CUSK-less class typechecks on 8.4, but not on 8.6+
This code will typecheck with GHC 8.0.2 through 8.4.4:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where
import Data.Kind
import Data.Proxy
class C f where
type T (x :: f a) :: Type
sT :: forall a (x :: f a).
Proxy x -> T x
However, it mysteriously does not typecheck on GHC 8.6.5 or later:
$ /opt/ghc/8.8.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:13:22: error:
• Expected kind ‘f a’, but ‘x’ has kind ‘f a1’
• In the first argument of ‘T’, namely ‘x’
In the type signature: sT :: forall a (x :: f a). Proxy x -> T x
In the class declaration for ‘C’
|
13 | Proxy x -> T x
| ^
I cannot think of a good reason why this shouldn't typecheck, especially since there appears to be no polymorphic recursion happening in C
, T
, or sT
.
Some observations:
-
Giving
C
a CUSK (i.e.,class C (f :: k -> Type) where ...
) makes it typecheck: -
Splitting up
C
into two classes like so also makes it typecheck:class C1 f where type T (x :: f a) :: Type class C2 f where sT :: forall a (x :: f a). Proxy x -> T x