S. Alexander Jacobson <[EMAIL PROTECTED]> wrote
> Haskell doesn't seem to allow
>
>> instance Num (Int->Int) where ...
> or
>> instance Stringable String where ...
>
>How come?
>
>PS I am sure this has been discussed before, but I missed it...
First, Num, needs Eq to be defined. Consider
module T where
instance Eq (Int->Int) where f==g = (f 0)==(g 0)
instance Num (Int->Int) where f+g = \x->(f x)+(g x)
Second, some compilers say at this:
"
(the instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
"
Maybe, they treat Int Int as a repeated variable?
At least, we may try to use some extension of Haskell-1.4.
For example in ghc, it helps -fglasgow-exts.
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
