Created
July 1, 2010 14:59
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module Main where | |
class Monad64 a ma mb | ma mb -> a where | |
(>>==) :: ma -> ( a -> mb) -> mb | |
data Maybe64 a = Just64 a | |
|Nothing64 | |
deriving Show | |
instance Monad64 a (Maybe64 a) (Maybe64 b) where | |
(>>==) m f = case m of Just64 a -> f a | |
_ -> Nothing64 | |
instance Monad64 a (Maybe64 a) (Maybe b) where | |
(>>==) m f = case m of Just64 a -> f a | |
_ -> Nothing | |
instance Monad64 a (Maybe a) ([b]) where | |
(>>==) m f = case m of Just a -> f a | |
_ -> [] | |
instance Monad64 a (Maybe [a]) (Maybe [b]) where | |
(>>==) m f = case m of Just xs -> Just [ k | x <- xs , let ks = (maybe [] id) (f x), k <- ks] | |
_ -> Nothing | |
main= print another >> print doSomething | |
doSomething = ((Just64 1) >>== \i -> | |
Just (i*2)) >>== \j -> | |
[j] | |
another= Just [1,2,3] >>== \a -> | |
if a < 3 then Just [a] else Nothing |
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I guess this gives a lot of flexibility that are not illustrated in this example, including defining monads on * type kinds. I know code can be much cleaner (maybe type families or other extensions) but I guess it shows the point.
It is just that I am wondering why should a category be represented as a * -> * kind...