diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
index d2dbafb3..5f545f9d 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_includes_test.res
@@ -27,7 +27,7 @@ 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.error->Reducer_ErrorValue.errorToString)
     }
   })
   let internalProject = project->Project.T.Private.castToInternalProject
@@ -63,7 +63,7 @@ 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.error->Reducer_ErrorValue.errorToString)
     }
   })
 
@@ -106,7 +106,7 @@ 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.error->Reducer_ErrorValue.errorToString)
     }
   })
   let internalProject = project->Project.T.Private.castToInternalProject
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_test.res
index 1dd215b9..f8e48aa5 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_test.res
@@ -11,7 +11,7 @@ open Expect.Operators
 
 let runFetchResult = (project, sourceId) => {
   Project.run(project, sourceId)
-  Project.getResult(project, sourceId)->InternalExpressionValue.toStringResult
+  Project.getResult(project, sourceId)->InternalExpressionValue.toStringWithSourceResult
 }
 
 let runFetchFlatBindings = (project, sourceId) => {
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_1_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_1_test.res
index 92a4932f..4e715969 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_1_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_1_test.res
@@ -50,7 +50,7 @@ Case "Running a single source".
 
       /* Let's display the result and bindings */
       (
-        result->InternalExpressionValue.toStringResult,
+        result->InternalExpressionValue.toStringWithSourceResult,
         bindings->InternalExpressionValue.toStringBindings,
       )->expect == ("Ok(3)", "@{}")
       /* You've got 3 with empty bindings. */
@@ -64,7 +64,7 @@ Case "Running a single source".
       let bindings = Project.getBindings(project, "main")->Bindings.removeResult
       /* Now you have external bindings and external result. */
       (
-        result->InternalExpressionValue.toStringResult,
+        result->InternalExpressionValue.toStringWithSourceResult,
         bindings->InternalExpressionValue.IEvBindings->InternalExpressionValue.toString,
       )->expect == ("Ok(3)", "@{}")
     })
@@ -80,7 +80,7 @@ Case "Running a single source".
       Project.runAll(project)
       let result = Project.getResult(project, "main")
       let _bindings = Project.getBindings(project, "main")
-      result->InternalExpressionValue.toStringResult->expect == "Ok(3)"
+      result->InternalExpressionValue.toStringWithSourceResult->expect == "Ok(3)"
     })
 
     test("shortcut", () => {
@@ -88,7 +88,7 @@ Case "Running a single source".
       /* Examples above was to prepare you for the multi source tutorial. */
       let (result, bindings) = Project.evaluate("1+2")
       (
-        result->InternalExpressionValue.toStringResult,
+        result->InternalExpressionValue.toStringWithSourceResult,
         bindings->Bindings.removeResult->InternalExpressionValue.toStringBindings,
       )->expect == ("Ok(3)", "@{}")
     })
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_2_multisource_test.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_2_multisource_test.res
index 80c73c9e..85e3b1a8 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_2_multisource_test.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_2_multisource_test.res
@@ -32,7 +32,7 @@ describe("ReducerProject Tutorial", () => {
       let bindings3 = Project.getBindings(project, "source3")->Bindings.removeResult
 
       (
-        result3->InternalExpressionValue.toStringResult,
+        result3->InternalExpressionValue.toStringWithSourceResult,
         bindings3->InternalExpressionValue.toStringBindings,
       )->expect == ("Ok(())", "@{x: 1,y: 2,z: 3}")
     })
@@ -58,7 +58,7 @@ describe("ReducerProject Tutorial", () => {
       let bindings3 = Project.getBindings(project, "source3")->Bindings.removeResult
 
       (
-        result3->InternalExpressionValue.toStringResult,
+        result3->InternalExpressionValue.toStringWithSourceResult,
         bindings3->InternalExpressionValue.toStringBindings,
       )->expect == ("Ok(())", "@{x: 1,y: 2,z: 3}")
     })
@@ -94,7 +94,7 @@ describe("ReducerProject Tutorial", () => {
       let bindings3 = Project.getBindings(project, "source3")->Bindings.removeResult
 
       (
-        result3->InternalExpressionValue.toStringResult,
+        result3->InternalExpressionValue.toStringWithSourceResult,
         bindings3->InternalExpressionValue.toStringBindings,
       )->expect == ("Ok(())", "@{x: 1,y: 2,z: 3}")
       /*
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 12c14468..3c4b4403 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
@@ -38,7 +38,7 @@ Here we will finally proceed to a real life scenario. */
       /* Parse includes has set the includes */
       switch Project.getIncludes(project, "main") {
       | Ok(includes) => includes->expect == ["common"]
-      | Error(err) => err->Reducer_ErrorValue.errorToString->fail
+      | Error(err) => err.error->Reducer_ErrorValue.errorToString->fail
       }
       /* If the includes cannot be parsed then you get a syntax error.
       Otherwise you get the includes.
@@ -87,7 +87,7 @@ Here we will finally proceed to a real life scenario. */
           let rIncludes = Project.getIncludes(project, sourceName)
           switch rIncludes {
           /* Maybe there is an include syntax error */
-          | Error(err) => err->Reducer_ErrorValue.errorToString->Js.Exn.raiseError
+          | Error(err) => err.error->Reducer_ErrorValue.errorToString->Js.Exn.raiseError
 
           | Ok(includes) =>
             Belt.Array.forEach(includes, newIncludeName => {
@@ -146,7 +146,7 @@ Here we will finally proceed to a real life scenario. */
       /* And see the result and bindings.. */
       test("recursive includes", () => {
         (
-          result->InternalExpressionValue.toStringResult,
+          result->InternalExpressionValue.toStringWithSourceResult,
           bindings->Bindings.removeResult->InternalExpressionValue.toStringBindings,
         )->expect == ("Ok(6)", "@{doubleX: 2,x: 1,y: 2,z: 3}")
         /* Everything as expected */
@@ -172,7 +172,7 @@ 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.error->Reducer_ErrorValue.errorToString->fail
       }
     })
   })
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_4_injecting_user_values.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_4_injecting_user_values.res
index 38d655c1..2465caf3 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_4_injecting_user_values.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_4_injecting_user_values.res
@@ -30,7 +30,7 @@ describe("ReducerProject Tutorial", () => {
     /* We can now run the project */
     Project.runAll(project)
     let result = Project.getResult(project, "main")
-    result->InternalExpressionValue.toStringResult->expect == "Ok(6)"
+    result->InternalExpressionValue.toStringWithSourceResult->expect == "Ok(6)"
   })
 })
 
diff --git a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_5_calling_functions.res b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_5_calling_functions.res
index 7e2471b5..c33ac869 100644
--- a/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_5_calling_functions.res
+++ b/packages/squiggle-lang/__tests__/ReducerProject/ReducerProject_tutorial_5_calling_functions.res
@@ -32,7 +32,7 @@ describe("ReducerProject Tutorial", () => {
 
   test("userResults", () => {
     let userResultsAsString = Belt.Array.map(userResults, aResult =>
-      aResult->InternalExpressionValue.toStringResult
+      aResult->InternalExpressionValue.toStringWithSourceResult
     )
     userResultsAsString->expect == ["Ok(2)", "Ok(4)", "Ok(6)", "Ok(8)", "Ok(10)"]
   })
diff --git a/packages/squiggle-lang/src/js/SqError.ts b/packages/squiggle-lang/src/js/SqError.ts
index 317fde4f..159900d0 100644
--- a/packages/squiggle-lang/src/js/SqError.ts
+++ b/packages/squiggle-lang/src/js/SqError.ts
@@ -1,17 +1,13 @@
 import * as RSErrorValue from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
 
 export class SqError {
-  constructor(private _value: RSErrorValue.reducerErrorValue) {}
+  constructor(private _value: RSErrorValue.reducerErrorValueWithSource) {}
 
   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));
+  getSource() {
+    return RSErrorValue.getSource(this._value);
   }
 }
diff --git a/packages/squiggle-lang/src/js/SqProject.ts b/packages/squiggle-lang/src/js/SqProject.ts
index 4c2dea7c..e85d5b32 100644
--- a/packages/squiggle-lang/src/js/SqProject.ts
+++ b/packages/squiggle-lang/src/js/SqProject.ts
@@ -1,5 +1,5 @@
 import * as RSProject from "../rescript/ForTS/ForTS_ReducerProject.gen";
-import { reducerErrorValue } from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
+import { reducerErrorValueWithSource } from "../rescript/ForTS/ForTS_Reducer_ErrorValue.gen";
 import { environment } from "../rescript/ForTS/ForTS_Distribution/ForTS_Distribution_Environment.gen";
 import { SqError } from "./SqError";
 import { SqModule } from "./SqModule";
@@ -50,7 +50,7 @@ export class SqProject {
     return resultMap2(
       RSProject.getIncludes(this._value, sourceId),
       (a) => a,
-      (v: reducerErrorValue) => new SqError(v)
+      (v: reducerErrorValueWithSource) => new SqError(v)
     );
   }
 
@@ -104,7 +104,7 @@ export class SqProject {
             items: [],
           })
         ),
-      (v: reducerErrorValue) => new SqError(v)
+      (v: reducerErrorValueWithSource) => new SqError(v)
     );
   }
 
diff --git a/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res b/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
index 18838899..e9a3892e 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS_ReducerProject.res
@@ -1,6 +1,7 @@
 @genType type reducerProject = ReducerProject_T.t //re-export
 
 type reducerErrorValue = ForTS_Reducer_ErrorValue.reducerErrorValue //use
+type reducerErrorValueWithSource = ForTS_Reducer_ErrorValue.reducerErrorValueWithSource //use
 
 type squiggleValue = ForTS_SquiggleValue.squiggleValue //use
 type squiggleValue_Module = ForTS_SquiggleValue_Module.squiggleValue_Module //use
@@ -102,7 +103,7 @@ To set the includes one first has to call "parseIncludes". The parsed includes o
 @genType
 let getIncludes = (project: reducerProject, sourceId: string): result<
   array<string>,
-  reducerErrorValue,
+  reducerErrorValueWithSource,
 > => project->T.Private.castToInternalProject->Private.getIncludes(sourceId)
 
 /* Other sources contributing to the global namespace of this source. */
@@ -200,7 +201,7 @@ Get the result after running this source file or the project
 @genType
 let getResult = (project: reducerProject, sourceId: string): result<
   squiggleValue,
-  reducerErrorValue,
+  reducerErrorValueWithSource,
 > => project->T.Private.castToInternalProject->Private.getResult(sourceId)
 
 /*
@@ -210,7 +211,7 @@ The source has to be include free
 */
 @genType
 let evaluate = (sourceCode: string): (
-  result<squiggleValue, reducerErrorValue>,
+  result<squiggleValue, reducerErrorValueWithSource>,
   squiggleValue_Module,
 ) => Private.evaluate(sourceCode)
 
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..ade7a50f 100644
--- a/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/ForTS/ForTS_Reducer_ErrorValue.res
@@ -1,12 +1,13 @@
 @genType type reducerErrorValue = Reducer_ErrorValue.errorValue //alias
+@genType type reducerErrorValueWithSource = Reducer_ErrorValue.errorValueWithSource //alias
 @genType type syntaxErrorLocation = Reducer_ErrorValue.syntaxErrorLocation //alias
 
 @genType
-let toString = (e: reducerErrorValue): string => Reducer_ErrorValue.errorToString(e)
+let toString = (e: reducerErrorValueWithSource): string => Reducer_ErrorValue.errorToString(e.error)
 
 @genType
-let getLocation = (e: reducerErrorValue): option<syntaxErrorLocation> =>
-  switch e {
+let getLocation = (e: reducerErrorValueWithSource): option<syntaxErrorLocation> =>
+  switch e.error {
   | RESyntaxError(_, optionalLocation) => optionalLocation
   | _ => None
   }
@@ -16,3 +17,6 @@ let createTodoError = (v: string) => Reducer_ErrorValue.RETodo(v)
 
