diff --git a/packages/squiggle-lang/__tests__/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res b/packages/squiggle-lang/__tests__/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res
index 0b4ab55a..535f9e56 100644
--- a/packages/squiggle-lang/__tests__/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res
+++ b/packages/squiggle-lang/__tests__/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn_test.res
@@ -29,3 +29,9 @@ describe("builtin exception", () => {
     expectEvalToBe("testZadanga(1)", "Error(JS Exception: Error: Undefined function testZadanga)")
   )
 })
+
+Skip.describe("error reporting from collection functions", () => {
+  testEval("arr = [normal(3,2)]; map(arr, zarathsuzaWasHere)", "") 
+  // FIXME: returns "Error(Function not found: map(Array,Symbol))"
+  // Actually this error is correct but not informative
+})
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res
index 203d2c37..72d599ca 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_Dispatch/Reducer_Dispatch_BuiltIn.res
@@ -149,7 +149,8 @@ let callInternal = (call: functionCall, environment, reducer: ExpressionT.reduce
   | ("reduceReverse", [EvArray(aValueArray), initialValue, EvLambda(aLambdaValue)]) =>
     doReduceReverseArray(aValueArray, initialValue, aLambdaValue)
   | ("reverse", [EvArray(aValueArray)]) => aValueArray->Belt.Array.reverse->EvArray->Ok
-  | call => callMathJs(call)
+  | (_, [EvBool(_)])  | (_, [EvNumber(_)]) | (_, [EvString(_)]) | (_, [EvBool(_), EvBool(_)])  | (_, [EvNumber(_), EvNumber(_)]) | (_, [EvString(_), EvString(_)]) => callMathJs(call)
+  | call => Error(REFunctionNotFound(call->functionCallToCallSignature->functionCallSignatureToString)) // Report full type signature as error
   }
 }
 
diff --git a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
index d36ca5c4..fc2f86f1 100644
--- a/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
+++ b/packages/squiggle-lang/src/rescript/Reducer/Reducer_ErrorValue.res
@@ -7,6 +7,7 @@ type errorValue =
   | REOperationError(Operation.operationError)
   | REExpressionExpected
   | REFunctionExpected(string)
+  | REFunctionNotFound(string)
   | REJavaScriptExn(option<string>, option<string>) // Javascript Exception
   | REMacroNotFound(string)
   | RENotAFunction(string)
@@ -29,6 +30,7 @@ let errorToString = err =>
   | REAssignmentExpected => "Assignment expected"
   | REExpressionExpected => "Expression expected"
   | REFunctionExpected(msg) => `Function expected: ${msg}`
+  | REFunctionNotFound(msg) => `Function not found: ${msg}`
   | REDistributionError(err) => `Distribution Math Error: ${DistributionTypes.Error.toString(err)}`
   | REOperationError(err) => `Math Error: ${Operation.Error.toString(err)}`
   | REJavaScriptExn(omsg, oname) => {
diff --git a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
index 730cac2a..958b99df 100644
--- a/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
+++ b/packages/squiggle-lang/src/rescript/ReducerInterface/ReducerInterface_ExpressionValue.res
@@ -104,3 +104,52 @@ type environment = DistributionOperation.env
 
 @genType
 let defaultEnvironment: environment = DistributionOperation.defaultEnv
+
+type expresionValueType =
+  | EvtArray
+  | EvtArrayString
+  | EvtBool
+  | EvtCall
+  | EvtDistribution
+  | EvtLambda
+  | EvtNumber
+  | EvtRecord
+  | EvtString
+  | EvtSymbol
+
+type functionCallSignature = CallSignature(string, array<expresionValueType>)
+type functionDefinitionSignature = FunctionDefinitionSignature(functionCallSignature, expresionValueType)
+
+let valueToValueType = (value) => switch value {
+  | EvArray(_) => EvtArray
+  | EvArrayString(_) => EvtArray
+  | EvBool(_) => EvtBool
+  | EvCall(_) => EvtCall
+  | EvDistribution(_) => EvtDistribution
+  | EvLambda(_) => EvtLambda
+  | EvNumber(_) => EvtNumber
+  | EvRecord(_) => EvtRecord
+  | EvString(_) => EvtArray
+  | EvSymbol(_) => EvtSymbol}
+
+let functionCallToCallSignature = (functionCall: functionCall): functionCallSignature => {
+  let (fn, args) = functionCall
+  CallSignature(fn, args->Js.Array2.map(valueToValueType))}
+
+let valueTypeToString = (valueType: expresionValueType): string =>
+  switch valueType {
+  | EvtArray => `Array`
+  | EvtArrayString => `ArrayString`
+  | EvtBool => `Bool`
+  | EvtCall => `Call`
+  | EvtDistribution => `Distribution`
+  | EvtLambda => `Lambda`
+  | EvtNumber => `Number`
+  | EvtRecord => `Record`
+  | EvtString => `String`
+  | EvtSymbol => `Symbol`
+  }
+
+let functionCallSignatureToString = (functionCallSignature: functionCallSignature): string => {
+  let CallSignature(fn, args) = functionCallSignature
+  `${fn}(${args->Js.Array2.map(valueTypeToString)->Js.Array2.toString})`}
\ No newline at end of file