Data.ByteString は結構充実している

競プロ典型 90 問 84 日目 - There are two types of characters（★3）

import qualified Data.ByteString.Char8 as C

main = (C.getLine *> C.getLine) >>= print . sol

sol s = comb (C.length s)-f s

comb n = n*(n-1) `div` 2

f = sum . map (comb . C.length) . C.group