[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 facundo.dominguez):

 > Are you saying that GHC 7.8's error message is better because it
 mentions b rather than b0?

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

 In contrast:
 {{{
 No instance for (Typeable b0) arising from a use of ‘second’
 }}}
 gives no clue of what the problem is. How does the user infer from this
 that the instance `First Int b Int` is missing? Consider that:

 * Even if `b0` is renamed to `b`, the user stays wondering which of all
 the `b`s in the program it might be. The class hierarchy might have
 multiple levels.
 * Even if we understood which class is introducing the constraint, we
 might not know which instantiation of the other type class parameters is
 being attempted.

 > What would you LIKE it to say?

 {{{
 No instance for (Typeable b0) arising from a use of ‘second’
 from instance 'Second Int Int' which needs missing instances
 'Typeable b' and 'First Int b Int'.
 }}}
 If there are more levels in the class hierarchy, the whole path to the
 missing instances should be reported from the method that the user called.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13582#comment:4>
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