[erlang-questions] dialyzer: user-defined types just synonyms?

[Kostis Sagonas]

Dmitry Belyaev wrote:
> I thought type specs are made for external typechecks. Ok. Now I have 
> this code:
> -module(test).
> 
> -export([start/0]).
> 
> -spec start() -> ok.
> start() ->
>    AtomBad = get_atom(1),
>    AtomGood = get_atom(2),
>    check_atom(AtomBad),
>    case check_atom(AtomGood) of
>        ok -> ok;
>        error -> error
>    end.
> 
> -spec get_atom(any()) -> good | bad.
> get_atom(1) ->
>    bad;
> get_atom(_) ->
>    good.
> 
> -spec check_atom(good | nice) -> ok | error.
> check_atom(Atom) ->
>    ok.
> 
> It doesn't warn me about check_atom(AtomBad).

It does; see below.  You are using an older Erlang/OTP.
Upgrade to R13B04.

Kostis

test.erl:12: The pattern 'error' can never match the type 'ok'
test.erl:21: The specification for test:check_atom/1 states that the 
function might also return 'error' but the inferred return is 'ok'