[GHC] #9613: when giving an error "No instance for C (a -> b)", suggest that a function may be underapplied

[GHC]

#9613: when giving an error "No instance for C (a -> b)", suggest that a function
may be underapplied
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              Reporter:  rwbarton    |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by MikeIzbicki):

 This is roughly equivalent to the patch I suggest in:
 https://phabricator.haskell.org/D201 (TcErrors.lhs, line 1094ish).

 The difference is that my patch doesn't suggest the underapplied function
 name or the instances that do exist.  It seems pretty straightforward to
 mention the existing instance, so I'll try to add that.  I'm pretty sure
 it's in general impossible to name the argument.  For example, in `show
 (length . head . tail)` which function do you say is underapplied?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9613#comment:1>
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