-- Computes the factorial of a non-negative number.
fac :: Integer -> Integer
fac n = if n==0 then 1 else n * fac (n-1)


-- Computes the n-th Fibonacci number. Mathematical definition!
fib1 :: Int -> Int
fib1 n = if n==0 then 0
                 else if n==1 then 1
                              else fib1 (n-1) + fib1 (n-2)


-- Computes the n-th Fibonacci number.
fib2 :: Int -> Int
fib2 n = fib2' 0 1 n
 where
  -- Computes the n-th Fibonacci number with the n-th and (n-1)-th Fib. number.
  fib2' :: Int -> Int -> Int -> Int
  fib2' fibn fibnp1 n = if n==0 then fibn
                                else fib2' fibnp1 (fibn + fibnp1) (n-1)

myconst42 = (let x = 10
                 y = 3
                 z = y*x
             in x + z)    + 2

-- Check whether a (non-negative) number is a prime number.
isPrime :: Int -> Bool
isPrime n = n /= 1 && checkPrime (div n 2)
 where
  checkPrime d = d==1 || mod n d /= 0 && checkPrime (d-1)