[GHC] #13546: Kind error with type equality

7 Apr 2017


      #13546: Kind error with type equality
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This code

 {{{#!hs
 {-# LANGUAGE TypeInType, TypeFamilies, GADTs, ConstraintKinds #-}

 import Data.Kind

 data Dict c where
   Dict :: c => Dict c

 data T (t :: k)

 type family UnT (a :: Type) :: k where
   UnT (T t) = t

 untt :: Dict (UnT (T "a") ~ "a")
 untt = Dict

 tunt :: Dict (T (UnT (T "a")) ~ T "a")
 tunt = Dict
 }}}

 fails with this error:

 {{{
 tunt.hs:17:8: error:
     • Couldn't match kind ‘k’ with ‘GHC.Types.Symbol’
       ‘k’ is a rigid type variable bound by
         the type signature for:
           tunt :: forall k. Dict T (UnT (T "a")) ~ T "a"
         at tunt.hs:16:1-38
       When matching types
         UnT (T "a") :: k
         "a" :: GHC.Types.Symbol
     • In the expression: Dict
       In an equation for ‘tunt’: tunt = Dict
     • Relevant bindings include
         tunt :: Dict T (UnT (T "a")) ~ T "a" (bound at tunt.hs:17:1)
    |
 17 | tunt = Dict
    |
 }}}

 Instead I would expect these reductions to take place:

 {{{
  1. T (UnT (T "a")) ~ T "a"
  2. T "a" ~ T "a"
  3. constraint satisfied (refl)
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13546
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