W2L1(Lecture3)-FP24-Vlad-Batch2Q3-WL

import Prelude hiding (map, filter, all, any, takeWhile, dropWhile)

map :: (a -> b) -> [a] -> [b]
-- List comprehension
map f xs = [f x | x <- xs]

filter :: (a -> Bool) -> [a] -> [a]
-- Explicit recursion
filter p [] = []
filter p (x : xs)
  | p x = (x : filter p xs)
  | otherwise =  filter p xs

all :: (a -> Bool) -> [a] -> Bool
-- Using the library function foldr
all p xs = foldr (\x acc -> (p x) && acc) True xs
-- Using recursion
all p [] = True
all p (x : xs)
  | p x = all p xs
  | otherwise = False

any :: (a -> Bool) -> [a] -> Bool
-- Using the library function foldr
any p xs = foldr (\x acc -> (p x) || acc) False xs
-- TODO: Using explicit recursion
any p [] = False -- At least (>=) 1 element must be `p x`
any p (x : xs)
  | p x = True
  | otherwise = any p xs

-- TODO: Using list comprehension


takeWhile :: (a -> Bool) -> [a] -> [a]
-- Explicit recursion
takeWhile p [] = []
takeWhile p (x : xs)
  | p x = x : takeWhile p xs
  | otherwise = []

dropWhile :: (a -> Bool) -> [a] -> [a]
-- Explicit recursion
dropWhile p [] = []
dropWhile p (x : xs)
  | p x = dropWhile p xs -- drop
  | otherwise = (x : xs)
-- TODO: Using foldr