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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
