monads

haskell/scratch/monadDumb.hs

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+-- error handling with a custom monad 
+-- say we want to divide a list of numbers by a divisor
+-- if the divisor is even, we throw an error, then apply ^2 to the rest of the numbers
+-- if the divisor is odd, we return the result
+-- if the divisor is 0, we throw an error
+
+-- custom monad
+data MyMonad a = DivByZeroError String | DivByEven a | DivByOdd a deriving (Show)
+
+-- instance of Functor
+instance Functor MyMonad where -- Functor is where we define fmap (i.e. map)
+    -- customMap, where we have 2 functions, one for even and one for odd
+    fmap f (DivByZeroError s) = DivByZeroError s
+    fmap f (DivByEven a) = DivByEven (f a)
+    fmap f (DivByOdd a) = DivByOdd (f a)
+
+-- instance of Applicative
+instance Applicative MyMonad where
+    (DivByZeroError s) <*> _ = DivByZeroError s
+    -- DivByEvenError -> ^2
+    (DivByEven a) <*> b = fmap a b -- 
+    (DivByOdd a) <*> b = fmap a b
+    pure = DivByOdd
+
+-- instance of Monad
+instance Monad MyMonad where
+    return = pure
+    (DivByZeroError s) >>= _ = DivByZeroError s
+    (DivByEven a) >>= f =
+        case f a of
+            DivByOdd b -> DivByEven b  -- propagate the "evenness context" forward
+            DivByEven b -> DivByEven b -- keep even state if it continues
+            DivByZeroError s -> DivByZeroError s
+    (DivByOdd a) >>= f = f a
+
+safeDivide :: Int -> [Int] -> MyMonad [Int]
+safeDivide 0 _ = DivByZeroError "Division by zero"
+safeDivide d xs
+    | even d    = DivByEven (map (^2) xs)
+    | otherwise = DivByOdd (map (`div` d) xs)
+
+main :: IO ()
+main = do
+    let nums = [69, 420, 666, 1337]
+    let divisor = 2
+    let result = safeDivide divisor nums
+
+    let nums2 = [1,2,3,4,5]
+    let divisor2 = 0
+    let result2 = safeDivide divisor2 nums2
+
+    let nums3 = [69, 420, 666, 1337]
+    let divisor3 = 3
+    let result3 = safeDivide divisor3 nums3
+
+    print result
+    print result2
+    print result3
+
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