[erlang-bugs] List comprehension generate "function clause" exception instead of "bad generator"

[Ivan Glushkov]

Hi.

We have found recently an interesting behaviour:
- erlang console generate "bad generator" exception for the code:
1> [ {A,B} || {A,B} <- aaa ].
** exception error: bad generator aaa

- if compiled, the code throw "function clause" exception:

$ cat a.erl
-module(a).
-export([a/1]).
a(List) -> [ {A,B} || {A,B} <- List ].

$ erl
1> c(a).
{ok,a}
2> a:a(aaa).
** exception error: no function clause matching
                    a:'-a/1-lc$^0/1-0-'(aaa)


The reason is that new anonymous function is created to handle the
list comprehension, but it expects only lists. So incorrect generator
also would cause a "function clause" exception.

I suppose that the reason of such an exception here is for the
similarity with the lists:map, it also generates a "function clause"
for unsupported arguments:

2> lists:map(fun(A) -> A end, aaa).
** exception error: no function clause matching
                    lists:map(#Fun<erl_eval.6.80247286>,aaa)



To my mind this behavior is not correct as somebody might expect "bad
generator" exception and he will try to catch it, but instead another
exception is thrown.

Ivan