diff --git a/packages/components/src/components/SquiggleErrorAlert.tsx b/packages/components/src/components/SquiggleErrorAlert.tsx
index 49179497..d03e237f 100644
--- a/packages/components/src/components/SquiggleErrorAlert.tsx
+++ b/packages/components/src/components/SquiggleErrorAlert.tsx
@@ -7,5 +7,9 @@ type Props = {
 };
 
 export const SquiggleErrorAlert: React.FC<Props> = ({ error }) => {
-  return <ErrorAlert heading="Error">{error.toString()}</ErrorAlert>;
+  return (
+    <ErrorAlert heading="Error">
+      <pre>{error.toString()}</pre>
+    </ErrorAlert>
+  );
 };
diff --git a/packages/components/src/components/SquiggleViewer/ExpressionViewer.tsx b/packages/components/src/components/SquiggleViewer/ExpressionViewer.tsx
index 8bd32292..829f323f 100644
--- a/packages/components/src/components/SquiggleViewer/ExpressionViewer.tsx
+++ b/packages/components/src/components/SquiggleViewer/ExpressionViewer.tsx
@@ -298,7 +298,9 @@ export const ExpressionViewer: React.FC<Props> = ({ value, width }) => {
           {() => (
             <div>
               <span>No display for type: </span>{" "}
-              <span className="font-semibold text-slate-600">{value.tag}</span>
+              <span className="font-semibold text-slate-600">
+                {(value as any).tag}
+              </span>
             </div>
           )}
         </VariableList>
diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res
index 0dadae86..4b9073dd 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_Peggy/Reducer_Peggy_TestHelpers.res
@@ -9,12 +9,12 @@ open Jest
 open Expect
 
 let expectParseToBe = (expr, answer) =>
-  Parse.parse(expr)->Parse.toStringResult->expect->toBe(answer)
+  Parse.parse(expr, "test")->Parse.toStringResult->expect->toBe(answer)
 
 let testParse = (expr, answer) => test(expr, () => expectParseToBe(expr, answer))
 
 let expectToExpressionToBe = (expr, answer, ~v="_", ()) => {
-  let rExpr = Parse.parse(expr)->Result.map(ToExpression.fromNode)
+  let rExpr = Parse.parse(expr, "test")->Result.map(ToExpression.fromNode)
   let a1 = rExpr->ExpressionT.toStringResultOkless
 
   if v == "_" {
@@ -22,6 +22,7 @@ let expectToExpressionToBe = (expr, answer, ~v="_", ()) => {
   } else {
     let a2 =
       rExpr
+      ->E.R2.errMap(e => e->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue)
       ->Result.flatMap(expr => Expression.BackCompatible.evaluate(expr))
       ->Reducer_Value.toStringResultOkless
     (a1, a2)->expect->toEqual((answer, v))
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
index 07660044..a6b35ec4 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
@@ -25,7 +25,7 @@ x=1`,
     let mainIncludes = Project.getIncludes(project, "main")
     switch mainIncludes {
     | Ok(includes) => expect(includes) == ["common"]
-    | Error(error) => fail(error->Reducer_ErrorValue.errorToString)
+    | Error(error) => fail(error->Reducer_ErrorValue.errorValueToString)
     }
   })
   test("past chain", () => {
@@ -60,7 +60,7 @@ x=1`,
     let mainIncludes = Project.getIncludes(project, "main")
     switch mainIncludes {
     | Ok(includes) => expect(includes) == ["common", "myModule"]
-    | Error(error) => fail(error->Reducer_ErrorValue.errorToString)
+    | Error(error) => fail(error->Reducer_ErrorValue.errorValueToString)
     }
   })
 
@@ -99,7 +99,7 @@ x=1`,
     let mainIncludes = Project.getIncludes(project, "main")
     switch mainIncludes {
     | Ok(includes) => expect(includes) == ["common", "common2", "myModule"]
-    | Error(error) => fail(error->Reducer_ErrorValue.errorToString)
+    | Error(error) => fail(error->Reducer_ErrorValue.errorValueToString)
     }
   })
   test("direct past chain", () => {
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_3_includes_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_3_includes_test.res
index 2c63dfff..995ce4d3 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_3_includes_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_3_includes_test.res
@@ -36,7 +36,7 @@ Here we will finally proceed to a real life scenario. */
       /* Parse includes has set the includes */
       switch project->Project.getIncludes("main") {
       | Ok(includes) => includes->expect == ["common"]
-      | Error(err) => err->Reducer_ErrorValue.errorToString->fail
+      | Error(err) => err->Reducer_ErrorValue.errorValueToString->fail
       }
       /* If the includes cannot be parsed then you get a syntax error.
       Otherwise you get the includes.
@@ -85,7 +85,7 @@ Here we will finally proceed to a real life scenario. */
           let rIncludes = project->Project.getIncludes(sourceName)
           switch rIncludes {
           /* Maybe there is an include syntax error */
-          | Error(err) => err->Reducer_ErrorValue.errorToString->Js.Exn.raiseError
+          | Error(err) => err->Reducer_ErrorValue.errorValueToString->Js.Exn.raiseError
 
           | Ok(includes) =>
             includes->Belt.Array.forEach(newIncludeName => {
@@ -169,7 +169,7 @@ Here we will finally proceed to a real life scenario. */
     test("getIncludes", () => {
       switch Project.getIncludes(project, "main") {
       | Ok(includes) => includes->expect == ["common"]
-      | Error(err) => err->Reducer_ErrorValue.errorToString->fail
+      | Error(err) => err->Reducer_ErrorValue.errorValueToString->fail
       }
     })
   })
diff --git a/packages/squiggle-lang/scripts/run.mjs b/packages/squiggle-lang/scripts/run.mjs
index 6dde265f..91058f33 100755
--- a/packages/squiggle-lang/scripts/run.mjs
+++ b/packages/squiggle-lang/scripts/run.mjs
@@ -1,11 +1,18 @@
 #!/usr/bin/env node
 import { run } from "./lib.mjs";
 
-const src = process.argv[2];
+import { Command } from "commander";
+
+const program = new Command();
+
+program.arguments("<string>");
+
+const options = program.parse(process.argv);
+
+const src = program.args[0];
 if (!src) {
   throw new Error("Expected src");
 }
-console.log(`Running ${src}`);
 
 const sampleCount = process.env.SAMPLE_COUNT;
 
diff --git a/packages/squiggle-lang/src/js/SqError.ts b/packages/squiggle-lang/src/js/SqError.ts
index 317fde4f..c0c509b4 100644
--- a/packages/squiggle-lang/src/js/SqError.ts
+++ b/packages/squiggle-lang/src/js/SqError.ts
@@ -1,16 +1,12 @@
 import * as RSErrorValue from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
 
 export class SqError {
-  constructor(private _value: RSErrorValue.reducerErrorValue) {}
+  constructor(private _value: RSErrorValue.reducerError) {}
 
   toString() {
     return RSErrorValue.toString(this._value);
   }
 
-  static createTodoError(v: string) {
-    return new SqError(RSErrorValue.createTodoError(v));
-  }
-
   static createOtherError(v: string) {
     return new SqError(RSErrorValue.createOtherError(v));
   }
diff --git a/packages/squiggle-lang/src/js/SqProject.ts b/packages/squiggle-lang/src/js/SqProject.ts
index 9e79b564..22f2f78a 100644
--- a/packages/squiggle-lang/src/js/SqProject.ts
+++ b/packages/squiggle-lang/src/js/SqProject.ts
@@ -1,5 +1,9 @@
 import * as RSProject from "../rescript/ForTS/ForTS_ReducerProject.gen";
-import { reducerErrorValue } from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
+import {
+  reducerError,
+  reducerErrorValue,
+  attachEmptyStackTraceToErrorValue,
+} from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
 import { environment } from "../rescript/ForTS/ForTS_Distribution/ForTS_Distribution_Environment.gen";
 import { SqError } from "./SqError";
 import { SqRecord } from "./SqRecord";
@@ -50,7 +54,8 @@ export class SqProject {
     return resultMap2(
       RSProject.getIncludes(this._value, sourceId),
       (a) => a,
-      (v: reducerErrorValue) => new SqError(v)
+      (v: reducerErrorValue) =>
+        new SqError(attachEmptyStackTraceToErrorValue(v))
     );
   }
 
@@ -104,7 +109,7 @@ export class SqProject {
             items: [],
           })
         ),
-      (v: reducerErrorValue) => new SqError(v)
+      (v: reducerError) => new SqError(v)
     );
   }
 
diff --git a/packages/squiggle-lang/src/rescript/FR/FR_Danger.res b/packages/squiggle-lang/src/rescript/FR/FR_Danger.res
index 2d60f556..7767f8c1 100644
--- a/packages/squiggle-lang/src/rescript/FR/FR_Danger.res
+++ b/packages/squiggle-lang/src/rescript/FR/FR_Danger.res
@@ -143,7 +143,7 @@ module Integration = {
       | Error(b) =>
         ("Integration error 2 in Danger.integrate. It's possible that your function doesn't return a number, try definining auxiliaryFunction(x) = mean(yourFunction(x)) and integrate auxiliaryFunction instead." ++
         "Original error: " ++
-        b->Reducer_ErrorValue.errorToString)
+        b->Reducer_ErrorValue.errorValueToString)
         ->Reducer_ErrorValue.REOther
         ->Error
       }
@@ -225,7 +225,7 @@ module Integration = {
                 reducer,
               )->E.R2.errMap(b =>
                 ("Integration error 7 in Danger.integrate. Something went wrong along the way: " ++
-                b->Reducer_ErrorValue.errorToString)->Reducer_ErrorValue.REOther
+                b->Reducer_ErrorValue.errorValueToString)->Reducer_ErrorValue.REOther
               )
             | _ =>
               "Integration error 8 in Danger.integrate. Remember that inputs are (function, number (min), number (max), number(increment))"
diff --git a/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res b/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
index ac9c167c..857044d7 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
@@ -1,5 +1,6 @@
 @genType type reducerProject = ReducerProject_T.project //re-export
 
+type reducerError = ForTS_Reducer_ErrorValue.reducerError //use
 type reducerErrorValue = ForTS_Reducer_ErrorValue.reducerErrorValue //use
 
 type squiggleValue = ForTS_SquiggleValue.squiggleValue //use
@@ -191,10 +192,8 @@ let getBindings = (project: reducerProject, sourceId: string): squiggleValue_Rec
 Get the result after running this source file or the project
 */
 @genType
-let getResult = (project: reducerProject, sourceId: string): result<
-  squiggleValue,
-  reducerErrorValue,
-> => project->Private.getResult(sourceId)
+let getResult = (project: reducerProject, sourceId: string): result<squiggleValue, reducerError> =>
+  project->Private.getResult(sourceId)
 
 /*
 This is a convenience function to get the result of a single source without creating a project. 
@@ -202,10 +201,8 @@ However, without a project, you cannot handle include directives.
 The source has to be include free
 */
 @genType
-let evaluate = (sourceCode: string): (
-  result<squiggleValue, reducerErrorValue>,
-  squiggleValue_Record,
-) => Private.evaluate(sourceCode)
+let evaluate = (sourceCode: string): (result<squiggleValue, reducerError>, squiggleValue_Record) =>
+  Private.evaluate(sourceCode)
 
 @genType
 let setEnvironment = (project: reducerProject, environment: environment): unit =>
@@ -213,24 +210,3 @@ let setEnvironment = (project: reducerProject, environment: environment): unit =
 
 @genType
 let getEnvironment = (project: reducerProject): environment => project->Private.getEnvironment
-
-/*
-Foreign function interface is intentionally demolished.
-There is another way to do that: Umur.
-Also there is no more conversion from javascript to squiggle values currently.
-If the conversion to the new project is too difficult, I can add it later.
-*/
-
-// let foreignFunctionInterface = (
-//   lambdaValue: squiggleValue_Lambda,
-//   argArray: array<squiggleValue>,
-//   environment: environment,
-// ): result<squiggleValue, reducerErrorValue> => {
-//   let accessors = ReducerProject_ProjectAccessors_T.identityAccessorsWithEnvironment(environment)
-//   Reducer_Expression_Lambda.foreignFunctionInterface(
-//     lambdaValue,
-//     argArray,
-//     accessors,
-//     Reducer_Expression.reduceExpressionInProject,
-//   )
-// }
diff --git a/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res b/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res
index d4a8059b..4edc4ed3 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res
@@ -1,18 +1,21 @@
+@genType type reducerError = Reducer_ErrorValue.error //alias
 @genType type reducerErrorValue = Reducer_ErrorValue.errorValue //alias
-@genType type syntaxErrorLocation = Reducer_ErrorValue.syntaxErrorLocation //alias
+@genType type location = Reducer_ErrorValue.location //alias
 
 @genType
-let toString = (e: reducerErrorValue): string => Reducer_ErrorValue.errorToString(e)
+let toString = (e: reducerError): string => Reducer_ErrorValue.errorToString(e)
 
 @genType
-let getLocation = (e: reducerErrorValue): option<syntaxErrorLocation> =>
-  switch e {
-  | RESyntaxError(_, optionalLocation) => optionalLocation
-  | _ => None
+let getLocation = (e: reducerError): option<location> =>
+  switch e.stackTrace {
+  | Some(stack) => Some(stack.location)
+  | None => None
   }
 
 @genType
-let createTodoError = (v: string) => Reducer_ErrorValue.RETodo(v)
+let createOtherError = (v: string): reducerError =>
+  Reducer_ErrorValue.REOther(v)->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue
 
 @genType
-let createOtherError = (v: string) => Reducer_ErrorValue.REOther(v)
+let attachEmptyStackTraceToErrorValue = (v: reducerErrorValue): reducerError =>
+  Reducer_ErrorValue.attachEmptyStackTraceToErrorValue(v)
diff --git a/packages/squiggle-lang/src/rescript/ForTS/ForTS_SquiggleValue/ForTS_SquiggleValue.res b/packages/squiggle-lang/src/rescript/ForTS/ForTS_SquiggleValue/ForTS_SquiggleValue.res
index 8331e776..ff029c95 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS_SquiggleValue/ForTS_SquiggleValue.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS_SquiggleValue/ForTS_SquiggleValue.res
@@ -1,5 +1,5 @@
 @genType type squiggleValue = Reducer_T.value //re-export
-type reducerErrorValue = ForTS_Reducer_ErrorValue.reducerErrorValue //use
+type reducerError = ForTS_Reducer_ErrorValue.reducerError //use
 
 @genType type squiggleValue_Array = Reducer_T.arrayValue //re-export recursive type
 @genType type squiggleValue_Record = Reducer_T.map //re-export recursive type
@@ -69,7 +69,7 @@ let toString = (variant: squiggleValue) => Reducer_Value.toString(variant)
 // This is a useful method for unit tests.
 // Convert the result along with the error message to a string.
 @genType
-let toStringResult = (variantResult: result<squiggleValue, reducerErrorValue>) =>
+let toStringResult = (variantResult: result<squiggleValue, reducerError>) =>
   Reducer_Value.toStringResult(variantResult)
 
 @genType
diff --git a/packages/squiggle-lang/src/rescript/ForTS/ForTS__Types.res b/packages/squiggle-lang/src/rescript/ForTS/ForTS__Types.res
index 1cd74827..949801c7 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS__Types.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS__Types.res
@@ -1,5 +1,5 @@
 @genType type reducerErrorValue = ForTS_Reducer_ErrorValue.reducerErrorValue //re-export
-@genType type syntaxErrorLocation = ForTS_Reducer_ErrorValue.syntaxErrorLocation //re-export
+@genType type location = ForTS_Reducer_ErrorValue.location //re-export
 
 @genType type reducerProject = ForTS_ReducerProject.reducerProject //re-export
 @genType type squiggleValue = ForTS_SquiggleValue.squiggleValue //re-export
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
index b6baeaf5..4a3d863f 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
@@ -1,6 +1,16 @@
-//TODO: Do not export here but in ForTS__Types
-@gentype.import("peggy") @genType.as("LocationRange")
-type syntaxErrorLocation
+// Do not gentype this, use LocationRange from peggy types instead
+// TODO - rename locationPoint -> location, location -> locationRange to match peggy
+@genType
+type locationPoint = {
+  line: int,
+  column: int,
+}
+@genType
+type location = {
+  source: string,
+  start: locationPoint,
+  end: locationPoint,
+}
 
 @genType.opaque
 type errorValue =
@@ -18,7 +28,7 @@ type errorValue =
   | REOperationError(Operation.operationError)
   | RERecordPropertyNotFound(string, string)
   | RESymbolNotFound(string)
-  | RESyntaxError(string, option<syntaxErrorLocation>)
+  | RESyntaxError(string, option<location>)
   | RETodo(string) // To do
   | REUnitNotFound(string)
   | RENeedToRun
@@ -28,7 +38,20 @@ type t = errorValue
 
 exception ErrorException(errorValue)
 
-let errorToString = err =>
+type rec stackTrace = {
+  location: location,
+  parent: option<stackTrace>,
+}
+
+@genType.opaque
+type error = {
+  error: t,
+  stackTrace: option<stackTrace>,
+}
+
+exception ExceptionWithStackTrace(error)
+
+let errorValueToString = (err: errorValue) =>
   switch err {
   | REArityError(_oFnName, arity, usedArity) =>
     `${Js.String.make(arity)} arguments expected. Instead ${Js.String.make(
@@ -80,4 +103,37 @@ let fromException = exn =>
   | _e => REOther("Unknown error")
   }
 
-let toException = (errorValue: t) => raise(ErrorException(errorValue))
+let rec stackTraceToString = ({location, parent}: stackTrace) => {
+  `  Line ${location.start.line->Js.Int.toString}, column ${location.start.column->Js.Int.toString}, source ${location.source}\n` ++
+  switch parent {
+  | Some(parent) => stackTraceToString(parent)
+  | None => ""
+  }
+}
+
+let errorToString = (err: error) =>
+  switch err.stackTrace {
+  | Some(stack) => "Traceback:\n" ++ stack->stackTraceToString
+  | None => ""
+  } ++
+  err.error->errorValueToString
+
+let toException = (errorValue: t) => errorValue->ErrorException->raise
+
+let attachEmptyStackTraceToErrorValue = (errorValue: t) => {
+  error: errorValue,
+  stackTrace: None,
+}
+
+let attachLocationToErrorValue = (errorValue: t, location: location): error => {
+  error: errorValue,
+  stackTrace: Some({location: location, parent: None}),
+}
+
+let raiseNewExceptionWithStackTrace = (errorValue: t, location: location) =>
+  errorValue->attachLocationToErrorValue(location)->ExceptionWithStackTrace->raise
+
+let raiseExtendedExceptionWithStackTrace = ({error, stackTrace}: error, location: location) =>
+  {error: error, stackTrace: Some({location: location, parent: stackTrace})}
+  ->ExceptionWithStackTrace
+  ->raise
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 c28e121e..5ab213a0 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
@@ -5,12 +5,19 @@ module T = Reducer_T
 
 type errorValue = Reducer_ErrorValue.errorValue
 
+let toLocation = (expression: T.expression): Reducer_ErrorValue.location => {
+  expression.ast.location
+}
+
+let throwFrom = (error: errorValue, expression: T.expression) =>
+  error->Reducer_ErrorValue.raiseNewExceptionWithStackTrace(expression->toLocation)
+
 /*
   Recursively evaluate the expression
 */
 let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
   // Js.log(`reduce: ${expression->Reducer_Expression_T.toString}`)
-  switch expression {
+  switch expression.content {
   | T.EBlock(statements) => {
       let innerContext = {...context, bindings: context.bindings->Bindings.extend}
       let (value, _) =
@@ -49,7 +56,7 @@ let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
           let (key, _) = eKey->evaluate(context)
           let keyString = switch key {
           | IEvString(s) => s
-          | _ => REOther("Record keys must be strings")->Reducer_ErrorValue.ErrorException->raise
+          | _ => REOther("Record keys must be strings")->throwFrom(expression)
           }
           let (value, _) = eValue->evaluate(context)
           (keyString, value)
@@ -73,7 +80,7 @@ let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
   | T.ESymbol(name) =>
     switch context.bindings->Bindings.get(name) {
     | Some(v) => (v, context)
-    | None => Reducer_ErrorValue.RESymbolNotFound(name)->Reducer_ErrorValue.ErrorException->raise
+    | None => RESymbolNotFound(name)->throwFrom(expression)
     }
 
   | T.EValue(value) => (value, context)
@@ -82,7 +89,7 @@ let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
       let (predicateResult, _) = predicate->evaluate(context)
       switch predicateResult {
       | T.IEvBool(value) => (value ? trueCase : falseCase)->evaluate(context)
-      | _ => REExpectedType("Boolean", "")->Reducer_ErrorValue.ErrorException->raise
+      | _ => REExpectedType("Boolean", "")->throwFrom(expression)
       }
     }
 
@@ -98,12 +105,18 @@ let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
         argValue
       })
       switch lambda {
-      | T.IEvLambda(lambda) => (
-          Lambda.doLambdaCall(lambda, argValues, context.environment, evaluate),
-          context,
-        )
-      | _ =>
-        RENotAFunction(lambda->Reducer_Value.toString)->Reducer_ErrorValue.ErrorException->raise
+      | T.IEvLambda(lambda) =>
+        try {
+          let result = Lambda.doLambdaCall(lambda, argValues, context.environment, evaluate)
+          (result, context)
+        } catch {
+        | Reducer_ErrorValue.ErrorException(e) => e->throwFrom(expression) // function implementation returned an error without location
+        | Reducer_ErrorValue.ExceptionWithStackTrace(e) =>
+          Reducer_ErrorValue.raiseExtendedExceptionWithStackTrace(e, expression->toLocation) // function implementation probably called a lambda that threw an exception
+        | Js.Exn.Error(obj) =>
+          REJavaScriptExn(obj->Js.Exn.message, obj->Js.Exn.name)->throwFrom(expression)
+        }
+      | _ => RENotAFunction(lambda->Reducer_Value.toString)->throwFrom(expression)
       }
     }
   }
@@ -113,18 +126,24 @@ module BackCompatible = {
   // Those methods are used to support the existing tests
   // If they are used outside limited testing context, error location reporting will fail
   let parse = (peggyCode: string): result<T.expression, errorValue> =>
-    peggyCode->Reducer_Peggy_Parse.parse->Result.map(Reducer_Peggy_ToExpression.fromNode)
+    peggyCode->Reducer_Peggy_Parse.parse("main")->Result.map(Reducer_Peggy_ToExpression.fromNode)
 
-  let evaluate = (expression: T.expression): result<T.value, errorValue> => {
+  let evaluate = (expression: T.expression): result<T.value, Reducer_ErrorValue.error> => {
     let context = Reducer_Context.createDefaultContext()
     try {
       let (value, _) = expression->evaluate(context)
       value->Ok
     } catch {
-    | exn => Reducer_ErrorValue.fromException(exn)->Error
+    | exn =>
+      exn
+      ->Reducer_ErrorValue.fromException
+      ->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue
+      ->Error // TODO - this could be done better (see ReducerProject)
     }
   }
 
-  let evaluateString = (peggyCode: string): result<T.value, errorValue> =>
-    parse(peggyCode)->Result.flatMap(evaluate)
+  let evaluateString = (peggyCode: string): result<T.value, Reducer_ErrorValue.error> =>
+    parse(peggyCode)
+    ->E.R2.errMap(e => e->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue)
+    ->Result.flatMap(evaluate)
 }
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 9f1ed5fb..bd0a044a 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
@@ -3,14 +3,18 @@ module T = Reducer_T
 
 type errorValue = BErrorValue.errorValue
 type expression = Reducer_T.expression
+type expressionContent = Reducer_T.expressionContent
 
-let eArray = (anArray: array<T.expression>) => anArray->T.EArray
+let eArray = (anArray: array<T.expression>): expressionContent => anArray->T.EArray
 
 let eBool = aBool => aBool->T.IEvBool->T.EValue
 
-let eCall = (fn: expression, args: array<expression>): expression => T.ECall(fn, args)
+let eCall = (fn: expression, args: array<expression>): expressionContent => T.ECall(fn, args)
 
-let eLambda = (parameters: array<string>, expr: expression) => T.ELambda(parameters, expr)
+let eLambda = (parameters: array<string>, expr: expression): expressionContent => T.ELambda(
+  parameters,
+  expr,
+)
 
 let eNumber = aNumber => aNumber->T.IEvNumber->T.EValue
 
@@ -18,13 +22,13 @@ let eRecord = (aMap: array<(T.expression, T.expression)>) => aMap->T.ERecord
 
 let eString = aString => aString->T.IEvString->T.EValue
 
-let eSymbol = (name: string): expression => T.ESymbol(name)
+let eSymbol = (name: string): expressionContent => T.ESymbol(name)
 
-let eBlock = (exprs: array<expression>): expression => T.EBlock(exprs)
+let eBlock = (exprs: array<expression>): expressionContent => T.EBlock(exprs)
 
-let eProgram = (exprs: array<expression>): expression => T.EProgram(exprs)
+let eProgram = (exprs: array<expression>): expressionContent => T.EProgram(exprs)
 
-let eLetStatement = (symbol: string, valueExpression: expression): expression => T.EAssign(
+let eLetStatement = (symbol: string, valueExpression: expression): expressionContent => T.EAssign(
   symbol,
   valueExpression,
 )
@@ -33,11 +37,8 @@ let eTernary = (
   predicate: expression,
   trueCase: expression,
   falseCase: expression,
-): expression => T.ETernary(predicate, trueCase, falseCase)
+): expressionContent => T.ETernary(predicate, trueCase, falseCase)
 
-let eIdentifier = (name: string): expression => name->T.ESymbol
+let eIdentifier = (name: string): expressionContent => name->T.ESymbol
 
