Parsing roman numerals in Haskell - from http://rosettacode.org/wiki/Roman_numerals/Decode#Haskell. See also https://gist.github.com/davidallsopp/ee1c52a32ed9da393529

unroman.hs

import Data.List (isPrefixOf)
 
numerals = [("M",1000),("CM",900),("D",500),("CD",400),("C",100),("XC",90),
           ("L",50),("XL",40),("X",10),("IX",9),("V",5),("IV",4),("I",1)]
 
toArabic :: String -> Int
toArabic "" = 0
toArabic str = num + toArabic xs
    where (num, xs):_ = [ (num, drop (length n) str) | (n,num) <- numerals, isPrefixOf n str ]
    
-- Seems gratuitously unclear for the sake of a one-liner. How about:

toArabic2 :: String -> Int
toArabic2 "" = 0
toArabic2 str = n + toArabic xs
    where xs = drop (length ch) str
          Just (ch, n) = find ((`isPrefixOf` str) . fst ) numerals

The :_ = pattern match is the same as taking the head.