module Main (main) where          -- not needed in interpreter, is the default in a module filemain :: IO ()                     -- the compiler can infer this type definitionmain = putStrLn "Hello, World!"

-- Type annotation (optional, same for each implementation)factorial :: (Integral a) => a -> a-- Using recursion (with the "ifthenelse" expression)factorial n = if n < 2              then 1              else n * factorial (n - 1)-- Using recursion (with pattern matching)factorial 0 = 1factorial n = n * factorial (n - 1)-- Using recursion (with guards)factorial n   | n < 2     = 1   | otherwise = n * factorial (n - 1)-- Using a list and the "product" functionfactorial n = product [1..n]-- Using fold (implements "product")factorial n = foldl (*) 1 [1..n]-- Point-free stylefactorial = foldr (*) 1 . enumFromTo 1