[GHC] #8592: FunDep error message regression

3 Dec 2013

      #8592: FunDep error message regression
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       Reporter:  nomeata                  |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.7
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 This code (provided by Cale Gibbard)
 {{{
 #!haskell
 {-# LANGUAGE FunctionalDependencies, FlexibleContexts #-}

 class C a b | a -> b where
   foo :: a -> b

 instance C Integer Integer where
   foo = id

 f :: Integer -> String
 f = foo
 }}}
 produced this error message in GHC 7.6:
 {{{
 fundep.hs:10:5:
     Couldn't match type `Integer' with `String'
     When using functional dependencies to combine
       C Integer Integer,
         arising from the dependency `a -> b'
         in the instance declaration at fundep.hs:6:10
       C Integer String, arising from a use of `foo' at fundep.hs:10:5-7
     In the expression: foo
     In an equation for `f': f = foo
 }}}
 but this one with GHC HEAD
 {{{
 FunDepError.hs:11:5:
     No instance for (C Integer String) arising from a use of ‛foo’
     In the expression: foo
     In an equation for ‛f’: f = foo
 }}}
 which seems to be a regression.

 It may be that either message is fine (it seems to be in this case), In
 that case, this bug is about the test suite not having a test case for
 where we do want to see the `When using functional dependencies to
 combine` message.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8592
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