[GHC] #10116: Closed type families: Warn if it doesn't handle all cases

[GHC]

#10116: Closed type families: Warn if it doesn't handle all cases
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        Reporter:  andrewthad        |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 I conjecture that getting this completely right for closed type families
 is a research project of its own, because closed type families allow for
 repeated variables.

 For example:

 {{{
 type family F (a :: Bool) (b :: Maybe Bool) :: Nat where
   F True  (Just False) = 0
   F False (Just True)  = 1
   F x     (Just x)     = 2
   F y     Nothing      = 3
 }}}

 Is that total? I do believe it is. But it's a little hard to figure it
 out!

 But we could certainly do ''something'' in this area.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10116#comment:3>
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