-- The factorial function:
fac :: Int -> Int
fac n = if n == 0 then 1
                  else n * fac (n - 1)

-- The last element of a list:
last xs | xs == _ ++ [e] = e
 where e free

-- The last element of a list with functional patterns:
last' (_ ++ [e]) = e


perm []                = []
perm (xs ++ [x] ++ ys) = x : perm (xs ++ ys)