-// let eTypeIdentifier = (name: string): expression =>
-//   name->T.IEvTypeIdentifier->T.EValue
-
-let eVoid: expression = T.IEvVoid->T.EValue
+let eVoid: expressionContent = T.IEvVoid->T.EValue
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 71995c0a..2aca6cdb 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
@@ -12,7 +12,7 @@ let semicolonJoin = values =>
   Converts the expression to String
 */
 let rec toString = (expression: t) =>
-  switch expression {
+  switch expression.content {
   | EBlock(statements) =>
     `{${Js.Array2.map(statements, aValue => toString(aValue))->semicolonJoin}}`
   | EProgram(statements) => Js.Array2.map(statements, aValue => toString(aValue))->semicolonJoin
@@ -31,13 +31,13 @@ let rec toString = (expression: t) =>
 let toStringResult = codeResult =>
   switch codeResult {
   | Ok(a) => `Ok(${toString(a)})`
-  | Error(m) => `Error(${Reducer_ErrorValue.errorToString(m)})`
+  | Error(m) => `Error(${Reducer_ErrorValue.errorValueToString(m)})`
   }
 
 let toStringResultOkless = codeResult =>
   switch codeResult {
   | Ok(a) => toString(a)
-  | Error(m) => `Error(${Reducer_ErrorValue.errorToString(m)})`
+  | Error(m) => `Error(${Reducer_ErrorValue.errorValueToString(m)})`
   }
 
 let inspect = (expr: t): t => {
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 4d8cc135..0cd28a86 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
@@ -12,21 +12,21 @@ zeroOMoreArgumentsBlockOrExpression = innerBlockOrExpression / lambda
 outerBlock 
   = statements:array_statements  finalExpression: (statementSeparator @expression)?
     { if (finalExpression) statements.push(finalExpression)
-      return h.nodeProgram(statements) }
+      return h.nodeProgram(statements, location()) }
   / finalExpression: expression
-    { return h.nodeProgram([finalExpression]) }
+    { return h.nodeProgram([finalExpression], location()) }
     
 innerBlockOrExpression  
   = quotedInnerBlock
   / finalExpression: expression
-    { return h.nodeBlock([finalExpression])}
+    { return h.nodeBlock([finalExpression], location())}
 
 quotedInnerBlock  
   = '{' _nl statements:array_statements  finalExpression: (statementSeparator @expression)  _nl '}'
       { if (finalExpression) statements.push(finalExpression)
-        return h.nodeBlock(statements) }
+        return h.nodeBlock(statements, location()) }
   / '{' _nl finalExpression: expression  _nl '}'
-      { return h.nodeBlock([finalExpression]) }
+      { return h.nodeBlock([finalExpression], location()) }
 
 array_statements
   = head:statement tail:(statementSeparator @array_statements )
@@ -42,16 +42,16 @@ statement
 voidStatement 
   = "call" _nl value:zeroOMoreArgumentsBlockOrExpression
     { var variable = h.nodeIdentifier("_", location());
-      return h.nodeLetStatement(variable, value); }
+      return h.nodeLetStatement(variable, value, location()); }
 
 letStatement 
   = variable:variable _ assignmentOp _nl value:zeroOMoreArgumentsBlockOrExpression
-    { return h.nodeLetStatement(variable, value) }
+    { return h.nodeLetStatement(variable, value, location()) }
 
 defunStatement
   = variable:variable '(' _nl args:array_parameters _nl ')' _ assignmentOp _nl body:innerBlockOrExpression
-    { var value = h.nodeLambda(args, body)
-      return h.nodeLetStatement(variable, value) }
+    { var value = h.nodeLambda(args, body, location())
+      return h.nodeLetStatement(variable, value, location()) }
 
   assignmentOp "assignment" = '='
 
@@ -67,16 +67,16 @@ ifthenelse
   = 'if' __nl condition:logicalAdditive 
       __nl 'then' __nl trueExpression:innerBlockOrExpression 
     __nl 'else' __nl falseExpression:(ifthenelse/innerBlockOrExpression)
-    { return h.nodeTernary(condition, trueExpression, falseExpression) }
+    { return h.nodeTernary(condition, trueExpression, falseExpression, location()) }
   
 ternary 
   = condition:logicalAdditive _ '?' _nl trueExpression:logicalAdditive _ ':' _nl falseExpression:(ternary/logicalAdditive)
-    { return h.nodeTernary(condition, trueExpression, falseExpression) }
+    { return h.nodeTernary(condition, trueExpression, falseExpression, location()) }
 
 logicalAdditive
   = head:logicalMultiplicative tail:(_ operator:logicalAdditiveOp _nl arg:logicalMultiplicative {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
   logicalAdditiveOp "operator" = '||'
@@ -85,21 +85,21 @@ logicalAdditive
 logicalMultiplicative
   = head:equality tail:(_ operator:logicalMultiplicativeOp _nl arg:equality {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
   logicalMultiplicativeOp "operator" = '&&'
 
 equality
   = left:relational _ operator:equalityOp _nl right:relational 
-  { return h.makeFunctionCall(h.toFunction[operator], [left, right])}
+  { return h.makeFunctionCall(h.toFunction[operator], [left, right], location())}
   / relational  
   
   equalityOp "operator" = '=='/'!='
 
 relational
   = left:additive _ operator:relationalOp _nl right:additive 
-  { return h.makeFunctionCall(h.toFunction[operator], [left, right])}
+  { return h.makeFunctionCall(h.toFunction[operator], [left, right], location())}
   / additive
 
   relationalOp "operator" = '<='/'<'/'>='/'>'
@@ -107,7 +107,7 @@ relational
 additive
   = head:multiplicative tail:(_ operator:additiveOp _nl arg:multiplicative {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
   additiveOp "operator" = '+' / '-' / '.+' / '.-'
@@ -115,7 +115,7 @@ additive
 multiplicative
   = head:power tail:(_ operator:multiplicativeOp _nl arg:power {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
   multiplicativeOp "operator" = '*' / '/' / '.*' / './'
@@ -123,7 +123,7 @@ multiplicative
 power
   = head:credibleInterval tail:(_ operator:powerOp _nl arg:credibleInterval {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
   powerOp "operator" = '^' / '.^'
@@ -131,7 +131,7 @@ power
 credibleInterval
   = head:chainFunctionCall tail:(__ operator:credibleIntervalOp __nl arg:chainFunctionCall {return {operator: operator, right: arg}})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right])
+      return h.makeFunctionCall(h.toFunction[element.operator], [result, element.right], location())
     }, head)}
 
     credibleIntervalOp "operator" = 'to'
@@ -139,7 +139,7 @@ credibleInterval
 chainFunctionCall
   = head:unary tail:(_ ('->'/'|>') _nl chained:chainedFunction {return chained})* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(element.fnName, [result, ...element.args])
+      return h.makeFunctionCall(element.fnName, [result, ...element.args], location())
     }, head)}
 
   chainedFunction
@@ -154,7 +154,7 @@ chainFunctionCall
 
 unary
   = unaryOperator:unaryOperator _nl right:(unary/postOperator)
-  { return h.makeFunctionCall(h.unaryToFunction[unaryOperator], [right])}
+  { return h.makeFunctionCall(h.unaryToFunction[unaryOperator], [right], location())}
   / postOperator
   
   unaryOperator "unary operator"
@@ -169,17 +169,17 @@ collectionElement
     tail:(
       _ '[' _nl arg:expression  _nl ']' {return {fn: h.postOperatorToFunction['[]'], args: [arg]}}
     / _ '(' _nl args:array_functionArguments  _nl ')' {return {fn: h.postOperatorToFunction['()'], args: args}}
-    / '.' arg:$dollarIdentifier {return {fn: h.postOperatorToFunction['[]'], args: [h.nodeString(arg)]}}
+    / '.' arg:$dollarIdentifier {return {fn: h.postOperatorToFunction['[]'], args: [h.nodeString(arg, location())]}}
     )* 
   { return tail.reduce(function(result, element) {
-      return h.makeFunctionCall(element.fn, [result, ...element.args])
+      return h.makeFunctionCall(element.fn, [result, ...element.args], location())
     }, head)}
 
     array_functionArguments 
       = head:expression tail:(_ ',' _nl @expression)* 
       { return [head, ...tail]; }
   / ""
-      {return [h.nodeVoid()];}
+      {return [h.nodeVoid(location())];}
 
 atom
   = '(' _nl expression:expression _nl ')' {return expression}
@@ -195,7 +195,7 @@ basicLiteral
   / voidLiteral
 
 voidLiteral 'void'
-  = "()" {return h.nodeVoid();}
+  = "()" {return h.nodeVoid(location());}
 
 variable = dollarIdentifierWithModule / dollarIdentifier
 
@@ -221,36 +221,36 @@ dollarIdentifier '$identifier'
   = ([\$_a-z]+[\$_a-z0-9]i*) {return h.nodeIdentifier(text(), location())} 
 
 moduleIdentifier 'identifier'
-  = ([A-Z]+[_a-z0-9]i*) {return h.nodeModuleIdentifier(text())} 
+  = ([A-Z]+[_a-z0-9]i*) {return h.nodeModuleIdentifier(text(), location())} 
 
 
 string 'string'
-  = characters:("'" @([^'])* "'") {return h.nodeString(characters.join(''))} 
-  / characters:('"' @([^"])* '"') {return h.nodeString(characters.join(''))}
+  = characters:("'" @([^'])* "'") {return h.nodeString(characters.join(''), location())} 
+  / characters:('"' @([^"])* '"') {return h.nodeString(characters.join(''), location())}
 
 number = number:(float / integer) unit:unitIdentifier?
   { 
     if (unit === null)
       { return number }
     else
-      { return h.makeFunctionCall('fromUnit_'+unit.value, [number]) 
+      { return h.makeFunctionCall('fromUnit_'+unit.value, [number], location()) 
       }
   }
 
 integer 'integer'
   = d+ !"\." ![e]i
-  { return h.nodeInteger(parseInt(text()))} 
+  { return h.nodeInteger(parseInt(text()), location())} 
   
 float 'float'
   = $(((d+ "\." d*) / ("\." d+)) floatExponent? / d+ floatExponent)
-  { return h.nodeFloat(parseFloat(text()))} 
+  { return h.nodeFloat(parseFloat(text()), location())} 
 