 @genType
 let createOtherError = (v: string) => Reducer_ErrorValue.REOther(v)
+
+@genType
+let getErrorSource = (err: reducerErrorValueWithSource) => err.sourceId
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
index b6baeaf5..4caa474c 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
@@ -81,3 +81,11 @@ let fromException = exn =>
   }
 
 let toException = (errorValue: t) => raise(ErrorException(errorValue))
+
+@genType.opaque
+type errorValueWithSource = {error: errorValue, sourceId: string}
+
+let errorAddSource = (r: result<'a, errorValue>, sourceId: string): result<
+  'a,
+  errorValueWithSource,
+> => r->E.R2.errMap(err => {error: err, sourceId: sourceId})
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
index 5531cbdf..b5130b38 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_InternalExpressionValue.res
@@ -131,6 +131,12 @@ let toStringResult = x =>
   | Error(m) => `Error(${ErrorValue.errorToString(m)})`
   }
 
+let toStringWithSourceResult = (x: result<t, ErrorValue.errorValueWithSource>) =>
+  switch x {
+  | Ok(a) => `Ok(${toString(a)})`
+  | Error(m) => `Error(${ErrorValue.errorToString(m.error)})`
+  }
+
 let toStringOptionResult = x =>
   switch x {
   | Some(a) => toStringResult(a)
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
index 948c14c3..ea29ce4a 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject.res
@@ -129,7 +129,7 @@ module Private = {
 
   let getResult = (project: t, sourceId: string): ProjectItem.T.resultArgumentType =>
     switch getResultOption(project, sourceId) {
-    | None => RENeedToRun->Error
+    | None => RENeedToRun->Error->ErrorValue.errorAddSource(sourceId)
     | Some(result) => result
     }
 
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
index 7676ddef..608645d6 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem.res
@@ -6,12 +6,14 @@ module ExpressionT = Reducer_Expression_T
 module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 module ProjectAccessorsT = ReducerProject_ProjectAccessors_T
 module ReducerFnT = ReducerProject_ReducerFn_T
+module ErrorValue = Reducer_ErrorValue
 module T = ReducerProject_ProjectItem_T
 
 type projectItem = T.projectItem
 type t = T.t
 
-let emptyItem = T.ProjectItem({
+let emptyItem = (id: string) => T.ProjectItem({
+  id: id,
   source: "",
   rawParse: None,
   expression: None,
@@ -25,6 +27,7 @@ let emptyItem = T.ProjectItem({
 // source -> rawParse -> includes -> expression -> continuation -> result
 
 let getSource = (T.ProjectItem(r)): T.sourceType => r.source
+let getId = (T.ProjectItem(r)): T.sourceType => r.id
 let getRawParse = (T.ProjectItem(r)): T.rawParseType => r.rawParse
 let getExpression = (T.ProjectItem(r)): T.expressionType => r.expression
 let getContinuation = (T.ProjectItem(r)): T.continuationArgumentType => r.continuation
@@ -36,7 +39,7 @@ let getDirectIncludes = (T.ProjectItem(r)): array<string> => r.directIncludes
 let getIncludesAsVariables = (T.ProjectItem(r)): T.importAsVariablesType => r.includeAsVariables
 
 let touchSource = (this: t): t => {
-  let T.ProjectItem(r) = emptyItem
+  let T.ProjectItem(r) = emptyItem(getId(this))
   T.ProjectItem({
     ...r,
     source: getSource(this),
@@ -48,7 +51,7 @@ let touchSource = (this: t): t => {
 }
 
 let touchRawParse = (this: t): t => {
-  let T.ProjectItem(r) = emptyItem
+  let T.ProjectItem(r) = emptyItem(getId(this))
   T.ProjectItem({
     ...r,
     source: getSource(this),
@@ -61,7 +64,7 @@ let touchRawParse = (this: t): t => {
 }
 
