GitLab

Consider

{-# LANGUAGE PolyKinds, ExplicitForAll #-}
f :: forall (m :: k -> *) (a :: k). m a -> m a
f = id

g = f

I would expect GHC to infer the same type for g as for f. However, it gives the AnyK kind, and g is not possible to use:

ghci -Wall X.hs

...

X.hs:5:1: Warning:
    Top-level binding with no type signature:
      g :: forall (m :: AnyK -> *) (a :: AnyK). m a -> m a
Ok, modules loaded: Main.
*Main> g "a"

<interactive>:2:1:
    Kind incompatibility when matching types:
      a0 :: AnyK
      Char :: *
    In the first argument of ‛print’, namely ‛it’
    In a stmt of an interactive GHCi command: print it

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture