[GHC] #10713: Type family not reducing over data family

30 Jul 2015

      #10713: Type family not reducing over data family
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              Reporter:  acowley     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
  (Type checker)                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Given this code,

 {{{
 type family TEq t s where
   TEq t t = 'True
   TEq t s = 'False
 data family T a
 }}}

 I expected this GHCi interaction to reduce:
 `:kind! TEq (T Int) (T Bool)`

 But it does not. It does reduce (to `'True`) if you instead ask,

 `:kind! TEq (T Int) (T Int)`

 Tested on GHC 7.10.2

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10713
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