[GHC] #3440: Improve error message for GADT failures

[GHC]

#3440: Improve error message for GADT failures
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    Reporter:  simonpj                  |        Owner:                                 
        Type:  bug                      |       Status:  merge                          
    Priority:  low                      |    Milestone:  7.0.1                          
   Component:  Compiler (Type checker)  |      Version:  6.10.4                         
    Keywords:                           |     Testcase:  indexed-types/should_fail/T3440
   Blockedby:                           |   Difficulty:  Unknown                        
          Os:  Unknown/Multiple         |     Blocking:                                 
Architecture:  Unknown/Multiple         |      Failure:  None/Unknown                   
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Changes (by simonpj):

  * status:  new => merge
  * testcase:  => indexed-types/should_fail/T3440


Comment:

 On reflection, the error message could be better thus
 {{{
 T3440.hs:11:22:
     Could not deduce (a1 ~ a) from the context (Fam a ~ Fam a1)
       `a1' is a rigid type variable bound by
            a pattern with constructor
            GADT :: forall a. a -> Fam a -> GADT (Fam a)
              at T3440.hs:11:9
       `a' is a rigid type variable bound by
           the type signature for `unwrap' at T3440.hs:10:21
 }}}
 Patch
 {{{
 Mon Nov  1 16:46:30 GMT 2010  simonpj at microsoft.com
   * Improve error messages

   In particular, instead of
      Cannot match 'a' with 'b'
   we get
      Could not deduce (a~b) from context (F a ~ b)
   or whatever

     M ./compiler/typecheck/TcErrors.lhs -19 +27
 }}}
 And testsuite
 {{{
 Mon Nov  1 16:51:07 GMT 2010  simonpj at microsoft.com
   * Follow change in typechecker error messages

     M ./tests/ghc-regress/indexed-types/should_fail/T4093a.stderr -7 +7
     M ./tests/ghc-regress/indexed-types/should_fail/T4093b.stderr -1 +3
     M ./tests/ghc-regress/indexed-types/should_fail/T4254.stderr -7 +7
     M ./tests/ghc-regress/typecheck/should_compile/FD1.stderr -1 +2
     M ./tests/ghc-regress/typecheck/should_compile/FD2.stderr -1 +2
     M ./tests/ghc-regress/typecheck/should_compile/T2494.stderr -1 +1
     M ./tests/ghc-regress/typecheck/should_fail/T2688.stderr -1 +1
     M ./tests/ghc-regress/typecheck/should_fail/tcfail068.stderr -29 +29
     M ./tests/ghc-regress/typecheck/should_fail/tcfail131.stderr -1 +1
 }}}
 I'm easy about whether this merges into the branch or not.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3440#comment:5>
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