Parallel list comprehensions

[Andrew Pimlott]

On Sat, Feb 04, 2006 at 06:31:56PM +0000, Jon Fairbairn wrote:
> There ought to be a list_product somewhere (I mean [1..]
> `list_product` [4..] ==
> [(1,4),(2,4),(1,5),(3,4),(2,5),(1,6),...]). Is there?

This is called "fair conjunction" in "Backtracking, Interleaving, and
Terminating Monad Transformers" [1].

    (>>-) :: [a] -> (a -> [b]) -> [b]
    []     >>- f = []
    (x:xs) >>- f = interleave (f x) (xs >>- f)

    interleave :: [a] -> [a] -> [a]
    interleave [] ys     = ys
    interleave (x:xs) ys = x : interleave ys xs

    t = take 6 ([1..] >>- \x -> 
                [4..] >>- \y -> 
                [(x, y)])

Andrew

[1] http://okmij.org/ftp/Computation/monads.html#LogicT