Dialyzer warning when returning a prolist

Hello,

I'm trying to figure out why dialyzer is complaining about a function when
I don't explicitly declare its return as a proplist.
I have this small module which returns a proplist in the form {atom(),
[atom()]}:

> -module(test).
>
> -export([test/0]).
> test() ->
>    [
>        {one_test, [a,b,c]},
>        {another_test, [d,e,f]}
>    ].
>

For function above, I made the following spec, but dialyzer has complained
(as stated below) when I enabled its underspecs warnings.

> -spec test() -> [{atom(), [atom()]}].
> % src/test.erl
> %    Type specification test:test() -> [{atom(),[atom()]}] is a supertype
> of the success typing: test:test() -> [{'another_test',['d' | 'e' |
> 'f',...]} | {'one_test',['a' | 'b' | 'c',...]},...]
>

Well... both lists are non-empty lists as stated by dialyzer, so I modified
the spec, without success:

> -spec test() -> [{atom(), [atom(), ...]}, ...].
> %src/test.erl
> %   Type specification test:test() -> [{atom(),[atom(),...]},...] is a
> supertype of the success typing: test:test() -> [{'another_test',['d' | 'e'
> | 'f',...]} | {'one_test',['a' | 'b' | 'c',...]},...]
>

Then I tried to explicitly declare it as a proplist, and dialyzer has
accepted it:

> -spec test() -> proplists:proplist().
> % OK, no warnings.
>

As it worked, I tried to do the same as proplists module does and created
two types, which I used on my new spec. And, surprisingly, dialyzer has
complained about it!

> % From <https://erlang.org/doc/man/proplists.html>
> -type property() :: atom() | tuple().
> -type proplist() :: [property()].
>

> -spec test() -> proplist().
> %src/test.erl
>
%   Type specification test:test() -> proplist() is a supertype of the
> success typing: test:test() -> [{'another_test',['d' | 'e' | 'f',...]} |
> {'one_test',['a' | 'b' | 'c',...]},...]
>

Therefore, I could not understand why declaring the return as
"proplists:proplist" is OK while declaring it as lists, nonempty lists or
creating the same types as proplists are not OK. Why is dialyzer
complaining about the other specs? Is dialyzer handling proplists in a
different way? Could someone help me to understand this behaviour?


Best Regards,
Paulo Zulato

--
«Quis custodiet ipsos custodes?»