[GHC] #9479: Report required constraints when reporting the type of a hole

[GHC]

#9479: Report required constraints when reporting the type of a hole
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        Reporter:                    |                Owner:
  dominiquedevriese                  |
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.8.3
  checker)                           |
      Resolution:                    |             Keywords:  holes
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 thomie):

 This is fixed in HEAD.

 {{{
 $ ghc-7.11.20151213 Test.hs
 [1 of 1] Compiling Test             ( Test.hs, Test.o )

 Test.hs:4:8: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘show’
       prevents the constraint ‘(Show a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance Show Module -- Defined in ‘GHC.Show’
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show TrName -- Defined in ‘GHC.Show’
         ...plus 25 others
         (use -fprint-potential-instances to see them all)
     • In the expression: show _h
       In an equation for ‘test’: test = show _h

 Test.hs:4:13: error:
     • Found hole: _h :: a0
       Where: ‘a0’ is an ambiguous type variable
       Or perhaps ‘_h’ is mis-spelled, or not in scope
     • In the first argument of ‘show’, namely ‘_h’
       In the expression: show _h
       In an equation for ‘test’: test = show _h
     • Relevant bindings include test :: String (bound at Test.hs:4:1)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9479#comment:8>
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