-- Factorial functions implemented in Haskell

-- recursive version defined by cases like in mathematics
fact1 :: Integer->Integer
fact1 0 = 1
fact1 1 = 1
fact1 n = n * fact1(n-1)

-- tail recursive version using where clause
fact2 :: Integer->Integer
fact2 0 = 1
fact2 n = tfact n 1 
 	  where tfact 1 m = m 
	        tfact n m = tfact (n-1) (m*n)

-- tail recursive version using a let clause
fact3 :: Integer->Integer
fact3 0 = 1
fact3 n = let tfact 1 m = m 
	      tfact n m = tfact (n-1) (m*n)
	  in
	    tfact n 1

-- using foldl over a list from 1..n
-- equivalent to a for loop in C
fact4 :: Integer->Integer
fact4 n = foldl (*) 1 [1..n]