[GHC] #12923: MultiParamTypeClasses + ExtendedDefaultRules

4 Dec 2016

      #12923: MultiParamTypeClasses + ExtendedDefaultRules
-------------------------------------+-------------------------------------
           Reporter:  amindfv        |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 -XExtendedDefaultRules allows us to avoid ambiguity for unannotated
 numeric literals:

 {{{#!hs

 {-# LANGUAGE ExtendedDefaultRules #-}

 data A = A deriving Show

 class ToA a where
    toA :: a -> A

 instance ToA Double where
    toA _ = A

 main = print (toA 5 :: A)

 }}}

 But if we have a multi-param typeclass, -XExtendedDefaultRules doesn't
 help us:

 {{{#!hs

 {-# LANGUAGE ExtendedDefaultRules #-}

 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}

 data A x = A deriving Show

 class ToA a x where
    toA :: a -> A x

 instance ToA Double x where
    toA _ = A

 main = print (toA 5 :: A Bool)

 }}}

 It would be really nice for an EDSL I'm working on to be able to use
 extended defaults here!

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12923
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