[GHC] #10832: Generalize injective type families

[GHC]

#10832: Generalize injective type families
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        Reporter:  jstolarek         |                Owner:  jstolarek
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.11
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  InjectiveFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #6018             |  Differential Rev(s):  Phab:D1287
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Are we exploring avenues like
 [https://ghc.haskell.org/trac/ghc/ticket/6018#comment:48 head-injectivity]
 or partial injectivity? Richard's example tells us `x ~ Just y` if `F x ~
 True`

 {{{#!hs
 type family
   F a where
   F (Just x) = True
   F Nothing  = False
 }}}

 but similarly

 {{{#!hs
 type family
   G a where
   G (Just x) = (x,   x)
   G Nothing  = (Int, Int)
 }}}

 means `x ~ Int` if `G (Just x) ~ (Int, Int)` — is that useful?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10832#comment:18>
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