[only replying to haskell-cafe]
On 10/20/07, Rodrigo Geraldo
Hi!
Suppose that the GHC's flag -fallow-incoherent-instances is enabled. In this situation, when a instance will be rejected? And if the flag -fallow-overlapping-instances is enabled. When a instance will be rejected?
Thanks!
Rodrigo
The following is mainly from the GHC Userguide: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html... Suppose you have: {-# OPTIONS_GHC -fglasgow-exts -fallow-overlapping-instances #-} class C a b where foo :: a -> b -> (a, b) instance C Int a where foo n x = (n+1, x) -- (A) instance C a Bool where foo x b = (x, not b) -- (B) instance C Int [a] where foo n xs = (n+1, xs) -- (C) instance C Int [Int] where foo n ns = (n+1, map (+1) ns) -- (D) f :: [b] -> [b] f xs = snd $ foo (1 :: Int) xs In the right hand sight of 'f', 'foo' is applied to an Int and a [b] so it seems that instance C should match. However GHC rejects this program because in a later call 'f' can be applied to a list of Ints (like in: g = f ([1,2,3] :: [Int])) by which 'b' instantiates to an Int, by which instance D should really match. If you enable -fallow-incoherent-instances then 'f' will use instance C without complaining about the problem of subsequent instantiations. However if you then define 'g' you will get the error: Couldn't match expected type `Int' against inferred type `[a]' In the first argument of `f', namely `([1, 2, 3] :: Int)' regards, Bas.