Misleading error message when partially applying a data type with a visible quantifier in its kind
I'm not sure if this is a bug or an intended design, so I'll ask here. I want this program to typecheck:
{-# LANGUAGE TypeInType #-}
module Bug where
import Data.Kind
data Hm (k :: Type) (a :: k) :: Type
class HasName (a :: k) where
getName :: proxy a -> String
instance HasName Hm where
getName _ = "Hm"This is rejected, however:
$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/ryanglscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:11:18: error:
• Expecting two more arguments to ‘Hm’
Expected kind ‘k0’, but ‘Hm’ has kind ‘forall k -> k -> *’
• In the first argument of ‘HasName’, namely ‘Hm’
In the instance declaration for ‘HasName Hm’
|
11 | instance HasName Hm where
| ^^The culprit appears to be the fact that Hm has kind forall k -> k -> *, which uses a visible quantifier. Does this prevent partial applications of Hm? Or is this simply a GHC bug?
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |