import Prelude hiding (take)

f x = 1

loop = loop

m = f loop  -- > (Haskell!!!) 1

-- Lazy Evaluation:
-- evaluate an expression only if needed (--> needed evaluation):
-- because of case distinction or pattern matching

-- Advantages:
-- * less computational work
-- * computing with infinite data structures

-- An infinite list of increasing numbers:
from :: Int -> [Int]
from n = n : from (n+1)

-- Returns the prefix with n elements of a list:
take :: Int -> [a] -> [a]
take n xs | n <= 0    = []
          | otherwise = case xs of []   -> []
                                   y:ys -> y : take (n - 1) ys


sieve :: [Int] -> [Int]
sieve (p:xs) = p : sieve (filter (\x -> x `mod ` p > 0) xs)


primes :: [Int]
primes = sieve (from 2)

-- principle: separate data generation (primes) and control (take, !!)