List Monad. MonadPlus. Guard for filter monadic operations.

category-theory/monad.hs

@@ -127,3 +127,60 @@ wopwop :: Maybe Char
 wopwop = do  
     (x:xs) <- Just ""  
     return x
+
+-- Monad instance for List
+-- instance Monad [] where  
+--    return x = [x]  
+--    xs >>= f = concat (map f xs)  
+--    fail _ = []
+
+-- examples on lists
+g1 = [3,4,5] >>= \x -> [x,-x]
+
+-- fail is defined as an empty list
+g2 = [] >>= \x -> ["bad","mad","rad"]  
+g3 = [1,2,3] >>= \_ -> []
+
+-- chaining monadic operations on list
+g4 = [1,2] >>= \n -> ['a','b'] >>= \ch -> return (n,ch)
+
+-- same as g4 written using do notation
+listOfTuples :: [(Int,Char)]  
+listOfTuples = do  
+    n <- [1,2]  
+    ch <- ['a','b']  
+    return (n,ch)
+
+-- same as g4 using List comprehension (syntax sugar for using lists as monads)
+listOfTuplesC = [ (n,ch) | n <- [1,2], ch <- ['a','b'] ]
+
+-- filtering a list
+filteredList = [ x | x <- [1..50], '7' `elem` show x ]
+
+-- MonadPlus definition
+class Monad m => MonadPlus m where  
+    mzero :: m a  
+    mplus :: m a -> m a -> m a
+
+-- MonadPlus instance of List
+instance MonadPlus [] where  
+    mzero = []  
+    mplus = (++)
+
+guard :: (MonadPlus m) => Bool -> m ()  
+guard True = return ()  
+guard False = mzero
+
+-- filter example using guard
+h1 = guard (5 > 2) :: [()]
+h2 = guard (5 > 2) >> return "cool" :: [String]  
+h3 = guard (1 > 2) >> return "cool" :: [String]
+
+-- filter list using Monad
+filteredListMonadic = [1..50] >>= (\x -> guard ('7' `elem` show x) >> return x)
+
+sevensOnly :: [Int]  
+sevensOnly = do  
+    x <- [1..50]  
+    guard ('7' `elem` show x) -- filter
+    return x