GitLab

{-# LANGUAGE ConstraintKinds       #-}
{-# LANGUAGE NamedWildCards        #-}
{-# LANGUAGE PartialTypeSignatures #-}

module Test where

class C a where
  meth :: a

unnamed :: _ => a
unnamed = meth

named :: _c => a
named = meth

The unnamed version causes no problems, however named causes the following error:

    * Could not deduce: _0
      from the context: _
        bound by the inferred type for `named':
                   forall a (_ :: Constraint). _ => a
        at Test.hs:14:1-12
    * In the ambiguity check for the inferred type for `named'
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the inferred type
        named :: forall a (_ :: Constraint). _ => a
   |
14 | named = meth
   | ^^^^^^^^^^^^

That is, in this example, _c is not a named wildcard, but is instead universally quantified. So GHC unsurprisingly throws the same error as if we had written

universal :: c => a
universal = meth

I would have expected that the named wildcard _c behave just like the wildcard _, except that it would give me the ability to refer to the inferred constraint at the type-level inside the body of named.

(I came across this while thinking about #19010.)