[Haskell-cafe] Problem with fundeps.

[Henning Thielemann]

On Sun, 2 Jan 2005 karczma at info.unicaen.fr wrote:

> I am afraid that something is wrong with my understanding of multi-
> param classes with dependencies. I tried to generalize one of my old
> packages for quantum *abstract* computations, where state vectors are
> defined as functional objects, whose codomain has some arithmetic.
> It is easy to see that you can define (f <+> g) = \x -> f x + g x
> etc. It should be possible to curry this further, so I defined 
> 
> class Vspace a v | v -> a
> where
>  (<+>) :: v -> v -> v
>  (*>)  :: a -> v -> v
>  -- etc. 
> 
> instance Vspace a a where
>  (<+>) = (+)
>  (*>)  = (*)
>  -- etc.  No problem. 
> 
> instance (Vspace a v) => Vspace a (c->v) where
>  f <+> g = \x -> f x <+> g x
>  (a *> f) x = a *> (f x)
>  -- ... 

I had the same problem with the same class
 http://www.haskell.org//pipermail/haskell-cafe/2004-March/005979.html
  and thus Dylan Thurston advised me to drop functional dependencies. 
Here are two implementations using multi-type classes without functional
dependencies. (They wait for unification, yet, sorry.) 
 http://cvs.haskell.org/darcs/numericprelude/VectorSpace.lhs
 http://cvs.haskell.org/darcs/numericprelude/physunit/VectorSpace.hs