On 21/12/05, Daniel Carrera
Hi all,
I'm a Haskell newbie and I don't really understand how Haskell deals with functions that really must have side-effects. Like a rand() function or getLine().
I know this has something to do with monads, but I don't really understand monads yet. Is there someone who might explain this in newbie terms? I don't need to understand the whole thing, I don't need a rand() function right this minute. I just want to understand how Haskell separates purely functional code from non-functional code (I understand that a rand() function is inevitably not functional code, right?)
Cheers, Daniel.
Haskell separates pure functions from computations where side effects must be considered by encoding those side effects as values of a particular type. Specifically, a value of type (IO a) is an action, which if executed would produce a value of type 'a'. Some examples: getLine :: IO String putStrLn :: String -> IO () -- note that the result value is an empty tuple. randomRIO :: (Random a) => (a,a) -> IO a Ordinary Haskell evaluation doesn't cause this execution to occur. A value of type (IO a) is almost completely inert. In fact, the only IO action which can really be said to run in a compiled Haskell program is main. Now, so far, all this is great, but without a way to chain actions together end-to-end, we can't do a whole lot. So Haskell provides us with a few primitives for composing and chaining together IO actions. A simple one of these is: (>>) :: IO a -> IO b -> IO b where if x and y are IO actions, then (x >> y) is the action that performs x, dropping the result, then performs y and returns its result. Great, we can now write programs which do multiple things: main = putStrLn "Hello" >> putStrLn "World" will print "Hello" and "World" on separate lines. However, we don't yet have a way to chain actions in which we are allowed to use the result of the first in order to affect what the second action will do. This is accomplished by the following operation, called 'bind': (>>=) :: IO a -> (a -> IO b) -> IO b Now, if x is an IO action with a result of type 'a', and f is a function from values of type 'a' to actions with a result of type 'b', then x >>= f is the action that performs x, and captures its result, passing it to f, which then computes a second action to be performed. That action is then carried out, and its result is the result of the overall computation. That's a mouthful, but once you see it in use, perhaps the idea will become clearer: main = putStrLn "Hello, what is your name?" >> getLine >>= \name -> putStrLn ("Hello, " ++ name ++ "!") This is most of what we need. In fact, this bind function is really successful, and we can define (>>) in terms of it: x >> y = x >>= const y In practice, it turns out to also be quite important to turn a value into an IO action which does nothing, and simply returns that value. This is quite handy near the end of a chain of actions, where we want to decide what to return ourselves, rather than leaving it up to the last action in the chain. So there's one more primitive, return :: a -> IO a which does just that. Note that there is no function: unsafe :: IO a -> a as this would defeat the referential transparency of Haskell -- applying 'unsafe' to the same IO action might return different things every time, and Haskell functions aren't allowed to behave that way. Now, I haven't really told you anything about monads in general yet. Most monads are actually rather unlike IO, but they do share the similar concepts of bind and return. For monads in general, see my tutorial MonadsAsContainers on the wiki :) Hope this helps - Cale