[GHC] #14942: QuantifiedConstraints: GHC can't infer

[GHC]

#14942: QuantifiedConstraints: GHC can't infer
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:  invalid           |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
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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Changes (by simonpj):

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


Comment:

 > If we comment out together's type signature GHC cannot infer it back,
 shouldn't it be able to though?

 No, that's by design.  Currently GHC makes no attempt to ''infer'' a type
 with a quantified constraint.  It's not impossible to do so -- even fairly
 easy -- but I worry that they'd then pop up unexpectedly in programs where
 there was simply a type error.

 For now, write a type signature; but re-raise it if that seems to be
 becoming onerous.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14942#comment:1>
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