[GHC] #12791: Superclass methods could be more aggressively specialised.

[GHC]

#12791: Superclass methods could be more aggressively specialised.
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This is nothing to do with specialisation. It has to do with how the
 constraint solver solves constraints.  But it's not easy to fix you
 problem.

 GHC's constraint solver uses the "given" constraint (here `Num Int` via a
 superclass of `C Int b`) where possible.  You may say "If there is an
 instance declaration, use that instead of the given constraint.  But no
 {{{
 f :: Ord [a] => ...
 f x = ..Need Eq [a]...
 }}}
 There is a top-level instance for `Eq [a]`, but if we use it we'll need
 `Eq a` and we haven't got that.  So we must satisfy the `Eq [a]` from the
 superclass of `Ord [a]`.

 I suppose we could make a special case when the instance declaration does
 not generate any new constraints, as is the case for `Num Int`.  Would
 that deal with your "ton of cases"?  Can you give more concrete examples?

 I worry that it might not be long before someone complains that GHC is
 bypassing the dictionary they have passed in.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12791#comment:5>
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