     floatExponent = [e]i '-'? d+
     d = [0-9]
     
 boolean 'boolean'
   = ('true'/'false')  ! [a-z]i ! [_$]
-  { return h.nodeBoolean(text() === 'true')}
+  { return h.nodeBoolean(text() === 'true', location())}
 
 valueConstructor
   = recordConstructor
@@ -261,15 +261,15 @@ valueConstructor
 lambda  
   = '{' _nl '|' _nl args:array_parameters _nl '|' _nl statements:array_statements  finalExpression: (statementSeparator @expression)  _nl '}'
       { statements.push(finalExpression) 
-        return h.nodeLambda(args, h.nodeBlock(statements)) }
+        return h.nodeLambda(args, h.nodeBlock(statements, location()), location()) }
   / '{' _nl '|' _nl args:array_parameters _nl '|' _nl finalExpression: expression  _nl '}'
-      { return h.nodeLambda(args, finalExpression) }
+      { return h.nodeLambda(args, finalExpression, location()) }
 
 arrayConstructor 'array'
   = '[' _nl ']'
-    { return h.constructArray([]); }
+    { return h.constructArray([], location()); }
   / '[' _nl args:array_elements _nl ']' 
-    { return h.constructArray(args); }
+    { return h.constructArray(args, location()); }
 
   array_elements 
     = head:expression tail:(_ ',' _nl @expression)* 
@@ -277,7 +277,7 @@ arrayConstructor 'array'
 
 recordConstructor  'record'
   = '{' _nl args:array_recordArguments _nl end_of_record
-  { return h.constructRecord(args); }
+  { return h.constructRecord(args, location()); }
 
   end_of_record 
     = '}'
@@ -289,7 +289,7 @@ recordConstructor  'record'
 
   keyValuePair 
     = key:expression _ ':' _nl value:expression 
-    { return h.nodeKeyValue(key, value)}
+    { return h.nodeKeyValue(key, value, location())}
 
 // Separators
 
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 9c59c504..c6e21c9e 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
@@ -1,19 +1,20 @@
 module Extra = Reducer_Extra
 open Reducer_ErrorValue
 
-type node = {"type": string}
+type node = {"type": string, "location": Reducer_ErrorValue.location}
 
-@module("./Reducer_Peggy_GeneratedParser.js") external parse__: string => node = "parse"
+@module("./Reducer_Peggy_GeneratedParser.js")
+external parse__: (string, {"grammarSource": string}) => node = "parse"
 
-type withLocation = {"location": Reducer_ErrorValue.syntaxErrorLocation}
+type withLocation = {"location": Reducer_ErrorValue.location}
 external castWithLocation: Js.Exn.t => withLocation = "%identity"
 
-let syntaxErrorToLocation = (error: Js.Exn.t): Reducer_ErrorValue.syntaxErrorLocation =>
+let syntaxErrorToLocation = (error: Js.Exn.t): Reducer_ErrorValue.location =>
   castWithLocation(error)["location"]
 
-let parse = (expr: string): result<node, errorValue> =>
+let parse = (expr: string, source: string): result<node, errorValue> =>
   try {
-    Ok(parse__(expr))
+    Ok(parse__(expr, {"grammarSource": source}))
   } catch {
   | Js.Exn.Error(obj) =>
     RESyntaxError(Belt.Option.getExn(Js.Exn.message(obj)), syntaxErrorToLocation(obj)->Some)->Error
@@ -34,27 +35,30 @@ type nodeLetStatement = {...node, "variable": nodeIdentifier, "value": node}
 type nodeModuleIdentifier = {...node, "value": string}
 type nodeString = {...node, "value": string}
 type nodeTernary = {...node, "condition": node, "trueExpression": node, "falseExpression": node}
-// type nodeTypeIdentifier = {...node, "value": string}
 type nodeVoid = node
 
-type peggyNode =
-  | PgNodeBlock(nodeBlock)
-  | PgNodeProgram(nodeProgram)
-  | PgNodeArray(nodeArray)
-  | PgNodeRecord(nodeRecord)
-  | PgNodeBoolean(nodeBoolean)
-  | PgNodeFloat(nodeFloat)
-  | PgNodeCall(nodeCall)
-  | PgNodeIdentifier(nodeIdentifier)
-  | PgNodeInteger(nodeInteger)
-  | PgNodeKeyValue(nodeKeyValue)
-  | PgNodeLambda(nodeLambda)
-  | PgNodeLetStatement(nodeLetStatement)
-  | PgNodeModuleIdentifier(nodeModuleIdentifier)
-  | PgNodeString(nodeString)
-  | PgNodeTernary(nodeTernary)
-  // | PgNodeTypeIdentifier(nodeTypeIdentifier)
-  | PgNodeVoid(nodeVoid)
+type astContent =
+  | ASTBlock(nodeBlock)
+  | ASTProgram(nodeProgram)
+  | ASTArray(nodeArray)
+  | ASTRecord(nodeRecord)
+  | ASTBoolean(nodeBoolean)
+  | ASTFloat(nodeFloat)
+  | ASTCall(nodeCall)
+  | ASTIdentifier(nodeIdentifier)
+  | ASTInteger(nodeInteger)
+  | ASTKeyValue(nodeKeyValue)
+  | ASTLambda(nodeLambda)
+  | ASTLetStatement(nodeLetStatement)
+  | ASTModuleIdentifier(nodeModuleIdentifier)
+  | ASTString(nodeString)
+  | ASTTernary(nodeTernary)
+  | ASTVoid(nodeVoid)
+
+type ast = {
+  location: location,
+  content: astContent,
+}
 
 external castNodeBlock: node => nodeBlock = "%identity"
 external castNodeProgram: node => nodeProgram = "%identity"
@@ -71,80 +75,87 @@ external castNodeLetStatement: node => nodeLetStatement = "%identity"
 external castNodeModuleIdentifier: node => nodeModuleIdentifier = "%identity"
 external castNodeString: node => nodeString = "%identity"
 external castNodeTernary: node => nodeTernary = "%identity"
-// external castNodeTypeIdentifier: node => nodeTypeIdentifier = "%identity"
 external castNodeVoid: node => nodeVoid = "%identity"
 
 exception UnsupportedPeggyNodeType(string) // This should never happen; programming error
-let castNodeType = (node: node) =>
-  switch node["type"] {
-  | "Block" => node->castNodeBlock->PgNodeBlock
-  | "Program" => node->castNodeBlock->PgNodeProgram
-  | "Array" => node->castNodeArray->PgNodeArray
-  | "Record" => node->castNodeRecord->PgNodeRecord
-  | "Boolean" => node->castNodeBoolean->PgNodeBoolean
-  | "Call" => node->castNodeCall->PgNodeCall
-  | "Float" => node->castNodeFloat->PgNodeFloat
-  | "Identifier" => node->castNodeIdentifier->PgNodeIdentifier
-  | "Integer" => node->castNodeInteger->PgNodeInteger
-  | "KeyValue" => node->castNodeKeyValue->PgNodeKeyValue
-  | "Lambda" => node->castNodeLambda->PgNodeLambda
-  | "LetStatement" => node->castNodeLetStatement->PgNodeLetStatement
-  | "ModuleIdentifier" => node->castNodeModuleIdentifier->PgNodeModuleIdentifier
-  | "String" => node->castNodeString->PgNodeString
-  | "Ternary" => node->castNodeTernary->PgNodeTernary
-  // | "TypeIdentifier" => node->castNodeTypeIdentifier->PgNodeTypeIdentifier
-  | "Void" => node->castNodeVoid->PgNodeVoid
+let nodeToAST = (node: node) => {
+  let content = switch node["type"] {
+  | "Block" => node->castNodeBlock->ASTBlock
+  | "Program" => node->castNodeBlock->ASTProgram
+  | "Array" => node->castNodeArray->ASTArray
+  | "Record" => node->castNodeRecord->ASTRecord
+  | "Boolean" => node->castNodeBoolean->ASTBoolean
+  | "Call" => node->castNodeCall->ASTCall
+  | "Float" => node->castNodeFloat->ASTFloat
+  | "Identifier" => node->castNodeIdentifier->ASTIdentifier
+  | "Integer" => node->castNodeInteger->ASTInteger
+  | "KeyValue" => node->castNodeKeyValue->ASTKeyValue
+  | "Lambda" => node->castNodeLambda->ASTLambda
+  | "LetStatement" => node->castNodeLetStatement->ASTLetStatement
+  | "ModuleIdentifier" => node->castNodeModuleIdentifier->ASTModuleIdentifier
+  | "String" => node->castNodeString->ASTString
+  | "Ternary" => node->castNodeTernary->ASTTernary
+  | "Void" => node->castNodeVoid->ASTVoid
   | _ => raise(UnsupportedPeggyNodeType(node["type"]))
   }
 
