{-# LANGUAGE ScopedTypeVariables #-}
without, the 'f's in the instance are independent.
Claus
----- Original Message -----
From: "Jacques Carette"
I was playing with some of Oleg's code (at end for convenience). After minor adjustments for ghc 6.10.1, it still didn't work. The error message is quite puzzling too, as it suggests adding exactly the constraint which is present... Any ideas?
Jacques
-- Oleg's definition of a vector space class, based on IsFunction and -- TypeCast. See http://okmij.org/ftp/Haskell/isFunction.lhs -- for the January 2003 message, which works in GHC 6.2.1 and 6.4 -- code below *works* in 6.8.1 AFAIK {-# LANGUAGE EmptyDataDecls, MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-}
module Q where
class Vspace a v | v -> a where (<+>) :: v -> v -> v (*>) :: a -> v -> v
instance (IsFunction v f, Vspace' f a v) => Vspace a v where (<+>) = doplus (undefined::f) (*>) = dostar (undefined::f)
class Vspace' f a v | f v -> a where doplus :: f -> v -> v -> v dostar :: f -> a -> v -> v
instance Num a => Vspace' HFalse a a where doplus _ = (+) dostar _ = (*) -- etc. No problem.
instance (IsFunction v f, Vspace' f a v, Vspace a v) => Vspace' HTrue a (c->v) where doplus _ f g = \x -> f x <+> g x dostar _ a f x = a *> (f x)
test1 = (1::Int) <+> 2 test2 = ((\x -> x <+> (10::Int)) <+> (\x -> x <+> (10::Int))) 1 test3 = ((\x y -> x <+> y) <+> (\x y -> (x <+> y) <+> x)) (1::Int) (2::Int)
test4 = ((\x y -> x <+> y) <+> (\x y -> ((2 *> x) <+> (3 *> y)))) (1::Int) (2::Int)
data HTrue data HFalse
class IsFunction a b | a -> b instance TypeCast f HTrue => IsFunction (x->y) f instance TypeCast f HFalse => IsFunction a f
-- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x
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