Evan Martin (evan) wrote in evan_tech,
Evan Martin


Beautiful Haskell implementation of math's "power set":
import Control.Monad

powerset :: [a] -> [[a]]
powerset = filterM (const [True, False])

And powerset [1,2,3] produces [[1,2,3],[1,2],[1,3],[1],[2,3],[2],[3],[]].

Oh man is that hard for me to grok, mostly because I haven't internalized the list monad.
(It helped me to write out the definition of filterM, which has type Monad m => (a -> m Bool) -> [a] -> m [a].)

But roughly, it's saying "for each assignment of true and false over the values of the list, grab the true values".

[via Adam Langley via a private mailing list]
Tags: haskell

Recent Posts from This Community

  • blog moved

    As described elsewhere, I've quit LiveJournal. If you're interested in my continuing posts, you should look at one of these (each contains feed…

  • dremel

    They published a paper on Dremel, my favorite previously-unpublished tool from the Google toolchest. Greg Linden discusses it: "[...] it is capable…

  • treemaps

    I finally wrote up my recent adventures in treemapping, complete with nifty clickable visualizations.

  • Post a new comment


    default userpic
    When you submit the form an invisible reCAPTCHA check will be performed.
    You must follow the Privacy Policy and Google Terms of use.