Factorial in Haskell

Example for versions GHC 6.10.4

This example performs a “fold” using the function foldl (which is a left-fold, but since multiplication is associative, left fold and right fold are the same) to fold multiplication over the list [1..n]. We provide the “initial value” 1, so that it will produce 1 when the list is empty. 

module Main where

factorial :: Integer -> Integer
factorial n = foldl (*) 1 [1..n]

line x = putStrLn $ show x ++ "! = " ++ factorial x

main = mapM_ line [0..16]