How to find line where error occurred in Mathematica notebook?

Question

I have a Mathematica file called myUsefulFunctions.m containing, for example, a function called mySuperUsefulFunction. Suppose I call mySuperUsefulFunction in a notebook and get

Answer 1

I don't know of a way to find the line in the file, which I assume was read without error.

You can however use Trace and related functions to see where in the evaluation chain the error occurs.

Example:

myFunc1[x_, y_] := x[[y]]
myFunc2[a_List, n_] := Pick[a, a, myFunc1[a, n]]

myFunc2[Range@10, #1]

During evaluation of In[4]:= Part::pspec: Part specification #1 is neither an integer nor a list of integers. >>

With Trace:

myFunc2[Range@10, #1] // Trace // Column

{Range[10], {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}}
myFunc2[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, #1]
Pick[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, myFunc1[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, #1]]
{myFunc1[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, #1], {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}[[#1]], {Message[Part::pspec, #1], {MakeBoxes[Part::pspec: Part specification #1 is neither an integer nor a list of integers. >>, StandardForm], RowBox[{RowBox[{Part, ::, "pspec"}], : , "\!\(\*StyleBox[\"\\\"Part specification \\\"\",  \"MT\"]\)\!\(\*StyleBox[\!\(#1\),  \"MT\"]\)\!\(\*StyleBox[\"\\\" is neither an integer nor a list of integers.\\\"\",  \"MT\"]\) \!\(\*ButtonBox[\">>\",  ButtonStyle->\"Link\",  ButtonFrame->None,  ButtonData:>\"paclet:ref/message/General/pspec\",  ButtonNote -> \"Part::pspec\"]\)"}]}, Null}, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}[[#1]]}

You can see that just before Message[Part::pspec, #1] is called, which results in a long mess of formatting, we had:

{myFunc1[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, #1], {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}[[#1]]

This shows that myFunc1[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, #1] is called, and this causes evaluation of {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}[[#1]] which is clearly in error.

Please see this question and its answers for a more convenient use of Trace:

https://stackoverflow.com/q/5459735/618728