[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)                           |
      Resolution:                    |             Keywords:  TypeFamilies
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 carter):

 I've hit needing this feature for what I was hoping would be pretty
 elementary code :(


 {{{

 type family ReverseTC (a :: [ k ]) (res :: [ k ] )  = result    where
     ReverseTC '[] res = res
     ReverseTC (a ': bs ) res = ReverseTC bs (a ':  res)

 type family Reverse  (a :: [k]) =  (result  :: [k])     where
   Reverse a = ReverseTC a '[]

 }}}

 I would like to explain to GHC that if i know  a and result , or res and
 result, i know the remaining variable, so that my "stack safe" Reverse
 computation can also be treated as Injective (which it is! )

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10832#comment:6>
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