#12923: MultiParamTypeClasses + ExtendedDefaultRules -------------------------------------+------------------------------------- Reporter: amindfv | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by amindfv: @@ -44,2 +44,3 @@ - It would be really nice for an EDSL I'm working on to be able to use - extended defaults here! + It would be really nice for an EDSL of mine to be able to use extended + defaults here! At the moment my only option has been to use incoherent + instances, which has resulted in incoherent behavior... New description: -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 of mine to be able to use extended defaults here! At the moment my only option has been to use incoherent instances, which has resulted in incoherent behavior... -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12923#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler