Re: [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
      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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Description changed by danilo2:

Old description:
...
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.
New description:

 Hello! Let's look at this small example:

 {{{
 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:

 {{{
     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#comment:1
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