[GHC] #13490: Ambiguous types with constraints on new variables

[GHC]

#13490: Ambiguous types with constraints on new variables
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             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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Comment (by simonpj):

 You are walking on thin ice.  Consider
 {{{
 f :: Eq [a] => [a] -> [a]
 f x = [x] == [x]
 }}}
 From the RHS we need `Eq [a]` which is provided by the type signature.
 But GHC could ''also'' solve `Eq [a]` by using `instance Eq a => Eq [a]`
 (which is usually the right thing to do).  Then it would need `Eq a`,
 ''which it cannot get from `Eq a`''.  To avoid this dead end GHC tries to
 solve "wanted" constraints from "givens" first.

 But what if it was
 {{{
 f :: (Eq b, b ~ [a]) => [a] -> [a]
 f x = [x] == [x]
 }}}
 Now it's not quite so obvious.  I would have expected this still to work,
 just, because equalities are always solved first.  But the ice is a bit
 thinner.

 I would have expected your example to work too, just.  And so it does in
 HEAD, and hence in 8.2.  But it'd be much more robust to put `(Typeable
 z)` in the context.  Is there some reason you can't do that?

 I'll add your example as a regression test and close as 'worksforme'.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13490#comment:1>
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