[GHC] #9948: Recommend class constraint instead of instance constraint

[GHC]

#9948: Recommend class constraint instead of instance constraint
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        Reporter:  crockeea          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      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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Comment (by crockeea):

 Replying to [comment:5 goldfire]:
 > However, we would, of course, still like to find an instance where
 possible.

 I see your point. So it's good that GHC looks at instances and tries to
 find a RHS match. If he constraints are also satisfied, then no errors
 should be reported. The case I'm talking about is when there is a matching
 instance [on the RHS], but the instance constraints are not satisfied.


 {{{
 data T a

 class Foo b where
   foo :: b -> Int

 instance (Integral a) => Foo (T a)

 f :: T a -> Int
 f = foo
 }}}

 I don't like that GHC behaves differently in the presence of a matching
 instance vs when there are no matching instances. One workaround for what
 I'm suggesting (that I find myself frequently using) is to comment out all
 instances of the class. Then GHC just says `no instance for `Foo a`
 arising from the use of `foo`` instead of recommending constraints from
 the matching instance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9948#comment:6>
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