[GHC] #9173: Better type error messages

[GHC]

#9173: Better type error messages
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        Reporter:  simonpj           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by schyler):

 Here's an error that a noob making a subtle typo might get from GHC:

 {{{
 Prelude> map 1 [1..5]

 <interactive>:2:1:
     Could not deduce (Num (a0 -> b))
       arising from the ambiguity check for ‛it’
     from the context (Num (a -> b), Num a, Enum a)
       bound by the inferred type for ‛it’:
                  (Num (a -> b), Num a, Enum a) => [b]
       at <interactive>:2:1-12
     The type variable ‛a0’ is ambiguous
     When checking that ‛it’
       has the inferred type ‛forall a b.
                              (Num (a -> b), Num a, Enum a) =>
                              [b]’
     Probable cause: the inferred type is ambiguous
 }}}

 I wholeheartedly agree that the information needs to be presented in a way
 that's easier to digest.

 For what should be a really simple error, I really have to stare that down
 to understand what it's actually trying to say ...

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9173#comment:3>
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