[GHC] #12587: InstanceSigs doesn't work with ambigous types

[GHC]

#12587: InstanceSigs doesn't work with ambigous types
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        Reporter:  vagarenko         |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 In your instance decl (I have renamed the forall'd variable).
 {{{
 instance Bar Int where
     bar :: forall c. (Foo c) => Int  -- error here
     bar = undefined
         where
             x :: c
             x = undefined
 }}}
 GHC is trying to check that the instance signature you ''provide'':
 {{{
 bar :: forall c. Foo c => Int
 }}}
 is more general than the one that is ''required'':
 {{{
 bar :: forall b. Foo b => Int
 }}}
 To to that it

 * instantiates the required one, giving `[W] Foo b0`
 * unifies provided and requied types `Int ~ Int`
 * checks that it can prove the required `[W] Foo b0` from the given `Foo
 c`.

 But it can't prove that because nothing tells GHC to instantiate `b0` to
 `c`.

 I think this is fair enough: it really is an ambiguous type.  The same
 thing
 would happen if you used an auxiliary function
 {{{
 instance Bar Int where
     bar = barInt

 barInt :: forall c. (Foo c) => Int  -- error here
 barInt = undefined
        where
             x :: c
             x = undefined
 }}}
 Indeed, it'd be surprising if this didn't work but the previous code did.

 So I think it's fine.  Yell if you disagree.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12587#comment:1>
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