On Mon, 2008-11-10 at 18:50 +0000, Duncan Coutts wrote: [...]
If you meant, why is it allowed rather than banned then I guess the answer is because it is orthogonal. The rules naturally handle that case and there was no particular reason to ban it, even if it is somewhat unusual.
"Unusual?" This is the motivation of list comprehensions. In naive set theory, set comprehensions are one way of an equivalence between predicates and sets. It's the Cartesian product aspect that should be considered unusual if anything. The binding aspect of list generators corresponds to naming the parameters of the predicate and then the Cartesian product aspect is simply the fact that a binary predicate, say, is a unary predicate on a binary Cartesian product.