[GHC] #9227: Coverage Condition cannot be turned off

23 Jun 2014

      #9227: Coverage Condition cannot be turned off
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       Reporter:  augustss          |             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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 Compile this program
 {{{
 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
 module Bug1 where

 class C a b | a -> b

 data A a = A
 data B a = B

 instance C (A a) (B b)
 }}}
 and observe the error message
 {{{
 Bug1.hs:9:10:
     Illegal instance declaration for `C (A a) (B b)'
       The coverage condition fails in class `C'
         for functional dependency: `a -> b'
       Reason: lhs type `A a' does not determine rhs type `B b'
     In the instance declaration for `C (A a) (B b)'
 }}}
 Then read manual, sec 7.6.3.3:
 {{{
 Both the Paterson Conditions and the Coverage Condition are lifted if you
 give the -XUndecidableInstances flag
 }}}
 Compile again with -XUndecidableInstances.  Observe the exact same error
 message.

 Either the compiler or the documentation is wrong.  I certainly hope it is
 the compiler, because it worked in 7.6 and was quite useful.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9227
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