maybemonad.hs

module MaybeMonad where

data Cow = Cow {
    name   :: String
  , age    :: Int
  , weight :: Int
} deriving (Eq, Show)

noEmpty :: String -> Maybe String
noEmpty "" = Nothing
noEmpty str = Just str

noNegative :: Int -> Maybe Int
noNegative n | n >= 0  = Just n
             | otherwise = Nothing

weightCheck :: Cow -> Maybe Cow
weightCheck c =
  let w = weight c
      n = name c
  in if n == "Bess" && w > 499
     then Nothing
     else Just c

mkSphericalCow :: String
                  -> Int
                  -> Int
                  -> Maybe Cow 

mkSphericalCow name' age' weight' =
  case noEmpty name' of
    Nothing -> Nothing
    Just nammy ->
      case noNegative age' of
        Nothing -> Nothing
        Just agey ->
          case noNegative weight' of
            Nothing -> Nothing
            Just weighty -> 
              weightCheck (Cow nammy agey weighty)







--------------------
--
mkSphericalCow' :: String -> Int -> Int -> Maybe Cow
mkSphericalCow' name' age' weight' = do
  nammy <- noEmpty name'
  agey <- noNegative age'
  weighty <- noNegative weight'
  weightCheck (Cow nammy agey weighty)


---------
-- Applicative vs Monad
---------

f :: Integer -> Maybe Integer
f 0 = Nothing
f n = Just n

g :: Integer -> Maybe Integer
g i = 
  if even i
  then Just (i + 1)
  else Nothing

h :: Integer -> Maybe String
h i = Just ("10191" ++ show i)

doSomething n = do
  a <- f n
  b <- g a
  c <- h b
  pure (a, b, c)


doSomething' n =
  let m = \(Just x) -> x 
  in pure (m $ f n, m $ g $ m $ f n, m $ h $ m $ g $ m $ f n)