Re: [GHC] #11534: Allow class associated types to reference functional dependencies

8 Feb 2016

      #11534: Allow class associated types to reference functional dependencies
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Consider the rather similar definitions of A and B below:

 {{{#!hs

 {-# language FunctionalDependencies, TypeFamilies, MultiParamTypeClasses,
 GeneralizedNewtypeDeriving #-}

 class A s m | m -> s
 instance A () Int

 class B m where
   type S m :: *

 instance B Int where
   type S Int = ()

 }}}

 Now,

 {{{#!hs
 newtype X = X Int deriving (A ())
 }}}

 works, but

 {{{#!hs
 newtype X = X Int deriving (B)
 }}}

 complains

 {{{
  • Can't make a derived instance of ‘B X’
         (even with cunning GeneralizedNewtypeDeriving):
         the class has associated types
  • In the newtype declaration for ‘X’
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11534#comment:18
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