[GHC] #9605: Misleading error message with forgotten "do"

[GHC]

#9605: Misleading error message with forgotten "do"
-------------------------------------+-------------------------------------
              Reporter:  owst        |            Owner:  Yuras
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #7869
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D556   |
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Comment (by Austin Seipp <austin@…>):

 In [changeset:"09b7943321f89b945d10f8a914f4c2cbf73dff91/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="09b7943321f89b945d10f8a914f4c2cbf73dff91"
 fix misleading error message regarding function arity

 Summary:
 The error reports something like:

   The function ‘f’ is applied to three arguments,
   but its type ‘Int -> Float -> Char -> Bool’ has only three

 The original type was "Monad m => Int -> Float -> m Bool", but
 "m" was unified with "-> Char".

 Now it looks better:

   The function ‘f’ is applied to three arguments,
   its type is ‘Int -> Float -> m0 Bool’,
   it is specialized to ‘Int -> Float -> Char -> Bool’

 Test Plan: T9605

 Reviewers: simonpj, austin

 Reviewed By: austin

 Subscribers: carter, thomie

 Differential Revision: https://phabricator.haskell.org/D556

 GHC Trac Issues: #9605
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9605#comment:3>
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