[Haskell-cafe] generalized list comprehensions

[Jonathan Cast]

On Mon, 2008-11-10 at 16:06 -0600, Derek Elkins wrote:
> 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.

Well, the Cartesian product case is one way of an equivalence between
relations and sets of pairs.  So I don't think it

    [ (x, y) | x <- xn, y <- ys ]

is any more unusual than

    [ x | x <- xn ]

jcc