choose.hs

module Main where

import qualified Data.Char

main = print go

go =
  let l = ['a', 'b', 'a', 'A', 'b', 'B']
  in foldr choose [] l
  where
    choose ch xs
     | null xs = [ch]
     | Data.Char.toLower ch == Data.Char.toLower (head xs) = tail xs
     | otherwise = ch : xs