Eqwalizer doesn't like lists:foldl?

lambdamix · February 13, 2025, 12:04pm

If you add eqwalizer_support dependency to your project (see the instructions in README - eqwalizer/README.md at main · WhatsApp/eqwalizer · GitHub) then your example is accepted.

eqwalizer_support ships a special module eqwalizer_specs (eqwalizer/eqwalizer_support/src/eqwalizer_specs.erl at main · WhatsApp/eqwalizer · GitHub) that provide more accurate specs for some stdlib functions (especially “generic” functions like lists:foldl).

github.com/WhatsApp/eqwalizer

eqwalizer_support/src/eqwalizer_specs.erl

f5bc995b7


      
          -spec 'lists:foldl'(fun((T, Acc) -> Acc), Acc, [T]) -> Acc.

Compare it to the OTP spec:

github.com/erlang/otp

lib/stdlib/src/lists.erl

53e675190


      
          -spec foldl(Fun, Acc0, List) -> Acc1 when
                Fun :: fun((Elem :: T, AccIn) -> AccOut),
                Acc0 :: term(),
                Acc1 :: term(),
                AccIn :: term(),
                AccOut :: term(),
                List :: [T],
                T :: term().

The original spec says that the result type is Acc1 :: term() - note that Acc1 is not connected to any other type variables, so it’s just term().

There is an EEP - Eep 0071 - Erlang/OTP - bringing some clarification how type variables should be considered for type checking and there is a PR to make the lists specs more accurate for type-checking - Update and uniformise lists specs by VLanvin · Pull Request #8736 · erlang/otp · GitHub.

There is also a task - add sanity check that eqwalizer_support is present · Issue #82 · WhatsApp/erlang-language-platform · GitHub - to add more signal about eqwalizer_specs in CLI/UI.