#12923: MultiParamTypeClasses + ExtendedDefaultRules -------------------------------------+------------------------------------- Reporter: amindfv | Owner: Type: feature request | Status: patch Priority: normal | Milestone: 8.2.1 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): Phab:D2822 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): On Phab you comment that this program is still rejected with ambiguous variables. {{{ {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} module T12924 where import GHC.TypeLits data A (b :: [Symbol]) = A deriving Show class Works a (b :: [Symbol]) where works :: a -> A b instance Works Integer a where works _ = A addA :: A a -> A a -> A a addA A A = A test2 :: A x test2 = addA (works 5) (works 5) }}} The reason is described in `Note [ApproximateWC]` in `TcSimplify`, item (2) in that note. It arose from Trac #8155. We have a constraint {{{ forall x. () => Num alpha, Works alpha x }}} but because we carefully make the `Works alpha x` prevent the `Num alpha` float out for defaulting, for reasons described in the Note. The rule is un-documented and indeed hard to explain. I'm quite inclined to back-pedal on the fix to #8155. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12923#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler