#9210: "overlapping instances" through FunctionalDependencies -------------------------------------------+------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.8.1 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- This program prints `("1",2)`, but if you reverse the order of the two instances, it prints `(1,"2")`. {{{ {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} -- extracted from http://lpaste.net/105656 import Control.Applicative import Data.Functor.Identity modify :: ((a -> Identity b) -> s -> Identity t) -> (a -> b) -> (s -> t) modify l f s = runIdentity (l (Identity . f) s) class Foo s t a b | a b s -> t where foo :: Applicative f => (a -> f b) -> s -> f t instance Foo (x, a) (y, a) x y where foo f (a,b) = (\fa -> (fa,b)) <$> f a instance Foo (a, x) (a, y) x y where foo f (a,b) = (\fb -> (a,fb)) <$> f b main = print $ modify foo (show :: Int -> String) (1 :: Int, 2 :: Int) }}} Note that the two instances involved `Foo (Int, Int) (String, Int) Int String` and `Foo (Int, Int) (Int, String) Int String` are not actually overlapping. But, they have the same `a`, `b`, and `s` fields and it seems that this makes GHC think that either one is equally valid, thanks to the fundep. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9210 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler