Остроумный заголовок

homework.hs

import Data.List
import Data.List.Split

-- Exercise 1 Hopscotch
skips :: [a] -> [[a]]
skips xs = zipWith every [1..(length xs)] (repeat xs) where
  every n xs = extractBy orderedPairs where
    extractBy    = fmap fst . filter ((== n) . snd)
    orderedPairs = zip xs $ cycle [1..n]

  -- every n xs = fmap (take 1 . reverse) . filter (\x -> length x == n) $ chunksOf n xs

  -- every n xs = case drop (n-1) xs of
  --   (y:ys) -> y : every n ys
  --   [] -> []

-- skips [] = []
-- skips xs = [xs] ++ [everySecond xs] ++ nths where
--   everySecond (x:y:xs) = y : (everySecond xs)
--   everySecond _ = []
--   nths = fmap (\x -> [x]) $ drop 2 xs

exercise1 = do
  print $ skips "ABCD"       == ["ABCD", "BD", "C", "D"]
  print $ skips "hello!"     == ["hello!", "el!", "l!", "l", "o", "!"]
  print $ skips [1]          == [[1]]
  print $ skips [True,False] == [[True,False], [False]]
  -- print $ skips []           == []


-- Exercise 2 Local maxima
localMaxima :: [Integer] -> [Integer]
localMaxima xs = foldMap maxima $ triples xs where
  triples xs = zip3 xs (drop 1 xs) (drop 2 xs)
  maxima (x,y,z) = if (x < y) && (y > z) then [y] else []
  -- triples l@(x:xs) = [take 3 l] ++ triples xs
  -- triples _ = []
  -- maxima (x:y:z:[]) = if (x < y) && (y > z) then [y] else []
  -- maxima _ = []

exercise2 = do
  print $ localMaxima [2,9,5,6,1] == [9,6]
  print $ localMaxima [2,3,4,1,5] == [4]
  print $ localMaxima [1,2,3,4,5] == []



-- Exercise 3 Histogram
histogram :: [Integer] -> String
histogram nums = unlines . transpose $ reverse . createRow <$> occurrences where
  occurrences = (\(x:xs) -> (show x, length xs)) <$> (group . sort . (++ [0..9])) nums
  maxHeight   = maximum $ snd <$> occurrences
  createRow (x,y) = concat $ legend ++ bar where
    legend = [x ++ "="]
    bar    = take maxHeight (replicate y "*" ++ repeat " ")

exercise3 = do
  putStrLn $ histogram [1,1,1,5]
  putStrLn $ histogram [1,4,5,4,6,6,3,4,2,4,9]



main :: IO ()
main = do
  exercise1
  exercise2
  putStrLn ""
  exercise3