[GHC] #14366: Type family equation refuses to unify wildcard type patterns

18 Oct 2017


      #14366: Type family equation refuses to unify wildcard type patterns
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  TypeInType                         |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This typechecks:

 {{{#!hs
 {-# 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`:

 {{{#!hs
 {-# 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:

 {{{#!hs
 {-# 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
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14366
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