sprintf.hs

import Data.List (isPrefixOf)

(%) :: String -> [String] -> String
"" % _ = ""
s % xs | "%s" `isPrefixOf` s = next ++ (drop 2 s) % queue
       | otherwise = (head s) : (tail s) % xs
       where next = head xs
             queue = tail xs

main = print ("Hello, %s! My name is %s!" % ["World", "Alex"])