[GHC] #9173: Better type error messages

[GHC]

#9173: Better type error messages
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
                                     |  TypeErrorMessages
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by vanto):

 I'm coming back about the solution provided by rwbarton.
 This solution is not appropriate in the case below.
 {{{
 let y = [True, 'a']
 }}}
 What is the actual type? What is the expected type?\\

 If I change the order of the values in the list like this:
 {{{
 let y = ['a', True]
 }}}
 What is the actual type? What is the expected type?\\\\

 An obvious answer is {{{The types must be identical in a list}}} or {{{The
 types are not equal}}}
 Here we see that there is no signature. If the signature had been {{{y ::
 [Char]}}}
 The obvious answer is the one I have given in comment 23, and which is
 {{{
 The Type of the result is not in accordance with the Type of the signature
 }}}
 So I no longer agree with rwbarton and bgamari.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9173#comment:27>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler