> Haskell doesn't seem to allow
>
> > instance Num (Int->Int) where ...
>
> or
>
> > instance Stringable String where ...
Haskell requires you to write instances of the form
instance context => T a1..an where ...
where T is a type constructor and a1..an are type variables.
This is a conservative, but sound, design choice. Haskell 2,
some versions of Hugs, and GHC 3.02 and later, all accept more
instance declarations.
There's extensive discussion in
http://www.dcs.gla.ac.uk/~simonpj/multi.ps.gz
and a summary of the state of play in
http://www.dcs.gla.ac.uk/~simonpj/multi-param.html
Simon
