diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_CallStack.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_CallStack.res
deleted file mode 100644
index 520ce6eb..00000000
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_CallStack.res
+++ /dev/null
@@ -1,36 +0,0 @@
-@genType.opaque
-type rec lambdaFrame = {location: Reducer_Peggy_Parse.location, name: string}
-@genType.opaque and ffiFrame = {name: string}
-@genType
-and frame =
-  | InLambda(lambdaFrame)
-  | InFFI(ffiFrame)
-
-let toStringFrame = (t: frame) =>
-  switch t {
-  | InLambda({location}) =>
-    `Line ${location.start.line->Js.Int.toString}, column ${location.start.column->Js.Int.toString}, source ${location.source}`
-  | InFFI({name}) => `Builtin ${name}`
-  }
-
-@genType.opaque
-type rec t = list<frame>
-
-let make = (): t => list{}
-
-let extend = (t: t, frame: frame) => t->Belt.List.add(frame)
-
-let toString = (t: t) =>
-  t->Belt.List.map(s => "  " ++ s->toStringFrame ++ "\n")->Belt.List.toArray->Js.Array2.joinWith("")
-
-@genType
-let toFrameArray = (t: t): array<frame> => t->Belt.List.toArray
-
-@genType
-let getTopFrame = (t: t): option<frame> => t->Belt.List.head
-
-let isEmpty = (t: t): bool =>
-  switch t->Belt.List.head {
-  | Some(_) => true
-  | None => false
-  }
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 030ca71d..f334335a 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
@@ -6,8 +6,11 @@ let toLocation = (expression: T.expression): SqError.location => {
   expression.ast.location
 }
 
-let throwFrom = (error: SqError.Message.t, context: T.context) =>
-  error->SqError.throwMessage(context)
+let throwFrom = (error: SqError.Message.t, expression: T.expression, context: T.context) =>
+  error->SqError.throwMessage(
+    context.callStack,
+    location: Some(expression->toLocation)
+  )
 
 /*
   Recursively evaluate the expression
@@ -53,7 +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")->throwFrom(context)
+          | _ => REOther("Record keys must be strings")->throwFrom(expression, context)
           }
           let (value, _) = eValue->evaluate(context)
           (keyString, value)
@@ -77,7 +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 => RESymbolNotFound(name)->throwFrom(context)
+    | None => RESymbolNotFound(name)->throwFrom(expression, context)
     }
 
   | T.EValue(value) => (value, context)
@@ -86,7 +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", "")->throwFrom(context)
+      | _ => REExpectedType("Boolean", "")->throwFrom(expression, context)
       }
     }
 
@@ -112,7 +115,7 @@ let rec evaluate: T.reducerFn = (expression, context): (T.value, T.context) => {
           let result = Reducer_Lambda.doLambdaCall(lambda, argValues, context, evaluate)
           (result, context)
         }
-      | _ => RENotAFunction(lambda->Reducer_Value.toString)->throwFrom(context)
+      | _ => RENotAFunction(lambda->Reducer_Value.toString)->throwFrom(expression, context)
       }
     }
   }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_FrameStack.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_FrameStack.res
new file mode 100644
index 00000000..217993f0
--- /dev/null
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_FrameStack.res
@@ -0,0 +1,40 @@
+type t = Reducer_T.frameStack
+
+module Frame = {
+  let toString = ({lambda, location}: Reducer_T.frame) =>
+    `${fromFrame} at ${location.start.line->Js.Int.toString}, column ${location.start.column->Js.Int.toString}`
+}
+
+let make = (): t => list{}
+
+let topFrameName = (t: t) =>
+  switch t->getTopFrame {
+  | Some({lambda}) =>
+    switch lambda {
+    | FnLambda({name}) => name
+    | FnBuiltin({name}) => name
+    }
+  | None => "<main>"
+  }
+
+let extend = (t: t, lambda: Reducer_T.lambdaValue, location: option<location>) =>
+  t->Belt.List.add({
+    lambda: lambda,
+    fromLocation: location,
+    fromFrame: t->topFrameName,
+  })
+
+let toString = (t: t) =>
+  t->Belt.List.map(s => "  " ++ s->toStringFrame ++ "\n")->Belt.List.toArray->Js.Array2.joinWith("")
+
+@genType
+let toFrameArray = (t: t): array<frame> => t->Belt.List.toArray
+
+@genType
+let getTopFrame = (t: t): option<frame> => t->Belt.List.head
+
+let isEmpty = (t: t): bool =>
+  switch t->Belt.List.head {
+  | Some(_) => true
+  | None => false
+  }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Lambda.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Lambda.res
index 81e30cfb..04adeea0 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Lambda.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Lambda.res
@@ -57,14 +57,6 @@ let makeFFILambda = (name: string, body: Reducer_T.lambdaBody): t => FnBuiltin({
   name: name,
 })
 
-let extendCallStack = (t: t, callStack: Reducer_CallStack.t): Reducer_CallStack.t => {
-  switch t {
-  | FnLambda({location}) =>
-    callStack->Reducer_CallStack.extend(InLambda({location: location, name: "TODO"})) // FIXME
-  | FnBuiltin({name}) => callStack->Reducer_CallStack.extend(InFFI({name: name}))
-  }
-}
-
 // this function doesn't scale to FunctionRegistry's polymorphic functions
 let parameters = (t: t): array<string> => {
   switch t {
@@ -76,13 +68,13 @@ let parameters = (t: t): array<string> => {
 let doLambdaCall = (t: t, args, context: Reducer_Context.t, reducer) => {
   let newContext = {
     ...context,
-    callStack: t->extendCallStack(context.callStack),
+    callStack: t->Reducer_CallStack.extend(t),
   }
 
-  SqError.contextualizeAndRethrow(() => {
+  SqError.rethrowWithStacktrace(() => {
     switch t {
     | FnLambda({body}) => body(args, newContext, reducer)
     | FnBuiltin({body}) => body(args, newContext, reducer)
     }
-  }, newContext)
+  }, newContext.callStack)
 }
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
index cb0edfd2..eab7bd55 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_T.res
@@ -50,10 +50,18 @@ and bindings = {
   parent: option<bindings>,
 }
 
+and frame = {
+  name: string,
+  location: option<Reducer_Peggy_Parse.location>, // can be empty for calls from builtin functions
+}
+
+and frameStack = list<frame>
+
 and context = {
   bindings: bindings,
   environment: environment,
-  callStack: Reducer_CallStack.t,
+  inFunction: option<string>, // used to build the next frame in frameStack
+  callStack: frameStack,
 }
 
 and reducerFn = (expression, context) => (value, context)
diff --git a/packages/squiggle-lang/src/rescript/SqError.res b/packages/squiggle-lang/src/rescript/SqError.res
index 3cf3dfe2..461e1feb 100644
--- a/packages/squiggle-lang/src/rescript/SqError.res
+++ b/packages/squiggle-lang/src/rescript/SqError.res
@@ -90,91 +90,68 @@ module Message = {
 @genType.opaque
 type t = {
   message: Message.t,
-  /*
-  Errors raised from internal functions can have empty location.
-
-  Also, location is not the same as the top of the stackTrace.
-  Consider this:
-  ```
-  f() = {
-    x = 5
-    y = z // no such var
-    x + y
-  }
-  ```
-  This code should report the location of assignment issue, but there's no function call there.
- */
-  location: option<location>,
-  stackTrace: Reducer_CallStack.t,
+  frameStack: Reducer_FrameStack.t,
 }
 
 exception SqException(t)
 
 // `context` should be specified for runtime errors, but can be left empty for errors from Reducer_Project and so on.
 // `location` can be empty for errors raised from FunctionRegistry.
-let fromMessage = (
-  message: Message.t,
-  context: option<Reducer_T.context>,
-  location: option<location>,
-): t => {
+let fromMessage = (message: Message.t, frameStack: Reducer_FrameStack.t): t => {
   message: message,
-  location: location,
-  stackTrace: switch context {
-  | Some(context) => context.callStack
-  | None => Reducer_CallStack.make()
-  },
+  stackTrace: stackTrace,
 }
 