-let rec pgToString = (peggyNode: peggyNode): string => {
+  {location: node["location"], content: content}
+}
+
+let nodeIdentifierToAST = (node: nodeIdentifier) => {
+  {location: node["location"], content: node->ASTIdentifier}
+}
+
+let nodeKeyValueToAST = (node: nodeKeyValue) => {
+  {location: node["location"], content: node->ASTKeyValue}
+}
+
+let rec pgToString = (ast: ast): string => {
   let argsToString = (args: array<nodeIdentifier>): string =>
-    args->Js.Array2.map(arg => PgNodeIdentifier(arg)->pgToString)->Js.Array2.toString
+    args->Belt.Array.map(arg => arg->nodeIdentifierToAST->pgToString)->Js.Array2.toString
 
   let nodesToStringUsingSeparator = (nodes: array<node>, separator: string): string =>
-    nodes->Js.Array2.map(toString)->Extra.Array.intersperse(separator)->Js.String.concatMany("")
+    nodes->Belt.Array.map(toString)->Extra.Array.intersperse(separator)->Js.String.concatMany("")
 
-  let pgNodesToStringUsingSeparator = (nodes: array<peggyNode>, separator: string): string =>
-    nodes->Js.Array2.map(pgToString)->Extra.Array.intersperse(separator)->Js.String.concatMany("")
+  let pgNodesToStringUsingSeparator = (nodes: array<ast>, separator: string): string =>
+    nodes->Belt.Array.map(pgToString)->Extra.Array.intersperse(separator)->Js.String.concatMany("")
 
-  switch peggyNode {
-  | PgNodeBlock(node)
-  | PgNodeProgram(node) =>
+  switch ast.content {
+  | ASTBlock(node)
+  | ASTProgram(node) =>
     "{" ++ node["statements"]->nodesToStringUsingSeparator("; ") ++ "}"
-  | PgNodeArray(node) => "[" ++ node["elements"]->nodesToStringUsingSeparator("; ") ++ "]"
-  | PgNodeRecord(node) =>
+  | ASTArray(node) => "[" ++ node["elements"]->nodesToStringUsingSeparator("; ") ++ "]"
+  | ASTRecord(node) =>
     "{" ++
     node["elements"]
-    ->Js.Array2.map(element => PgNodeKeyValue(element))
+    ->Belt.Array.map(element => element->nodeKeyValueToAST)
     ->pgNodesToStringUsingSeparator(", ") ++ "}"
-  | PgNodeBoolean(node) => node["value"]->Js.String.make
-  | PgNodeCall(node) =>
+  | ASTBoolean(node) => node["value"]->Js.String.make
+  | ASTCall(node) =>
     "(" ++ node["fn"]->toString ++ " " ++ node["args"]->nodesToStringUsingSeparator(" ") ++ ")"
-  | PgNodeFloat(node) => node["value"]->Js.String.make
-  | PgNodeIdentifier(node) => `:${node["value"]}`
-  | PgNodeInteger(node) => node["value"]->Js.String.make
-  | PgNodeKeyValue(node) => toString(node["key"]) ++ ": " ++ toString(node["value"])
-  | PgNodeLambda(node) =>
-    "{|" ++ node["args"]->argsToString ++ "| " ++ node["body"]->toString ++ "}"
-  | PgNodeLetStatement(node) =>
-    pgToString(PgNodeIdentifier(node["variable"])) ++ " = " ++ toString(node["value"])
-  | PgNodeModuleIdentifier(node) => `@${node["value"]}`
-  | PgNodeString(node) => `'${node["value"]->Js.String.make}'`
-  | PgNodeTernary(node) =>
+  | ASTFloat(node) => node["value"]->Js.String.make
+  | ASTIdentifier(node) => `:${node["value"]}`
+  | ASTInteger(node) => node["value"]->Js.String.make
+  | ASTKeyValue(node) => toString(node["key"]) ++ ": " ++ toString(node["value"])
+  | ASTLambda(node) => "{|" ++ node["args"]->argsToString ++ "| " ++ node["body"]->toString ++ "}"
+  | ASTLetStatement(node) =>
+    pgToString(node["variable"]->nodeIdentifierToAST) ++ " = " ++ toString(node["value"])
+  | ASTModuleIdentifier(node) => `@${node["value"]}`
+  | ASTString(node) => `'${node["value"]->Js.String.make}'`
+  | ASTTernary(node) =>
     "(::$$_ternary_$$ " ++
     toString(node["condition"]) ++
     " " ++
     toString(node["trueExpression"]) ++
     " " ++
     toString(node["falseExpression"]) ++ ")"
-  // | PgNodeTypeIdentifier(node) => `#${node["value"]}`
-  | PgNodeVoid(_node) => "()"
+  | ASTVoid(_node) => "()"
   }
 }
-and toString = (node: node): string => node->castNodeType->pgToString
+and toString = (node: node): string => node->nodeToAST->pgToString
 
 let toStringResult = (rNode: result<node, errorValue>): string =>
   switch rNode {
   | Ok(node) => toString(node)
-  | Error(error) => `Error(${errorToString(error)})`
+  | Error(error) => `Error(${errorValueToString(error)})`
   }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression.res
index ef8ed75b..6c187476 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/Reducer_Peggy_ToExpression.res
@@ -3,62 +3,72 @@ module ExpressionT = Reducer_Expression_T
 module Parse = Reducer_Peggy_Parse
 
 type expression = Reducer_T.expression
+type expressionContent = Reducer_T.expressionContent
 
 let rec fromNode = (node: Parse.node): expression => {
-  let caseBlock = nodeBlock =>
-    ExpressionBuilder.eBlock(nodeBlock["statements"]->Js.Array2.map(fromNode))
+  let ast = Parse.nodeToAST(node)
 
-  let caseProgram = nodeProgram =>
-    ExpressionBuilder.eProgram(nodeProgram["statements"]->Js.Array2.map(fromNode))
+  let content: expressionContent = {
+    let caseBlock = nodeBlock =>
+      ExpressionBuilder.eBlock(nodeBlock["statements"]->Js.Array2.map(fromNode))
 
-  let caseLambda = (nodeLambda: Parse.nodeLambda): expression => {
-    let args =
-      nodeLambda["args"]->Js.Array2.map((argNode: Parse.nodeIdentifier) => argNode["value"])
-    let body = nodeLambda["body"]->fromNode
+    let caseProgram = nodeProgram =>
+      ExpressionBuilder.eProgram(nodeProgram["statements"]->Js.Array2.map(fromNode))
 
-    ExpressionBuilder.eLambda(args, body)
+    let caseLambda = (nodeLambda: Parse.nodeLambda): expressionContent => {
+      let args =
+        nodeLambda["args"]->Js.Array2.map((argNode: Parse.nodeIdentifier) => argNode["value"])
+      let body = nodeLambda["body"]->fromNode
+
+      ExpressionBuilder.eLambda(args, body)
+    }
+
+    let caseRecord = (nodeRecord): expressionContent => {
+      nodeRecord["elements"]
+      ->Js.Array2.map(keyValueNode => (
+        keyValueNode["key"]->fromNode,
+        keyValueNode["value"]->fromNode,
+      ))
+      ->ExpressionBuilder.eRecord
+    }
+
+    switch ast.content {
+    | ASTBlock(nodeBlock) => caseBlock(nodeBlock)
+    | ASTProgram(nodeProgram) => caseProgram(nodeProgram)
+    | ASTArray(nodeArray) =>
+      ExpressionBuilder.eArray(nodeArray["elements"]->Js.Array2.map(fromNode))
+    | ASTRecord(nodeRecord) => caseRecord(nodeRecord)
+    | ASTBoolean(nodeBoolean) => ExpressionBuilder.eBool(nodeBoolean["value"])
+    | ASTCall(nodeCall) =>
+      ExpressionBuilder.eCall(fromNode(nodeCall["fn"]), nodeCall["args"]->Js.Array2.map(fromNode))
+    | ASTFloat(nodeFloat) => ExpressionBuilder.eNumber(nodeFloat["value"])
+    | ASTIdentifier(nodeIdentifier) => ExpressionBuilder.eSymbol(nodeIdentifier["value"])
+    | ASTInteger(nodeInteger) => ExpressionBuilder.eNumber(Belt.Int.toFloat(nodeInteger["value"]))
+    | ASTKeyValue(nodeKeyValue) =>
+      ExpressionBuilder.eArray([fromNode(nodeKeyValue["key"]), fromNode(nodeKeyValue["value"])])
+    | ASTLambda(nodeLambda) => caseLambda(nodeLambda)
+    | ASTLetStatement(nodeLetStatement) =>
+      ExpressionBuilder.eLetStatement(
+        nodeLetStatement["variable"]["value"],
+        fromNode(nodeLetStatement["value"]),
+      )
+    | ASTModuleIdentifier(nodeModuleIdentifier) =>
+      ExpressionBuilder.eIdentifier(nodeModuleIdentifier["value"])
+    | ASTString(nodeString) => ExpressionBuilder.eString(nodeString["value"])
+    | ASTTernary(nodeTernary) =>
+      ExpressionBuilder.eTernary(
+        fromNode(nodeTernary["condition"]),
+        fromNode(nodeTernary["trueExpression"]),
+        fromNode(nodeTernary["falseExpression"]),
+      )
+    // | PgNodeTypeIdentifier(nodeTypeIdentifier) =>
+    //   ExpressionBuilder.eTypeIdentifier(nodeTypeIdentifier["value"])
+    | ASTVoid(_) => ExpressionBuilder.eVoid
+    }
   }
 
