[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                  |           Status:  new
                  Type:  feature     |        Milestone:
  request                            |          Version:  7.8.3
              Priority:  low         |         Keywords:  holes
             Component:  Compiler    |     Architecture:  Unknown/Multiple
  (Type checker)                     |       Difficulty:  Unknown
            Resolution:              |       Blocked By:
      Operating System:              |  Related Tickets:
  Unknown/Multiple                   |
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by phadej):

 With `-fdefer-type-errors` GHC (both 7.8 and 7.10) tries a bit longer and
 finds the missing instance:

 {{{
 [1 of 1] Compiling Test             ( /Users/ogre/show.hs,
 /Users/ogre/show.o )

 /Users/ogre/show.hs:4:8: Warning:
     No instance for (Show a0) arising from a use of ‘show’
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
       instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
       instance Show Ordering -- Defined in ‘GHC.Show’
       instance Show Integer -- Defined in ‘GHC.Show’
       ...plus 22 others
     In the expression: show _h
     In an equation for ‘test’: test = show _h

 /Users/ogre/show.hs:4:13: Warning:
     Found hole ‘_h’
       with type: a0
     Where: ‘a0’ is an ambiguous type variable
     Relevant bindings include
       test :: String (bound at /Users/ogre/show.hs:4:1)
     In the first argument of ‘show’, namely ‘_h’
     In the expression: show _h
     In an equation for ‘test’: test = show _h
 }}}

 So should we make holeReporter to try to find //Dicts// errors too, if
 `-fdefer-type-errors` is not set? IMHO the //No instance// error is what
 we need here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9479#comment:5>
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