On Tuesday 21 September 2010 15:37:35, Martin Tomko wrote:
Hi edgar, the whole of my code and the input data are one contained world, I am not using mondas - as I don't understand them properly,
Understanding will improve if you start to use them (don't go in too deep too fast, just start tinkering a little with Monads in Maybe and [], take a look at Control.Monad.State, build up experience and understanding).
and they seem not to be necessary for the simple algebra I am trying to develop. Do I really need to use them here?
Not really. Monads make many things more convenient, but you can do almost all without them. In the case at hand, using Maybe's Monad instance could add some convenience and elegance, but it'd not make a huge difference.
Now, to your example: >>>Try for example in ghci: fromJust $ Just 1 >>= (\x -> return $ x + 1) (I also had to search for definition of the $ operator, totally avoided in the two books I have, and seems to be just syntactic sugar instead of parentheses. Argh.)
Apart from avoiding parentheses, ($) has a few other uses, it can be passed to some higher order functions and it's nice to use for sections, map ($ 4) [sin, cos, (^2)] zipWith ($) [sin, cos, (^2)] [1,2,3] In both cases, ($) could be replaced with id, since ($) :: (a -> b) -> a -> b f $ x = f x , ($) *is* the identity function, restricted to function types and with a different fixity, but somehow map ($ 4) looks less intriguing than map (`id` 4), doesn't it?
this seems to be equivalent to fromJust (Just 1),
It's fromJust (Just 2). foo :: Int -> Maybe Int foo x = Just (x+1) The function (\x -> return $ x+1) is just foo, only more general (works for any Num type and any Monad). Just val >>= func = func val, so Just 1 >>= foo = foo 1 = Just (1+1) = Just 2 Nothing >>= _ = Nothing
where I would assuem a result of 1. But the example seems to be dependent on whatever x is entered by keyboard. Am I right?
No, the x is (bound to) the argument of Just on the left of the (>>=), in this case 1. Keyboard input lives in the IO Monad, there's no such stuff available in the Maybe Monad. In getLine >>= \line -> putStrLn (reverse line) the name line is bound to the keyboard input. Just "A line" >>= \line -> putStrLn (reverse line) doesn't type-check, because the Monad to the left of (>>=) is different from the Monad on the right (left: Maybe, right: IO).
Cheers Martin