「積み木の水槽」問題の Haskell 回答 (問題 http://nabetani.sakura.ne.jp/hena/ord13blocktup/ , 回答 https://rosettacode.org/wiki/Water_collected_between_towers#Haskell)

WaterTowers.hs

import           Data.Bool (bool)
import           Data.List (scanl1, scanr1)

main :: IO ()
main = print $ all ((==) <$> snd <*> (show . solve . fst)) testSet

solve = sum . fmap (sum . (zipWith (-) =<< (zipWith min . scanl1 max <*> scanr1 max)) . fmap (read . (: []))) . words . fmap (bool ' ' <*> (/=) '0')

testSet :: [(String, String)]
testSet =
  [ ("83141310145169154671122", "24") -- 0
  , ("923111128", "45") -- 1
  , ("923101128", "1") -- 2
  , ("903111128", "9") -- 3
  , ("3", "0") -- 4
  , ("31", "0") -- 5
  , ("412", "1") -- 6
  , ("3124", "3") -- 7
  , ("11111", "0") -- 8
  , ("222111", "0") -- 9
  , ("335544", "0") -- 10
  , ("1223455321", "0") -- 11
  , ("000", "0") -- 12
  , ("000100020003121", "1") -- 13
  , ("1213141516171819181716151413121", "56") -- 14
  , ("712131415161718191817161514131216", "117") -- 15
  , ("712131405161718191817161514031216", "64") -- 16
  , ("03205301204342100", "1") -- 17
  , ("0912830485711120342", "18") -- 18
  , ("1113241120998943327631001", "20") -- 19
  , ("7688167781598943035023813337019904732", "41") -- 20
  , ("2032075902729233234129146823006063388", "79") -- 21
  , ("8323636570846582397534533", "44") -- 22
  , ("2142555257761672319599209190604843", "41") -- 23
  , ("06424633785085474133925235", "51") -- 24
  , ("503144400846933212134", "21") -- 25
  , ("1204706243676306476295999864", "21") -- 26
  , ("050527640248767717738306306596466224", "29") -- 27
  , ("5926294098216193922825", "65") -- 28
  , ("655589141599534035", "29") -- 29
  , ("7411279689677738", "34") -- 30
  , ("268131111165754619136819109839402", "102") -- 31
  ]