+let fromMessageWithoutFrameStack = (message: Message.t) =>
+  fromMessage(message, Reducer_FrameStack.make())
+
 @genType
 let getTopFrame = (t: t): option<Reducer_CallStack.frame> =>
-  t.stackTrace->Reducer_CallStack.getTopFrame
+  t.stackTrace->Reducer_FrameStack.getTopFrame
 
 @genType
-let getStackTrace = (t: t): Reducer_CallStack.t => t.stackTrace
+let getFrameStack = (t: t): Reducer_FrameStack.t => t.frameStack
 
 @genType
 let toString = (t: t): string => t.message->Message.toString
 
 @genType
-let createOtherError = (v: string): t => Message.REOther(v)->fromMessage()
+let createOtherError = (v: string): t => Message.REOther(v)->fromMessageWithoutFrameStack
 
 @genType
 let getFrameArray = (t: t): array<Reducer_CallStack.frame> =>
   t.stackTrace->Reducer_CallStack.toFrameArray
 
 @genType
-let toStringWithStackTrace = (t: t) =>
-  if t.stackTrace->Reducer_CallStack.isEmpty {
-    "Traceback:\n" ++ t.stackTrace->Reducer_CallStack.toString
-  } else {
-    ""
-  } ++
-  t->toString
-
+let toStringWithStacktrace = (t: t) =>
+  t->toString ++ if t.frameStack->Reducer_FrameStack.isEmpty {
+      "Traceback:\n" ++ t.frameStack->Reducer_FrameStack.toString
+    }
 let throw = (t: t) => t->SqException->raise
 
-let throwMessage = (message: Message.t, context: Reducer_T.context, location: location) =>
-  fromMessage(message, context, location)->throw
+let throwMessage = (message: Message.t, frameStack: Reducer_FrameStack.t) =>
+  fromMessage(message, frameStack)->throw
 
 // this shouldn't be used for most runtime errors - the resulting error would have an empty stacktrace
 let fromException = exn =>
   switch exn {
   | SqException(e) => e
-  | Message.MessageException(e) => e->fromMessage
-  | Js.Exn.Error(obj) => REJavaScriptExn(obj->Js.Exn.message, obj->Js.Exn.name)->fromMessage
-  | _ => REOther("Unknown exception")->fromMessage
+  | Message.MessageException(e) => e->fromMessage(Reducer_CallStack.make())
+  | Js.Exn.Error(obj) =>
+    REJavaScriptExn(obj->Js.Exn.message, obj->Js.Exn.name)->fromMessageWithoutStacktrace
+  | _ => REOther("Unknown exception")->fromMessageWithoutStacktrace
   }
 
 // converts raw exceptions into exceptions with stacktrace attached
 // already converted exceptions won't be affected
-let contextualizeAndRethrow = (fn: unit => 'a, context: Reducer_T.context) => {
+let rethrowWithStacktrace = (fn: unit => 'a, stackTrace: Reducer_CallStack.t) => {
   try {
     fn()
   } catch {
-  | SqException(e) => e->throw
-  | Message.MessageException(e) => e->throwMessage(context)
+  | SqException(e) => e->throw // exception already has a stacktrace
+  | Message.MessageException(e) => e->throwMessage(stackTrace) // probably comes from FunctionRegistry, adding stacktrace
   | Js.Exn.Error(obj) =>
-    REJavaScriptExn(obj->Js.Exn.message, obj->Js.Exn.name)->throwMessage(context)
-  | _ => REOther("Unknown exception")->throwMessage(context)
+    REJavaScriptExn(obj->Js.Exn.message, obj->Js.Exn.name)->throwMessage(stackTrace)
+  | _ => REOther("Unknown exception")->throwMessage(stackTrace)
   }
 }