[GHC] #9269: Type families returning quantified types

5 Jul 2014


      #9269: Type families returning quantified types
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       Reporter:  pumpkin           |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Is there a fundamental reason for not being able to use quantification in
 a type family? I'd very much like to be able to do it, obviously turning
 on RankNTypes if necessary.

 I'm looking for things like this:

 {{{#!haskell
 type family Foo (x :: Bool) where
   Foo True = forall a. [a]
   Foo False = ()
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9269
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