[erlang-questions] Line number in errors: badmatch, case clause, badarith

[Thomas Lindgren]

----- Original Message ----
> From: caio ariede <caio.ariede@REDACTED>
> To: Erlang-Questions Questions <erlang-questions@REDACTED>
> Sent: Tue, January 12, 2010 3:30:00 PM
> Subject: Re: [erlang-questions] Line number in errors: badmatch, case clause,  badarith
> 
> It works with this type of error too?
> 
> > test:foo().
> ** exception error: no function clause matching test:bar(1)
> 
>   1 -module(test).
>   2 -compile(export_all).
>   3
>   4 foo() ->
>   5     bar(1).
>   6
>   7 bar(0) -> 0.
> 
> There is a way to get what line (5) the error occurred?


Not quite, the parse transform rewrites bar/1 into something like this (the details may differ somewhat):

  bar(0) -> 0;
  bar(N) -> exit({function_clause, {?MODULE, ?LINE}, [N]}).

So you can see where the error occurred and what the bad value was. The ?LINE used is that of the first clause of bar/1. The same is done for case, if, funs and wherever possible. For builtin functions (including +) you get the equivalent. The transform is not perfect, however. It only operates on Erlang source code, so some places are impossible to instrument.

I guess if you use error/1 instead of exit/1, you will get a backtrace, but not line numbers for the calls in the stack. That would probably need some compiler support and a modification of how error/1 is implemented.

Best,
Thomas