Daniel McAllansmith
Unless I've missed it, there is no typeclass for positive integers in GHC. Is there any particular reason it doesn't exist?
Also, it seems Word would be a far better type in the likes of (!!), length, etc. Is it just tradition that resulted in the use of Int?
There is a good argument that what you really want here is the lazy infinite natural numbers. Think Peano: data Natural = Zero | Succ Natural The clear correspondance between lazy lists and the size/index of a lazy list makes this type fairly compelling. It can be jolly useful, particularly with infinite streams, to be able to compute (length xs > n) without forcing the evaluation of the list to more than n places. Of course, if the naturals were actually represented in Peano (unary) form, that would be somewhat inefficient, but there are various strategies that can be used to make the representation smaller whilst retaining their nice properties. Regards, Malcolm