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

[Michael McDaniel]

On Fri, Mar 13, 2009 at 02:02:27PM -0700, Anthony Molinaro wrote:
> 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.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


  you could replace the macro ...


-export([is_dict/1]).

is_dict(D) ->
     is_tuple (D) andalso element (1, D) =:= dict
.


~M


> 
> 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
> > 
> > --
> > ------------------------------------------------------------------------
> > Anthony Molinaro                                           <nym@REDACTED>
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> Anthony Molinaro                                           <nym@REDACTED>
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-- 
Michael McDaniel
Portland, Oregon, USA
http://autosys.us