#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: | ---------------------------------+------------------------------------------ Changes (by simonpj): * difficulty: => Unknown Comment: I think the issue is this. Consider this: {{{ class C a where op :: a -> a instance (?x:[a]) => C [a] where op _ = ?x foo y xs = let ?x = [y] in (length xs, op xs) }}} From the instance declaration we get a dictionary-constructing function with type {{{ dfunCList :: forall a. (?x:a) -> C [a] }}} What type should be inferred for `foo`? I can think of three. I give there with their translation into System F so you can see what proof strategy I've used to solve the constraints. {{{ foo1 :: C [a] => b -> [a] -> [a] foo1 a (d:C [a]) (y:b) (xs:[a]) = let _x = [y] in (length a xs, op a d xs) foo2 :: (?x:a} => b -> [a] -> [a] foo2 a b (?x:a) (y:b) (xs:[a]) = let _x = [y] in (length a xs, op a (dfunCList a ?x) xs) foo3 :: a -> [a] -> [a] foo3 a (y:a) (xs:[a]) = let ?x = [y] in (length a xs, op a (dfunCList a ?x) xs) }}} Notice that there is no most-general type. It all depends when we make use of the `dfunCList` dictionary construction, and, even when we have done so, whether we take advantage of the local binding for `?x`. I suppose we could just say "GHC will randomly decide which of these three to pick, but its hardly satisfactory! Adding implicit parameters as superclasses is at least as problematic, perhaps more so. In short, I'm unconvinced! Simon -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7624#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler