[GHC] #12803: Functional dependencies and type families

[GHC]

#12803: Functional dependencies and type families
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 Consider
 {{{
 type family F a :: *

 class C a b | a -> b

 instance C p (F q) => C p [q]
 }}}
 Is the instance decl OK under the liberal coverage condition?

 That is, does `(C t [s1], C t [s2])` mean that `(s1 ~ s2)`?

 NO!  The context of the instance declaration means that `p -> F q` but
 that does not imply `p -> q` unless `F` is injective!

 So this program is wrongly accepted.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12803>
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