For some recent supervision work on my Security lectures, I was given the task of decoding a string encrypted with a simple shift cipher. This cipher, given a key, simply moves each letter on in the alphabet by an amount given by the key, wrapping at the end. Being a good functional programmer, I decided to implement a brute force solver for this in Haskell, and I’m rather pleased by the terseness of the result, so I felt compelled to share it.

import List(elemIndex)

shift i x = (cycle ['A'..'Z'])!!(maybe (error "Bad character " ++ (show x))
                                       (+i) (elemIndex x ['A'..'Z']))
main = sequence_ (map (\i -> putStrLn $ map (shift i) cipherText) [0..25])

If you ignore the boilerplate, there’s only two lines of actual code there :-). I happen to think it’s rather understandable, though I’d be interested to get the opinion of someone who isn’t a Haskell hacker on the veracity of that statement. A hint you may need understanding it is that cycle a_list just returns the list which is a_list concatenated infinite times, and that !! is Haskells list indexing operator.

On this note, I saw an interesting blog post at retrospections the other day which compared the LOC count for a number of open source source control programs: Darcs (Haskell) and Mercurial (Python) were tied at 20KLOC, with the next best being Bazaar-NG (Python) at 47KLOC. Obviously this isn’t a strictly apples-to-apples comparison, though, so take it with a grain of salt!