[GHC] #10570: Too restrictive liberate coverage condition

25 Jun 2015

      #10570: Too restrictive liberate coverage condition
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              Reporter:  danilo2     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              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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 Hello! Let's look at this small example:

 {{{haskell
 class ConsByIdx2 x a m cls | x -> m where
     consByIdx2 :: x -> a -> m cls

 instance ConsByIdx2 Int a Proxy cls where
     consByIdx2 _ _ = Proxy
 }}}

 It fails to compile with the following error:

 {{{haskell
     Illegal instance declaration for ‘ConsByIdx2 Int a Proxy cls’
       The liberal coverage condition fails in class ‘ConsByIdx2’
         for functional dependency: ‘x -> m’
       Reason: lhs type ‘Int’ does not determine rhs type ‘Proxy’
     In the instance declaration for ‘ConsByIdx2 Int a Proxy cls’
 }}}

 But Int determines the Proxy in a nice way.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10570
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