: NameSpace.t = {
+        name: nameSpace,
+        functions: t->E.A2.filter(r => r.nameSpace == nameSpace),
+      }
+      (nameSpace, NameSpace.toModule(foo))
     })
     E.A.reduce(nameSpaceBindings, prevBindings, (acc, (name, fn)) =>
       acc->Reducer_Module.defineModule(name, fn)
diff --git a/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res b/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
index 436ed1fe..b7b925ed 100644
--- a/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
+++ b/packages/squiggle-lang/src/rescript/SquiggleLibrary/SquiggleLibrary_Math.res
@@ -19,6 +19,7 @@ let mathBindings: Bindings.t =
   ->E.A2.fmap(((name, v)) => (name, ReducerInterface_InternalExpressionValue.IEvNumber(v)))
   ->Bindings.fromArray
 
+//TODO: This should be in a different place.
 let makeBindings = (previousBindings: Bindings.t): Bindings.t =>
   previousBindings
   ->Bindings.defineModule("Math", mathBindings)