[GHC] #12704: Check if constraint synonym satisfies functional dependencies

14 Oct 2016

      #12704: Check if constraint synonym satisfies functional dependencies
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           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When we resolve #12679, it will be possible to implement an abstract type
 class using a constraint synonym. However, in my implementation, I didn't
 implement functional dependency checking:

 {{{
 signature H where
   class F a b | a -> b

 -- This should be an invalid implementation
 module H where
   type F a b = (Eq a, Eq b)
 }}}

 The check is annoying fiddly: we have to descend into the constraint
 synonym, collect all the implied fundeps, and then see if we have the ones
 we need, so I didn't implement it. Maybe some time.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12704
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