[GHC] #12239: Dependent type family does not reduce

[GHC]

#12239: Dependent type family does not reduce
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       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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 When a type family is used in a kind of an argument for another type
 family, it does not reduce. Here's example code:


 {{{
 {-# LANGUAGE TypeInType, GADTs, TypeFamilies #-}

 import Data.Kind (Type)

 data N = Z | S N

 data Fin :: N -> Type where
   FZ :: Fin (S n)
   FS :: Fin n -> Fin (S n)

 type family FieldCount (t :: Type) :: N

 type family FieldType (t :: Type) (i :: Fin (FieldCount t)) :: Type

 data T

 type instance FieldCount T = S (S (S Z))

 type instance FieldType T FZ = Int
 type instance FieldType T (FS FZ) = Bool
 type instance FieldType T (FS (FS FZ)) = String
 }}}

 Unfortunately, I get these errors during typechecking:

 {{{
 [1 of 1] Compiling Main             ( fieldtype.hs, fieldtype.o )

 fieldtype.hs:19:27: error:
     • Expected kind ‘Fin (FieldCount T)’,
         but ‘'FZ’ has kind ‘Fin ('S n0)’
     • In the second argument of ‘FieldType’, namely ‘FZ’
       In the type instance declaration for ‘FieldType’

 fieldtype.hs:20:28: error:
     • Expected kind ‘Fin (FieldCount T)’,
         but ‘'FS 'FZ’ has kind ‘Fin ('S ('S n0))’
     • In the second argument of ‘FieldType’, namely ‘FS FZ’
       In the type instance declaration for ‘FieldType’

 fieldtype.hs:21:28: error:
     • Expected kind ‘Fin (FieldCount T)’,
         but ‘'FS ('FS 'FZ)’ has kind ‘Fin ('S ('S ('S n0)))’
     • In the second argument of ‘FieldType’, namely ‘FS (FS FZ)’
       In the type instance declaration for ‘FieldType’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12239>
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