-- List comprehensions
div3 = [(x, x `div` 3) 
    | x <- [1..20], x `mod` 3 == 0]
    
-- Python
-- [(x, x // e) for x in range(1, 20) if x % 3 == 0]    

cart a b = 
    [(x, z) | x <- a, even x, 
     y <- b, let z = y : "::"]
-- cart [1..5] ['a'..'d']

mapLC f l = [f x | x <- l]
filterLC f l = [x | x <- l, f x]


ones = 1 : ones
naturals = build 0
    where build n = n : build (n + 1)
naturalsA = let build n = n : build (n + 1)
   in build 0
naturals2 = 0 : zipWith (+) ones naturals2
naturalsLC = 0 : [n + 1 | n <- naturalsLC]


primes1 = 2 : [n | n <- [3..], 
    
    -- n este prim dacă nu are divizori primi
    null [primediv | 
    
      -- primediv este un număr prim, din cele mai mici decât
      -- radical din n
      primediv <- takeWhile (< (round $ sqrt $ fromIntegral n)) primes1,
      n `mod` primediv == 0]]


primes2 = sieve [2..]
    where sieve (p : numbers) = p : sieve [x | x <- numbers, x `mod` p /= 0]


--inc x = x + 1
--inc x = (+1) x
inc = (+1)

--r = map (\x -> x + 1) [1..]
r = map (+1) [1..]

lista = ["un", "sir", "care", "", "contine",
 "cuvinte", "", "vide"]
 
--nevide = filter (\s -> not (null s)) lista
--nevide = filter (\s -> not $ null s) lista
--nevide = filter (\s -> (not . null) s) lista
nevide = filter (not . null) lista

matrix = [[1,2,3], [4,5,6], [7,8,9], [10,11,12]]

p :: Show a => [[a]] -> String
p = concatMap ((++ "\n") . printRow)
    where
       printRow = concatMap ((++" ") . show)
{-
transpose m =
  foldr (\ rowM partialT ->
    zipWith (:) rowM partialT)
    (map (\_ -> []) (head m))
    m
-}

transpose m =
  foldr (zipWith (:))
    (map (\_ -> []) (head m))
    m

-- putStr $ p $ transpose matrix


--paren s = "(" ++ s ++ ")" -- mult mai lizibil
--paren s = ("("++) ((++")") s)
paren =  ("("++) . (++")") -- ilizibil