[GHC] #13582: Confusing error message with multiparameter type classes.

[GHC]

#13582: Confusing error message with multiparameter type classes.
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        Reporter:                    |                Owner:  (none)
  facundo.dominguez                  |
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  type-checking
                                     |  errors multiparameter type classes
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 simonpj):

 >  No. It is better because it gives you a clue of how to fix it: Add an
 instance of First Int b Int.

 Alas, all that will happen then is that the missing `Typeable b`
 constraint will be reported.  There really are two missing constraints,
 but GHC tries to avoid saturating the user by reporting only one per birth
 site.

 Reporting the entire path to a constraint is quite painful/voluminous, but
 I can see the point.  Good ideas in here for better error messages.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13582#comment:5>
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