non-question, instance decl

[Ketil Z. Malde]

Hi,

experimenting with QuickCheck, I write

>   instance (Arbitrary e) => Arbitrary (Array Int e) where
>        arbitrary = undefined

and I get:

|    Illegal instance declaration for `Arbitrary (Array Int e)'
|	(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)
|    In the instance declaration for `Arbitrary (Array Int e)'
|Failed, modules loaded: QuickCheck.

Okay, after experimenting a bit (more fun than reading documentation)
I see what's happening:  I can't instantiate a halfway specialized
type, but 'Ix i => Arbitrary (Array i e)' is Okay.

Unfortunately, I really depend on i being Int in my (real) definition
of 'arbitrary'.  I got around it by liberal doses of 'fromInteger' and
a 'Num' context -- ugly, but it seems to work.

I suppose there's a solid reason why my previous attempt is illegal?

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants