GitLab

The following code typechecks:

{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import Data.Kind
import Data.Proxy

class C a where
  c :: Proxy a

type family F

data D :: F -> Type

instance C (D :: F -> Type) where
  c = undefined

However, if we try to actually //use// that C D instance, like so:

c' :: Proxy (D :: F -> Type)
c' = c @(D :: F -> Type)

Then GHC gives a rather flummoxing error message:

$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:20:6: error:
    • No instance for (C D) arising from a use of ‘c’
    • In the expression: c @(D :: F -> Type)
      In an equation for ‘c'’: c' = c @(D :: F -> Type)
   |
20 | c' = c @(D :: F -> Type)
   |      ^^^^^^^^^^^^^^^^^^^

But that error is clearly misleading, as we defined such a C D instance directly above it!

The use of the type family F in the kind of D appears to play an important role in this bug. If I change F to be a data type (e.g., data F), then c' is accepted.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture