[erlang-questions] Typespecs of functions that may fail

Stavros Aronis aronisstav@REDACTED
Tue Jan 17 16:31:22 CET 2017

Yes. Dialyzer collects all local calls to a non-exported function and
analyses its body with the additional constraint that the arguments given
to the function are only the ones in those calls. In such cases it may be
able to infer types that are stronger than the relevant specs (you can get
a warning about the spec itself by using the "-Wunderspecs" option).

You can eliminate the warning by e.g. exporting 'is_relation/2' or by using
it locally in situations where it may also return `{true, ...}`.



On Tue, Jan 17, 2017 at 4:22 PM Bastien Chamagne <bchamagne@REDACTED>

> Thank you for the quick reply Stavros!
> You mean that dialyzer inference is stronger than the defined typespec?
> On 01/17/2017 04:08 PM, Stavros Aronis wrote:
> Hi Bastien,
> can it be the case that "is_relation/2" is only used "locally" (i.e.
> within the module itself) and all such calls to "is_relation/2" will return
> 'false'?
> This looks like a case where dialyzer has inferred such a thing and thus
> complains about the pattern that can't match.
> Regards,
> Stavros
> On Tue, Jan 17, 2017 at 3:47 PM, Bastien Chamagne <bchamagne@REDACTED>
> wrote:
> Hello,
> I'm trying to use dialyzer and I don't know how to type functions that are
> written in "happy path" (may crash).
> Here's a sample function that may or may not crash depending if Field
> correspond to a relation field on given Model:
>     -spec is_relation(field(), model()) -> {true, relation_type()} | false.
>     %% @doc Sets a relation on the given model.
>     -spec set_relation(field(), model(), model()) -> model() | no_return().
>     set_relation(Field, RelatedModel, Model) ->
>         % On next line, I'm OK that this function crashes if I try to set
> a relation on a non-relation field
>         {true, RelationType} = is_relation(Field, Model),
>         set_relation(Field, RelatedModel, Model, RelationType).
> Here's what dialyzer says:
>     model.erl:180: Function set_relation/3 has no local return
>     model.erl:181: The pattern {'true', RelationType} can never match the
> type 'false'
> Is there anything that I can do to inform dialyzer that this is a OK
> behaviour for me?
> Cheers!
> *ps: I'm using erlang 17 if that matters.*
> *ps: Sorry if it's a duplicate, I can't find the previous message on the
> archive.*
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