[erlang-questions] Bad warning when using short-circuit booleanexpressions

[Anthony Molinaro]

I understand that element/2 is expecting a tuple, but the andalso
is short circuiting when D is not a tuple, so element is always called
with a tuple in this case.  The warning is therefore misleading, since
it states "this expression will fail" when it does not fail.  This may
just be impossible to detect because of the lack of types, in that case
I would be curious how to achieve the same functionality (testing that
an erlang term is a dict in a guard), without the warning.

Thanks,

-Anthony

On Fri, Mar 13, 2009 at 03:06:45PM -0500, Sibley, Jamaal wrote:
>  Sorry, I didn't see the macro.  Element/2 is expecting a tuple as the
> second argument. 
> 
> -----Original Message-----
> From: erlang-questions-bounces@REDACTED
> [mailto:erlang-questions-bounces@REDACTED] On Behalf Of Anthony
> Molinaro
> Sent: Friday, March 13, 2009 1:37 PM
> To: erlang-questions@REDACTED
> Subject: [erlang-questions] Bad warning when using short-circuit
> booleanexpressions
> 
> Hi,
> 
>   I have this code
> 
> -module (is_dict).
> 
> -include_lib ("eunit/include/eunit.hrl").
> 
> -define (is_dict (D), is_tuple (D) andalso element (1, D) =:= dict).
> 
> is_dict_test () ->
>   ?assertEqual (false, ?is_dict (a)),
>   ?assertEqual (false, ?is_dict ({a, b})),
>   ?assertEqual (false, ?is_dict ([{a, b}])),
>   ?assertEqual (true, ?is_dict (dict:new())),
>   ?assertEqual (true, ?is_dict (dict:from_list ([{a, b}]))).
> 
> which gives
> 
> Erlang (BEAM) emulator version 5.6.5 [source] [smp:2] [async-threads:0]
> [kernel-poll:false]
> 
> Eshell V5.6.5  (abort with ^G)
> 1> c (is_dict).
> ./is_dict.erl:8: Warning: this expression will fail with a 'badarg'
> exception
> ./is_dict.erl:10: Warning: this expression will fail with a 'badarg'
> exception {ok,is_dict}
> 2> is_dict:test ().
>   Test successful.
> ok
> 3> 
> 
> The warnings seem wrong as the expression does not fail (or maybe I'm
> just doing something wrong with this test?).
> 
> -Anthony
> 
> --
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> Anthony Molinaro                                           <nym@REDACTED>
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-- 
------------------------------------------------------------------------
Anthony Molinaro                                           <nym@REDACTED>