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_test.res
similarity index 83%
rename from packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_defineFFI_test.res
rename to packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_test.res
index a06c7a57..69fca252 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_defineFFI_test.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_Module/Reducer_Module_test.res
@@ -22,6 +22,13 @@ module FooImplementation = {
   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`
+  // You can also define functions that has their internal errors
+  let makeReturningError = (_a: float, _b: float, _environment): result<float, ErrorValue.t> =>
+    if false {
+      0.->Ok
+    } else {
+      ErrorValue.RETodo("test error")->Error
+    }
 }
 
 // There is a potential for type modules to define lift functions
@@ -38,6 +45,17 @@ module FooFFI = {
     | [IEvNumber(a), IEvNumber(b)] => FooImplementation.makeBar(a, b, environment)->IEvString->Some
     | _ => None
     }
+  let makeReturningError: ExpressionT.optionFfiFnReturningResult = (
+    args: array<InternalExpressionValue.t>,
+    environment,
+  ) =>
+    switch args {
+    | [IEvNumber(a), IEvNumber(b)] =>
+      FooImplementation.makeReturningError(a, b, environment)
+      ->Belt.Result.map(v => v->InternalExpressionValue.IEvNumber)
+      ->Some
+    | _ => None
+    }
 }
 
 let fooModule: Module.t =
@@ -47,6 +65,7 @@ let fooModule: Module.t =
   ->Module.defineBool("fooBool", FooImplementation.fooBool)
   ->Module.defineFunction("makeFoo", FooFFI.makeFoo)
   ->Module.defineFunction("makeBar", FooFFI.makeBar)
+  ->Module.defineFunctionReturningResult("makeReturningError", FooFFI.makeReturningError)
 
 let makeBindings = (prevBindings: Bindings.t): Bindings.t =>
   prevBindings->Module.defineModule("Foo", fooModule)
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 dee70c88..bf4d0170 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
@@ -72,6 +72,10 @@ type ffiFn = (
 ) => result<internalExpressionValue, Reducer_ErrorValue.errorValue>
 
 type optionFfiFn = (array<internalExpressionValue>, environment) => option<internalExpressionValue>
+type optionFfiFnReturningResult = (
+  array<internalExpressionValue>,
+  environment,
+) => option<result<internalExpressionValue, Reducer_ErrorValue.errorValue>>
 
 type expressionOrFFI =
   | NotFFI(expression)
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 62f9d8c7..34c304f9 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
@@ -126,6 +126,17 @@ let functionNotFoundErrorFFIFn = (functionName: string): ExpressionT.ffiFn => {
   }
 }
 
+let convertOptionToFfiFnReturningResult = (
+  myFunctionName: string,
+  myFunction: ExpressionT.optionFfiFnReturningResult,
+): ExpressionT.ffiFn => {
+  (args: array<InternalExpressionValue.t>, environment) => {
+    myFunction(args, environment)->Belt.Option.getWithDefault(
+      functionNotFoundErrorFFIFn(myFunctionName)(args, environment),
+    )
+  }
+}
+
 let convertOptionToFfiFn = (
   myFunctionName: string,
   myFunction: ExpressionT.optionFfiFn,
@@ -159,4 +170,15 @@ let defineFunction = (nameSpace: t, identifier: string, value: ExpressionT.optio
   nameSpace->define(identifier, convertOptionToFfiFn(identifier, value)->eLambdaFFIValue)
 }
 
-let emptyStdLib: t = emptyModule->defineBool("stdlib", true)
+let defineFunctionReturningResult = (
+  nameSpace: t,
+  identifier: string,
+  value: ExpressionT.optionFfiFnReturningResult,
+): t => {
+  nameSpace->define(
+    identifier,
+    convertOptionToFfiFnReturningResult(identifier, value)->eLambdaFFIValue,
+  )
+}
+
+let emptyStdLib: t = emptyModule->defineBool("_standardLibrary", true)