Explicitly quantifying a kind variable causes a telescope to fail to kind-check
Consider the following program:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -fprint-explicit-kinds #-}
module Bug where
import Data.Kind
import Data.Type.Equality
type family Foo1 (e :: (a :: k) :~: (a :: k)) :: Type where
Foo1 (e :: a :~: a) = a :~: a
type family Foo2 (k :: Type) (e :: (a :: k) :~: (a :: k)) :: Type where
Foo2 k (e :: a :~: a) = a :~: aFoo2 is wholly equivalent to Foo1, except that in Foo2, the k kind variable is explicitly quantified. However, Foo1 typechecks, but Foo2 does not!
$ /opt/ghc/8.2.2/bin/ghci Bug.hs -fprint-explicit-kinds
GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/ryanglscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:13:10: error:
• Couldn't match kind ‘k’ with ‘k1’
When matching the kind of ‘a’
Expected kind ‘(:~:) k a a’, but ‘e’ has kind ‘(:~:) k a a’
• In the second argument of ‘Foo2’, namely ‘(e :: a :~: a)’
In the type family declaration for ‘Foo2’
|
13 | Foo2 k (e :: a :~: a) = a :~: a
| ^^^^^^^^^^^^^^(Moreover, there seems to be a tidying bug, since GHC claims that (:~:) k a a is not the same kind as (:~:) k a a.)
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.2.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |