{- This is a simple factorial program which uses the I/O system
    to read the input and print the result -}
 
 module Main where
 
 fact :: Integer -> Integer    
 fact 0 = 1
 fact (n+1) = (n+1)*fact n
 fact _ = error "Negative argument to factorial"
 
 main = appendChan stdout "Type in N: " abort $
        readChan stdin abort $ \ input ->
        appendChan stdout (show (fact (read (head (lines input))))) abort done