Warning emitted about type variable being used only once when it is used more than once

[lpil]

Describe the bug

Hello! I have typespecs that feature a type variable used multiple times, but OTP27 is emitting a warning saying they are used only once.

$ erlc wibble.erl
foo.erl:5:27: Warning: type variable 'X' is only used once (is unbound)
%    5| -spec wibble(atom()) -> {ok, X} | {error, X}.
%     |                           ^

To Reproduce

Compile a module with a function with this typespec:

-spec wibble(atom()) -> {ok, X} | {error, X}.

Expected behavior

Any resulting warning not to say the type variable is only used once.

OR

No warning to be omitted as the type variable to be used twice

Affected versions

OTP 27.0

OTP 26.2.5 does not emit a warning here.

Thanks,
Louis

[dvic]

OTP 26.2.5 does not emit a warning here.

Just to avoid any confusion: I pointed @lpil in the wrong direction, OTP 26.2.5 actually does emit a warning, it's OTP < 26 (I tested 25.3.2.12) that does not emit a warning.

[essen]

This is intentional. See #6915 for a previous ticket with links and/or explanations.

[lpil]

Aye! I'm suggesting that the warning message is confusing or possibly incorrect? It says that the type variable is used once, but it is used twice in this case.

[jhogberg]

It isn't used twice, though: each of those is a separate X that happens to share the same name. In other languages (e.g. C or Rust) it would be analogous to having variables declared in two different blocks with the same name.

[lpil]

I see! I presumed it worked like value variables did in Erlang. What's the syntax for having the same variable used twice?

[jhogberg]

{X, X} would use it twice. Disjunctions add (inner) scopes, so {X, {a, X} | b} should also work without a warning.

[lpil]

What's the correct syntax for the typespec I have here? Both X should stand in for the same type.

-spec wibble(atom()) -> {ok, X} | {error, X}.

[jhogberg]

There is no way to express that. :/

[essen]

I do

-spec wibble(atom()) -> {ok, X} | {error, X} when X::type().

Because even if Dialyzer doesn't care, to the human reader that's what it says.

[lpil]

If I had a -type result(Data, Error) :: {ok, Data} | {error, Error}. and did result(X, X) would that do it?

[jhogberg]

Nope. :/

[lpil]

Ah, ok. Should I close this issue then? Seems I have misunderstood how typespecs work.

[jhogberg]

Yes, let's. Variables in typespecs have been quite vague on how they work, and we're going to send in an EEP with clarifications to the type language Soon™.