[Haskell-cafe] Combinations

[Henning Thielemann]

On Tue, 6 Jun 2006 developer at imaginando.net wrote:

> Hi,
>
> I need a functions which takes as argument a list of lists like this one:
>
> [[1,2],[3],[4]]
>
> and gives me a list of list with all the possible combinations like this one:
>
> [[1,3,4],[2,3,4]]
>
> In this case there are only 2 combinations but if there was more than one
> element in the other lists the results were much more.

Using the List instance of monads you can simplify things.

  foldr (Monad.liftM2 (:)) [[]] [[1,2],[3],[4]]


> I'm a little stuck in this simple problem.
>
> First i was thinking in cartesian product but then i realized that i may
> not be what i really need.

cartesian product can be computed by
  liftM2 (,)