Evan Martin (evan) wrote in evan_tech,
Evan Martin
evan
evan_tech

run length encoding and arrows

Run length encoding in Haskell casually tosses in this bit:
encode = map (length &&& head) . group
Which produces RLE'd tuples:
> encode "aaaabbaaa"
[(4,'a'),(2,'b'),(3,'a')]


But what's this &&& bit? It's from Control.Arrow, the implementation of arrows, which are reportedly a generalization of monads. The last time I tried to look at this was before I grokked monads so it's probably worth another look, but the superficial understanding needed here is that &&& runs two functions "in parallel" (not as in simultaneously, just in that they're not interacting) and produces a pair of their two outputs.

That is,
> :t length &&& head
length &&& head :: [a] -> (Int, a)

where the first component of the tuple is the length and the second is the head of the list.

What a peculiar but also imaginably useful operator! (Of course, in the arrows docs the application of this just to functions is the most basic of the many generalizations.)
Tags: haskell
Subscribe

  • megaupload captcha

    Someone make a Javascript-based captcha cracker for megaupload. It's strange to see those captchas again because I idly myself wrote a…

  • zombie ghosd

    I was tickled to discover another IBM developerworks article on one of my abandoned hacks and that both it and its predecessor have been translated…

  • gat, a git clone in haskell

    I've been pretty busy with work lately, so I may as well dump this on the internet before it gets too dusty. Though I think I understand Git decently…

  • Post a new comment

    Error

    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.
  • 6 comments