[erlang-questions] List comprehension filter expressions not throwing exceptions

Hynek Vychodil <>
Thu Sep 30 10:44:12 CEST 2010


On Thu, Sep 30, 2010 at 6:43 AM, Sam Bobroff <> wrote:
> Hi all,
>
> I've discovered (after some rather painful debugging) that badrecord
> exceptions (and others, actually) are sometimes silently converted to
> false when they're generated inside a list comprehension filter
> expression (as if they were a guard?).
>
> I couldn't find any reference to this behaviour in the manual, so
> perhaps it's a bug or perhaps there needs to be a note in the manual. It
> certainly surprised me.
>
> Here's some code to reproduce the behaviour:
>
> -module(lc).
> -export([a/0, b/0]).
> -record(rec, {x}).
>
> a() ->
>        [E || E <- [1], E#rec.x =:= 1].
>
> b() ->
>        [E || E <- [1], check_element(E)].
>
> check_element(E) ->
>        E#rec.x =:= 1.
>
> Tested on R13B04, R14A or R14B, I get:
>
> $ erl
> Erlang R14A (erts-5.8) [source] [smp:2:2] [rq:2] [async-threads:0]
> [hipe] [kernel-poll:false]
>
> Eshell V5.8  (abort with ^G)
> 1> c(lc).
> {ok,lc}
> 2> lc:a().
> []
> 3> lc:b().
> ** exception error: {badrecord,rec}
>     in function  lc:check_element/1
>     in call from lc:'-b/0-lc$^0/1-0-'/1
> 4>
>
> Where I would expect both a() and b() to crash with badrecord. As the
> code shows, executing the same expression wrapped in a function call
> *does* cause the exception to be raised, so it's not as simple as all
> exceptions being converted.
>
> Peace,
> Sam.

Hi Sam, I think it is expected behavior that lists comprehension tests
should behave same as 'when' clause in function or 'case' clauses.
I.e. it is intended to handle exceptions as false. Call to
check_element/1 cant be converted to 'when' so this doesn't catch
exception.

> --
> Sam Bobroff |  | M5 Networks
> Why does my email have those funny headers? Because I use PGP to sign
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>
>



-- 
--Hynek (Pichi) Vychodil

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