import Data.List (isPrefixOf)

mapping = [("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) <- mapping, isPrefixOf n str ]