main :: [sys_message]
main = [ Stdout (show n ++ ": " ++ toroman n ++ "\n")
       | n <- [1990, 2008, 1666, 2023]]

toroman :: num->[char]
toroman 0 = ""
toroman n = d ++ toroman (n - v)
            where digits = [("M",1000),("CM",900),("D",500),("CD",400),
                            ("C",100),("XC",90),("L",50),("XL",40),
                            ("X",10),("IX",9),("V",5),("IV",4),
                            ("I",1)]
                  (d, v) = hd [(d,v) | (d,v) <- digits; v <= n]