[GHC] #15244: Ambiguity checks in QuantifiedConstraints

[GHC]

#15244: Ambiguity checks in QuantifiedConstraints
-------------------------------------+-------------------------------------
        Reporter:  bitwiseshiftleft  |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
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):

 {{{
 bar :: (forall t . Eq (a t), forall t . Eq (b t), a ~ b) => a t -> b t ->
 Bool
 }}}
 You'd never write this, correct?  Because if `a~b` then the two quantified
 constraints are identical.

 But your point is that in
 {{{
 class (forall t . Eq (c t)) => Blah c

 baz :: (Blah a, Blah b, a ~ b) => a t -> b t -> Bool
 }}}
 you can't avoid having two identical superclass constraints.

 In general if we have two quantified constraints
 {{{
 forall a. Q1 => t a
 forall a. Q2 => t a
 }}}
 and we have to solve `t Int`, say, GHC doesn't know which to pick.
 (Solving Q1 might be easier than Q2; we don't know.)  So it picks neither.
 And that is what is happening here.

 But in this case the quantified constraints are ''identical''; but GHC
 doesn't currently spot that.  It would not be hard to combine identical
 quantified constraints, which would solve this problem.

 Is this an application that matters to you?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15244#comment:2>
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