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

[GHC]

#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
      Resolution:                    |                Keywords:  data
                                     |  families, closed type families
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by oerjan:

Old description:

> 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.)

New description:

 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
 }}}

 This gives the error

 {{{
 Test.hs:16:8: Warning:
     Couldn't match type ‘Equals (ID Bool) Int’ with ‘'False’
     Expected type: Booly (Equals (ID Bool) Int)
       Actual type: Booly 'False
     In the expression: Falsey
     In an equation for ‘test’: test = Falsey
 }}}

 This gives no error with an ordinary type instead of a data family.

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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10910#comment:1>
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