Generate markdown TOC for [Qiita](http://qiita.com/)

Toc.hs

module Main where
import System.Environment (getArgs)

main = getArgs >>= \(f:_) -> readFile f >>= putStrLn . fromString

fromString = unlines . reverse . tocStr . lines

tocStr input = tocStr' input (repeat 0) []
    where
    tocStr' [] _ rs = rs
    tocStr' (l:ls) counts rs
        | marks == 0 = tocStr' ls counts rs
        | otherwise  = let title = drop marks l in 
            tocStr' ls (upd marks counts) (concat [ replicate (marks - 1) ' ', "- [", title, "](#",show marks, "-", show ((counts !! marks) + 1), ")"]:rs) 
        where 
        marks = length (takeWhile ('#' ==) l)
        upd m cs = let (a, (b:bs)) = splitAt m cs in a ++ ((b+1):bs)