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

[Robert Virding]

The reason for this behaviour is that the compiler specially handles filters which are also valid guard tests. After a generator then all the *directly* following filters which are valid guard tests are handled as guards to the generators and not as "normal" tests. While this is more efficient it does lead to the inconsistencies you discovered in the case of errors in the filter tests.

It seemed like a good idea at the time. I don't know if it is possible to change this today, if one wanted to.

Robert


----- "Sam Bobroff" <sam@REDACTED> 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.
> -- 
> Sam Bobroff | sam@REDACTED | M5 Networks
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