[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
       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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 Generating better type-error messages is a hoary chestnut, but Herbert
 brought my attention to
 [https://pay.reddit.com/r/haskell/comments/26tcrk/curious_with_a_bit_of_beginner_ranting_about_some/
 this thread on reddit], which has the following idea.

 At the moment, from
 {{{
 module Foo where
   addThree = \x -> x + 3 :: Int
   y = addThree $ Just 5
 }}}
 we get this error:
 {{{
 Foo.hs:2:20
     Couldn't match expected type `Int' with actual type `Maybe a0'
     In the return type of a call of `Just'
     In the second argument of `($)', namely `Just 5'
     In the expression: addThree $ Just 5
 }}}
 Maybe we could generate this instead
 {{{
 Foo.hs:2:20
   inferred: "Just 5" has type "Maybe a0"
   expected: second argument of "($)" must have type "Int"
   in the expression: addThree $ Just 5
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9173>
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