[erlang-questions] Dialyzer warning has me baffled.

[Brujo Benavides]

Hi bengt,

	This would be my guess:

	The spec for sets:is_dets_file/1 says…
	
	-spec is_dets_file(Filename) -> boolean() | {'error', Reason} when
	      Filename :: file:name(),
	      Reason :: term().

	That means that, for it to work, its parameter must be of type file:name().

	Therefore, in your code, dialyzer infers that your variable O is, in fact, a file:name() which of course doesn’t match with {error, Reason}.

	When you remove the first argument for key/2, you’re basically removing that restriction… dialyzer can no longer infer anything about the type of O and therefore is silent.

	Hope this helps.

Brujo Benavides <http://about.me/elbrujohalcon>



> On 17 Aug 2018, at 12:56, bengt e <cean.ebengt@REDACTED> wrote:
> 
> Greetings,
> 
> This is on Erlang 20.3. I am at work, so if anybody could test on 21 that would be nice.
> 
> The source code below (the smallest I could make the original and still get the warning) gives me the Dialyzer warning (line 19 is first/2):
> src/test_dialyzer.erl
>   19
> : The pattern <_, {'error', _Reason}> can never match the type <boolean() | {'error',_},[atom() | [any()] | char()]>
> 
> If I remove the first argument to key/2, Dialyzer is silent.
> Is there anybody that would like to explain why I get the warning?
> Is it some kind of interference between type specs? Ie, is_dets_file/1 hides {error, Reason} from oldest/1 ?
> 
> 
> Best Wishes,
> bengt
> 
> 
> -module( test_dialyzer ).
> 
> %% API exports
> -export( [key/1] ).
> 
> %%====================================================================
> %% API functions
> %%====================================================================
> 
> -spec( key(Name::file:filename()) -> ok ).
> key( Name ) ->
> 	O = oldest( file:list_dir(Name) ),
> 	key( dets:is_dets_file(O), O ).
> 
> %%====================================================================
> %% Internal functions
> %%====================================================================
> 
> key( _, {error, _Reason} ) -> ok;
> key( _, _ ) -> ok.
> 
> oldest( {ok, [H | _]} ) -> H;
> oldest( {error, Reason} ) -> {error, Reason}.
> 
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