-  let caseRecord = (nodeRecord): expression => {
-    nodeRecord["elements"]
-    ->Js.Array2.map(keyValueNode => (
-      keyValueNode["key"]->fromNode,
-      keyValueNode["value"]->fromNode,
-    ))
-    ->ExpressionBuilder.eRecord
-  }
-
-  switch Parse.castNodeType(node) {
-  | PgNodeBlock(nodeBlock) => caseBlock(nodeBlock)
-  | PgNodeProgram(nodeProgram) => caseProgram(nodeProgram)
-  | PgNodeArray(nodeArray) =>
-    ExpressionBuilder.eArray(nodeArray["elements"]->Js.Array2.map(fromNode))
-  | PgNodeRecord(nodeRecord) => caseRecord(nodeRecord)
-  | PgNodeBoolean(nodeBoolean) => ExpressionBuilder.eBool(nodeBoolean["value"])
-  | PgNodeCall(nodeCall) =>
-    ExpressionBuilder.eCall(fromNode(nodeCall["fn"]), nodeCall["args"]->Js.Array2.map(fromNode))
-  | PgNodeFloat(nodeFloat) => ExpressionBuilder.eNumber(nodeFloat["value"])
-  | PgNodeIdentifier(nodeIdentifier) => ExpressionBuilder.eSymbol(nodeIdentifier["value"])
-  | PgNodeInteger(nodeInteger) => ExpressionBuilder.eNumber(Belt.Int.toFloat(nodeInteger["value"]))
-  | PgNodeKeyValue(nodeKeyValue) =>
-    ExpressionBuilder.eArray([fromNode(nodeKeyValue["key"]), fromNode(nodeKeyValue["value"])])
-  | PgNodeLambda(nodeLambda) => caseLambda(nodeLambda)
-  | PgNodeLetStatement(nodeLetStatement) =>
-    ExpressionBuilder.eLetStatement(
-      nodeLetStatement["variable"]["value"],
-      fromNode(nodeLetStatement["value"]),
-    )
-  | PgNodeModuleIdentifier(nodeModuleIdentifier) =>
-    ExpressionBuilder.eIdentifier(nodeModuleIdentifier["value"])
-  | PgNodeString(nodeString) => ExpressionBuilder.eString(nodeString["value"])
-  | PgNodeTernary(nodeTernary) =>
-    ExpressionBuilder.eTernary(
-      fromNode(nodeTernary["condition"]),
-      fromNode(nodeTernary["trueExpression"]),
-      fromNode(nodeTernary["falseExpression"]),
-    )
-  // | PgNodeTypeIdentifier(nodeTypeIdentifier) =>
-  //   ExpressionBuilder.eTypeIdentifier(nodeTypeIdentifier["value"])
-  | PgNodeVoid(_) => ExpressionBuilder.eVoid
+  {
+    ast: ast,
+    content: content,
   }
 }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/helpers.ts b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/helpers.ts
index f24fd819..6e9fe80c 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/helpers.ts
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Peggy/helpers.ts
@@ -34,83 +34,91 @@ export const postOperatorToFunction = {
   "[]": "$_atIndex_$",
 };
 
-type NodeBlock = {
+type Node = {
+  location: LocationRange;
+};
+
+type NodeBlock = Node & {
   type: "Block";
   statements: AnyPeggyNode[];
 };
 
-type NodeProgram = {
+type NodeProgram = Node & {
   type: "Program";
   statements: AnyPeggyNode[];
 };
 
-type NodeArray = {
+type NodeArray = Node & {
   type: "Array";
   elements: AnyPeggyNode[];
 };
 
-type NodeRecord = {
+type NodeRecord = Node & {
   type: "Record";
   elements: NodeKeyValue[];
 };
 
-type NodeCall = {
+type NodeCall = Node & {
   type: "Call";
   fn: AnyPeggyNode;
   args: AnyPeggyNode[];
 };
 
-type NodeFloat = {
+type NodeFloat = Node & {
   type: "Float";
   value: number;
 };
 
-type NodeInteger = {
+type NodeInteger = Node & {
   type: "Integer";
   value: number;
 };
 
-type NodeIdentifier = {
+type NodeIdentifier = Node & {
   type: "Identifier";
   value: string;
 };
 
-type NodeLetStatement = {
+type NodeLetStatement = Node & {
   type: "LetStatement";
   variable: NodeIdentifier;
   value: AnyPeggyNode;
 };
 
-type NodeLambda = {
+type NodeLambda = Node & {
   type: "Lambda";
   args: AnyPeggyNode[];
   body: AnyPeggyNode;
 };
 
-type NodeTernary = {
+type NodeTernary = Node & {
   type: "Ternary";
   condition: AnyPeggyNode;
   trueExpression: AnyPeggyNode;
   falseExpression: AnyPeggyNode;
 };
 
-type NodeKeyValue = {
+type NodeKeyValue = Node & {
   type: "KeyValue";
   key: AnyPeggyNode;
   value: AnyPeggyNode;
 };
 
-type NodeString = {
+type NodeString = Node & {
   type: "String";
   value: string;
   location?: LocationRange;
 };
 
-type NodeBoolean = {
+type NodeBoolean = Node & {
   type: "Boolean";
   value: boolean;
 };
 
+type NodeVoid = Node & {
+  type: "Void";
+};
+
 export type AnyPeggyNode =
   | NodeArray
   | NodeRecord
@@ -125,47 +133,78 @@ export type AnyPeggyNode =
   | NodeTernary
   | NodeKeyValue
   | NodeString
-  | NodeBoolean;
+  | NodeBoolean
+  | NodeVoid;
 
-export function makeFunctionCall(fn: string, args: AnyPeggyNode[]) {
+export function makeFunctionCall(
+  fn: string,
+  args: AnyPeggyNode[],
+  location: LocationRange
+) {
   if (fn === "$$_applyAll_$$") {
-    return nodeCall(args[0], args.splice(1));
+    return nodeCall(args[0], args.splice(1), location);
   } else {
-    return nodeCall(nodeIdentifier(fn), args);
+    return nodeCall(nodeIdentifier(fn, location), args, location);
   }
 }
 
-export function constructArray(elements: AnyPeggyNode[]) {
-  return { type: "Array", elements };
+export function constructArray(
+  elements: AnyPeggyNode[],
+  location: LocationRange
+): NodeArray {
+  return { type: "Array", elements, location };
 }
-export function constructRecord(elements: AnyPeggyNode[]) {
-  return { type: "Record", elements };
+export function constructRecord(
+  elements: NodeKeyValue[],
+  location: LocationRange
+): NodeRecord {
+  return { type: "Record", elements, location };
 }
 
-export function nodeBlock(statements: AnyPeggyNode[]): NodeBlock {
-  return { type: "Block", statements };
+export function nodeBlock(
+  statements: AnyPeggyNode[],
+  location: LocationRange
+): NodeBlock {
+  return { type: "Block", statements, location };
 }
-export function nodeProgram(statements: AnyPeggyNode[]): NodeProgram {
-  return { type: "Program", statements };
+export function nodeProgram(
+  statements: AnyPeggyNode[],
+  location: LocationRange
+): NodeProgram {
+  return { type: "Program", statements, location };
 }
-export function nodeBoolean(value: boolean): NodeBoolean {
-  return { type: "Boolean", value };
+export function nodeBoolean(
+  value: boolean,
+  location: LocationRange
+): NodeBoolean {
+  return { type: "Boolean", value, location };
 }
-export function nodeCall(fn: AnyPeggyNode, args: AnyPeggyNode[]): NodeCall {
-  return { type: "Call", fn, args };
+export function nodeCall(
+  fn: AnyPeggyNode,
+  args: AnyPeggyNode[],
+  location: LocationRange
+): NodeCall {
+  return { type: "Call", fn, args, location };
 }
-export function nodeFloat(value: number): NodeFloat {
-  return { type: "Float", value };
+export function nodeFloat(value: number, location: LocationRange): NodeFloat {
+  return { type: "Float", value, location };
 }
-export function nodeIdentifier(value: string): NodeIdentifier {
-  return { type: "Identifier", value };
+export function nodeIdentifier(
+  value: string,
+  location: LocationRange
+): NodeIdentifier {
+  return { type: "Identifier", value, location };
 }
-export function nodeInteger(value: number): NodeInteger {
-  return { type: "Integer", value };
+export function nodeInteger(
+  value: number,
+  location: LocationRange
+): NodeInteger {
+  return { type: "Integer", value, location };
 }
 export function nodeKeyValue(
   key: AnyPeggyNode,
-  value: AnyPeggyNode
+  value: AnyPeggyNode,
+  location: LocationRange
 ): NodeKeyValue {
   if (key.type === "Identifier") {
     key = {
@@ -173,43 +212,43 @@ export function nodeKeyValue(
       type: "String",
     };
   }
-  return { type: "KeyValue", key, value };
+  return { type: "KeyValue", key, value, location };
 }
 export function nodeLambda(
   args: AnyPeggyNode[],
-  body: AnyPeggyNode
+  body: AnyPeggyNode,
+  location: LocationRange
 ): NodeLambda {
-  return { type: "Lambda", args, body };
+  return { type: "Lambda", args, body, location };
 }
 export function nodeLetStatement(
   variable: NodeIdentifier,
-  value: AnyPeggyNode
+  value: AnyPeggyNode,
+  location: LocationRange
 ): NodeLetStatement {
-  return { type: "LetStatement", variable, value };
+  return { type: "LetStatement", variable, value, location };
 }
-export function nodeModuleIdentifier(value: string) {
-  return { type: "ModuleIdentifier", value };
+export function nodeModuleIdentifier(value: string, location: LocationRange) {
+  return { type: "ModuleIdentifier", value, location };
 }
-export function nodeString(value: string): NodeString {
-  return { type: "String", value };
+export function nodeString(value: string, location: LocationRange): NodeString {
+  return { type: "String", value, location };
 }
 export function nodeTernary(
   condition: AnyPeggyNode,
   trueExpression: AnyPeggyNode,
-  falseExpression: AnyPeggyNode
+  falseExpression: AnyPeggyNode,
+  location: LocationRange
 ): NodeTernary {
   return {
     type: "Ternary",
     condition,
     trueExpression,
     falseExpression,
+    location,
   };
 }
 
-export function nodeTypeIdentifier(typeValue: string) {
-  return { type: "TypeIdentifier", value: typeValue };
-}
-
-export function nodeVoid() {
-  return { type: "Void" };
+export function nodeVoid(location: LocationRange): NodeVoid {
+  return { type: "Void", location };
 }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
index 85546905..2a704b23 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
@@ -22,7 +22,7 @@ and lambdaValue = {
   body: lambdaBody,
 }
 @genType.opaque and lambdaDeclaration = Declaration.declaration<lambdaValue>
-and expression =
+and expressionContent =
   | EBlock(array<expression>)
   // programs are similar to blocks, but don't create an inner scope. there can be only one program at the top level of the expression.
   | EProgram(array<expression>)
@@ -35,6 +35,11 @@ and expression =
   | ELambda(array<string>, expression)
   | EValue(value)
 
+and expression = {
+  ast: Reducer_Peggy_Parse.ast,
+  content: expressionContent,
+}
+
 and namespace = Belt.Map.String.t<value>
 and bindings = {
   namespace: namespace,
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Value.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Value.res
index cd1eefef..f7681ee7 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Value.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Value.res
@@ -79,24 +79,12 @@ let toStringResult = x =>
   | Error(m) => `Error(${ErrorValue.errorToString(m)})`
   }
 
-let toStringOptionResult = x =>
-  switch x {
-  | Some(a) => toStringResult(a)
-  | None => "None"
-  }
-
-let toStringResultOkless = (codeResult: result<t, ErrorValue.errorValue>): string =>
+let toStringResultOkless = (codeResult: result<t, ErrorValue.error>): string =>
   switch codeResult {
   | Ok(a) => toString(a)
   | Error(m) => `Error(${ErrorValue.errorToString(m)})`
   }
 
-let toStringResultRecord = x =>
-  switch x {
-  | Ok(a) => `Ok(${toStringMap(a)})`
-  | Error(m) => `Error(${ErrorValue.errorToString(m)})`
-  }
-
 type internalExpressionValueType =
   | EvtArray
   | EvtBool
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
index 1942a136..082b620c 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
@@ -117,7 +117,7 @@ let getResultOption = (project: t, sourceId: string): ProjectItem.T.resultType =
 
 let getResult = (project: t, sourceId: string): ProjectItem.T.resultArgumentType =>
   switch getResultOption(project, sourceId) {
-  | None => RENeedToRun->Error
+  | None => RENeedToRun->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue->Error
   | Some(result) => result
   }
 
@@ -171,7 +171,7 @@ let linkDependencies = (project: t, sourceId: string): Reducer_T.namespace => {
             "__result__",
             switch project->getResult(id) {
             | Ok(result) => result
-            | Error(error) => error->Reducer_ErrorValue.ErrorException->raise
+            | Error(error) => error->Reducer_ErrorValue.ExceptionWithStackTrace->raise
             },
           ),
         ])
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
index 93d434a3..4313f6e1 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
@@ -4,8 +4,9 @@ module T = ReducerProject_ProjectItem_T
 type projectItem = T.projectItem
 type t = T.t
 
-let emptyItem: projectItem = {
+let emptyItem = (sourceId: string): projectItem => {
   source: "",
+  sourceId: sourceId,
   rawParse: None,
   expression: None,
   continuation: Reducer_Namespace.make(),
@@ -18,6 +19,7 @@ let emptyItem: projectItem = {
 // source -> rawParse -> includes -> expression -> continuation -> result
 
 let getSource = (r: t): T.sourceType => r.source
+let getSourceId = (r: t): T.sourceType => r.sourceId
 let getRawParse = (r: t): T.rawParseType => r.rawParse
 let getExpression = (r: t): T.expressionType => r.expression
 let getContinuation = (r: t): T.continuationArgumentType => r.continuation
@@ -29,7 +31,7 @@ let getDirectIncludes = (r: t): array<string> => r.directIncludes
 let getIncludesAsVariables = (r: t): T.importAsVariablesType => r.includeAsVariables
 
 let touchSource = (this: t): t => {
-  let r = emptyItem
+  let r = emptyItem(this->getSourceId)
   {
     ...r,
     source: getSource(this),
@@ -41,8 +43,9 @@ let touchSource = (this: t): t => {
 }
 
 let touchRawParse = (this: t): t => {
+  let r = emptyItem(this->getSourceId)
   {
-    ...emptyItem,
+    ...r,
     source: getSource(this),
     continues: getContinues(this),
     includes: getIncludes(this),
@@ -148,7 +151,8 @@ let parseIncludes = (this: t): t => {
     }
   }
 }
-let doRawParse = (this: t): T.rawParseArgumentType => this->getSource->Reducer_Peggy_Parse.parse
+let doRawParse = (this: t): T.rawParseArgumentType =>
+  this->getSource->Reducer_Peggy_Parse.parse(this.sourceId)
 
 let rawParse = (this: t): t =>
   this->getRawParse->E.O2.defaultFn(() => doRawParse(this))->setRawParse(this, _)
@@ -167,7 +171,7 @@ let buildExpression = (this: t): t => {
   }
 }
 
-let failRun = (this: t, e: Reducer_ErrorValue.errorValue): t =>
+let failRun = (this: t, e: Reducer_ErrorValue.error): t =>
   this->setResult(e->Error)->setContinuation(Reducer_Namespace.make())
 
 let doRun = (this: t, context: Reducer_T.context): t =>
@@ -181,12 +185,24 @@ let doRun = (this: t, context: Reducer_T.context): t =>
         ->setResult(result->Ok)
         ->setContinuation(contextAfterEvaluation.bindings->Reducer_Bindings.locals)
       } catch {
-      | Reducer_ErrorValue.ErrorException(e) => this->failRun(e)
-      | _ => this->failRun(RETodo("unhandled rescript exception"))
+      | Reducer_ErrorValue.ErrorException(e) =>
+        this->failRun(e->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue)
+      | Reducer_ErrorValue.ExceptionWithStackTrace(e) => this->failRun(e)
+      | _ =>
+        this->failRun(
+          RETodo(
+            "unhandled rescript exception",
+          )->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue,
+        )
       }
-    | Error(e) => this->failRun(e)
+    | Error(e) => this->failRun(e->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue)
     }
-  | None => this->failRun(RETodo("attempt to run without expression"))
+  | None =>
+    this->failRun(
+      RETodo(
+        "attempt to run without expression",
+      )->Reducer_ErrorValue.attachEmptyStackTraceToErrorValue,
+    )
   }
 
 let run = (this: t, context: Reducer_T.context): t => {
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res
index bf1cbfcb..5d32a161 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res
@@ -11,7 +11,7 @@ type expressionType = option<expressionArgumentType>
 type continuationArgumentType = Reducer_T.namespace
 type continuationType = option<continuationArgumentType>
 type continuationResultType = option<result<continuationArgumentType, errorValue>>
-type resultArgumentType = result<Reducer_T.value, errorValue>
+type resultArgumentType = result<Reducer_T.value, error>
 type resultType = option<resultArgumentType>
 type continuesArgumentType = array<string>
 type continuesType = array<string>
@@ -21,6 +21,7 @@ type importAsVariablesType = array<(string, string)>
 
 type projectItem = {
   source: sourceType,
+  sourceId: string,
   rawParse: rawParseType,
   expression: expressionType,
   continuation: continuationArgumentType,
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
index 0eac623a..bb0ae2ee 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
@@ -13,4 +13,4 @@ type t = project
 let getSourceIds = (project: t): array<string> => Belt.MutableMap.String.keysToArray(project.items)
 
 let getItem = (project: t, sourceId: string) =>
-  Belt.MutableMap.String.getWithDefault(project.items, sourceId, ProjectItem.emptyItem)
+  Belt.MutableMap.String.getWithDefault(project.items, sourceId, ProjectItem.emptyItem(sourceId))