ghci> :set -XOverloadedStrings ghci> import Data.Monoid ghci> let hdrs = H.fromList [("Accept", "text/plain"), ("Accept", "text/html")] ghci> let f (cntr, acc) _ val = (cntr+1, val <> ";" <> acc) ghci> H.foldl' f (0, "") hdrs (2,"text/html;text/plain;")