 let touchExpression = (this: t): t => {
-  let T.ProjectItem(r) = emptyItem
+  let T.ProjectItem(r) = emptyItem(getId(this))
   T.ProjectItem({
     ...r,
     source: getSource(this),
@@ -104,7 +107,7 @@ let setResult = (T.ProjectItem(r): t, result: T.resultArgumentType): t => T.Proj
 let cleanResults = touchExpression
 
 let clean = (this: t): t => {
-  let T.ProjectItem(r) = emptyItem
+  let T.ProjectItem(r) = emptyItem(getId(this))
   T.ProjectItem({
     ...r,
     source: getSource(this),
@@ -143,7 +146,7 @@ let parseIncludes = (this: t): t => {
   let T.ProjectItem(r) = this
   let rRawImportAsVariables = getSource(this)->ReducerProject_ParseIncludes.parseIncludes
   switch rRawImportAsVariables {
-  | Error(e) => resetIncludes(this)->setIncludes(Error(e))
+  | Error(e) => resetIncludes(this)->setIncludes(Error(e)->ErrorValue.errorAddSource(getId(this)))
   | Ok(rawImportAsVariables) => {
       let includes = rawImportAsVariables->Belt.Array.map(((_variable, file)) => file)->Ok
       let includeAsVariables =
@@ -190,14 +193,16 @@ let doBuildResult = (
 ): T.resultType =>
   this
   ->getExpression
-  ->Belt.Option.map(
-    Belt.Result.flatMap(_, expression =>
+  ->Belt.Option.map(expressionResult =>
+    expressionResult
+    ->Belt.Result.flatMap(_, expression =>
       try {
         Reducer_Expression.reduceExpressionInProject(expression, aContinuation, accessors)->Ok
       } catch {
       | exn => Reducer_ErrorValue.fromException(exn)->Error
       }
-    ),
+    )
+    ->ErrorValue.errorAddSource(getId(this))
   )
 
 let buildResult = (this: t, aContinuation: T.continuation, accessors: ProjectAccessorsT.t): 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 caa80da4..2a8336ce 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_ProjectItem_T.res
@@ -4,6 +4,7 @@ module InternalExpressionValue = ReducerInterface_InternalExpressionValue
 open Reducer_ErrorValue
 
 type sourceArgumentType = string
+type idArgumentType = string
 type sourceType = string
 type rawParseArgumentType = result<Parse.node, errorValue>
 type rawParseType = option<rawParseArgumentType>
@@ -15,16 +16,17 @@ type continuationType = option<continuationArgumentType>
 type continuationResultType = option<result<continuationArgumentType, errorValue>>
 type bindingsArgumentType = InternalExpressionValue.nameSpace
 type bindingsType = option<bindingsArgumentType>
-type resultArgumentType = result<InternalExpressionValue.t, errorValue>
+type resultArgumentType = result<InternalExpressionValue.t, errorValueWithSource>
 type resultType = option<resultArgumentType>
 type continuesArgumentType = array<string>
 type continuesType = array<string>
 type includesArgumentType = string
-type includesType = result<array<string>, errorValue>
+type includesType = result<array<string>, errorValueWithSource>
 type importAsVariablesType = array<(string, string)>
 
 type projectItem =
   | ProjectItem({
+      id: string,
       source: sourceType,
       rawParse: rawParseType,
       expression: expressionType,
diff --git a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
index e1ec91f4..6ec15cb8 100644
--- a/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
+++ b/packages/squiggle-lang/src/rescript/ReducerProject/ReducerProject_T.res
@@ -33,5 +33,5 @@ module Private = {
   let getSourceIds = (this: t): array<string> => Belt.Map.String.keysToArray(this["items"])
 
   let getItem = (this: t, sourceId: string) =>
-    Belt.Map.String.getWithDefault(this["items"], sourceId, ProjectItem.emptyItem)
+    Belt.Map.String.getWithDefault(this["items"], sourceId, ProjectItem.emptyItem(sourceId))
 }