On Thu, May 20, 2010 at 06:35:34AM -0400, Walck, Scott wrote:
Hi folks,
NewVectorShort.hs:19:0: Type synonym `Vector2D' should have 1 argument, but has been given 0 In the instance declaration for `BasicVector Vector2D' Failed, modules loaded: none.
The problem is simply that type synonyms must always be fully applied, so given type Vector2D a = (a,a) you cannot declare an instance for Vector2D, since Vector2D is not applied to an argument. The solution is to make Vector2D a newtype: newtype Vector2D a = V2D (a,a) Of course, this means you'll need to wrap and unwrap V2D constructors in various places, which can be a bit annoying, but such is the price of progress. For another take on encoding vector stuff in Haskell, see the vector-space package on Hackage. -Brent
I don't understand how what I'm trying to do is different from, say, the Monad instance for Maybe. (Maybe a) is a type, and (Vector2D a) is a type.
Thanks,
Scott
{-# LANGUAGE TypeSynonymInstances #-}
infixl 6 <+> infixl 6 <-> infixl 7 *> infixl 7 <*
class BasicVector v where (<+>) :: v a -> v a -> v a (<->) :: v a -> v a -> v a (*>) :: Num a => a -> v a -> v a (<*) :: Num a => v a -> a -> v a v1 <-> v2 = v1 <+> fromInteger (-1) *> v2 v1 <* c = c *> v1 c *> v1 = v1 <* c
type Vector2D a = (a,a)
instance BasicVector Vector2D where (ax,ay) <+> (bx,by) = (ax+bx,ay+by) c *> (ax,ay) = (c*ax,c*ay)
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