[GHC] #10910: Data families seem not to be "surely apart" from anything

23 Sep 2015


      #10910: Data families seem not to be "surely apart" from anything
-------------------------------------+-------------------------------------
              Reporter:  oerjan      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:  data        |  Operating System:  Unknown/Multiple
  families, closed type families     |
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 The following code is not compiling (GHC 7.10.2), it looks as if data
 families are not considered "surely apart" from any type, despite being
 injective.

 {{{#!hs
 {-# LANGUAGE TypeFamilies, DataKinds, GADTs #-}

 type family Equals x y where
     Equals x x = True
     Equals x y = False

 data Booly b where
     Truey :: Booly True
     Falsey :: Booly False

 data family ID a

 data instance ID Bool = IB

 test :: Booly (Equals (ID Bool) Int)
 test = Falsey
 }}}

 (Inspired and simplified from http://stackoverflow.com/questions/32733368
 /type-inequalities-in-the-presence-of-data-families#32733368.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10910
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