Type family equation refuses to unify wildcard type patterns
This typechecks:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
import Data.Kind
import Data.Type.Equality
type family Cast (e :: a :~: b) (x :: a) :: b where
Cast Refl x = x
However, if you try to make the kinds a
and b
explicit arguments to Cast
:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
import Data.Kind
import Data.Type.Equality
type family Cast (a :: Type) (b :: Type) (e :: a :~: b) (x :: a) :: b where
Cast _ _ Refl x = x
Then GHC gets confused:
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/ryanglscott/.ghci
[1 of 1] Compiling Main ( Bug.hs, interpreted )
Bug.hs:9:12: error:
• Expected kind ‘_ :~: _1’, but ‘'Refl’ has kind ‘_ :~: _’
• In the third argument of ‘Cast’, namely ‘Refl’
In the type family declaration for ‘Cast’
|
9 | Cast _ _ Refl x = x
| ^^^^
A workaround is to explicitly write out what should be inferred by the underscores:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
import Data.Kind
import Data.Type.Equality
type family Cast (a :: Type) (b :: Type) (e :: a :~: b) (x :: a) :: b where
Cast a a Refl x = x
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |