[erlang-questions] Tricking dialyzer with an opaque type

[Tobias Lindahl]

It seems to me that the gb_sets:from_list/1 should give the correct type
for your function even if the spec is not taken into account.

In that case, the value you are getting warnings for comes from somewhere
else. Could it be that you are deconstructing the opaque type somewhere?


2017-06-25 18:56 GMT+02:00 Paul Guyot <pguyot@REDACTED>:

> Hello,
>
> I do have a parse transform that replaces the body of a 0-arity function
> with its result, as evaluated at compile time.
> I am looking for a way to avoid an opaque violation warning from dialyzer
> when this function returns, say, a gb_sets:set(). The spec is properly
> copied into the parsed tree, but dialyzer just ignores it.
>
> Typically, such a function would load data from a file and create a
> gb_sets:set() from this data.
>
> -spec f() -> gb_sets:set(unicode:unicode_binary()).
> f() ->
>         {ok, Content} = file:read_file(?FILENAME),
>         List0 = binary:split(Content, <<"\n">>, [global]),
>         List1 = lists:filter(fun(W) -> W =/= <<>> end, List0),
>         gb_sets:from_list(List1).
>
> With the parse_transform, dialyzer complains on usage, e.g. :
>
> [warning] some_module.erl:123 the call gb_sets:add(<VALUE>,Var::{1..1114111,{_,_,_}})
> does not have an opaque term of type gb_sets:set(_) as 2nd argument
>
> Yet, of course, some_module.erl does not violate the opacity of
> gb_sets:set() itself.
> I can trick dialyzer with an expensive identity function:
>
>         gb_sets:add(Value, binary_to_term(term_to_binary(Var)))
>
> Any better idea?
>
> Paul
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