[erlang-questions] Dialyzer warning has me baffled.

[bengt e]

Thank you very much. That makes sense. I can fix this.

My mistake was to think that Dialyzer knew the return value of oldest/1,
since it is there, in the file. Whereas, to me, the spec for is_dets_file/1
is 'just a spec'. dets:is_dest_file( {not, "afile", 1} ) will return
{error, Reason} and not crash.

On Fri, Aug 17, 2018 at 6:05 PM, Brujo Benavides <elbrujohalcon@REDACTED>
wrote:

> 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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>
>
>
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