do

do.hs

import Monad
import Control.Monad

foo :: Maybe String
foo = Just 3 >>= (\ x -> Just "!" >>= \ y -> Just $ show x ++ y)

bar :: Maybe String
bar = do
  x <- Just 3
  y <- Just "!"
  return $ show x ++ y

marySue :: Maybe Bool
marySue = do
  x <- Just 9
  return $ x > 8

routine :: Maybe Pole
routine = do
  start <- return (0, 0)
  first <- landLeft 2 start
  second <- landRight 2 first
  landLeft 1 second

justH :: Maybe Char
justH = do
  (x : xs) <- Just "hello"
  return x

wopwop :: Maybe Char
wopwop = do
  (x : xs) <- Just ""
  return x

listOfTuples :: [(Int, Char)]
listOfTuples = do
  n <- [1,2]
  ch <- ['a', 'b']
  return (n, ch)

sevensOnly :: [Int]
sevensOnly = do
  x <- [1..50]
  guard ('7' `elem` show x)
  return x

type KnightPos = (Int, Int)

moveKnight :: KnightPos -> [KnightPos]
moveKnight (c, r) = do
  (c', r') <- [(c + i, r + j) | i <- [-2, 2], j <- [-1, 1]]
              ++ [(c + i, r + j) | i <- [-1, 1], j <- [-2, 2]]
  guard (c' `elem` [1..8] && r' `elem` [1..8])
  return (c', r')

in3 :: KnightPos -> [KnightPos]
in3 start = do
  first <- moveKnight start
  second <- moveKnight first
  moveKnight second

canReachIn3 :: KnightPos -> KnightPos -> Bool
canReachIn3 start end = end `elem` in3 start