#7624: Handling ImplicitParams in Instance Declaration ---------------------------------+------------------------------------------ Reporter: philipjf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | Version: 7.6.1 Keywords: ImplicitParams | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Documentation bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Comment(by philipjf): Hm... I'm not sure that is different from what happen currently with local evidence and/or implicitParams {{{ {-# LANGUAGE GADTs, ImplicitParams #-} class C a where op :: a -> a data X a where X :: C a => X a fooGADT X xs = (length xs, op xs) fooIP xs ys = let ?op = const ys in (length xs, ?op xs) }}} GHC infers types for both of these terms, and has rules to do it. We have `fooGADT :: C [a] => X t -> [a] -> (Int, [a])` inferred, but this type will be rejected if we give it as an annotation because it has a non type variable argument, and is no more general than `fooGADT :: X [a] -> [a] -> (Int, [a])`. Actually, giving a type annotation is not good enough in the presence of local type information to determine behavior {{{ fooGADT' :: X [a] -> X [a] -> [a] -> (Int,[a]) fooGADT' X X xs = (length xs, op xs) }}} or the really unclear {{{ data X' a where X' :: (?op :: a -> a) => X' a fooGADT'' :: X' [a] -> X' [a] -> [a] -> (Int,[a]) fooGADT'' X' X' xs = (length xs, ?op xs) whatShouldThisDo :: [a] -> [a] -> (Int, [a]) whatShouldThisDo xs ys = let a = (let ?op = const ys in X') b = (let ?op = id in X') in fooGADT'' a b xs }}} The point is, I think this is a general problem, and not one unique to allowing implicits in instances. With orphan instnaces you don't need implicit params at all to make this problem show up. Allowing implicits in class constraints seems more suspect though. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7624#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler