foldl' is:exact -package:containers -package:base -package:vector package:snap-core

Strict left fold over all key-value pairs in the headers map. Example:

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;")