Introduction to IO
(This page is intended as a quick introduction to how IO is treated in Haskell. It doesn't describe everything there is to know, but should give some idea of how things work.)
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
getLine :: IO String putStrLn :: String -> IO () -- note that the result value is an empty tuple. randomRIO :: (Random a) => (a,a) -> IO a -- in practice you will avoid IO and prefer randomR
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
Armed with this knowledge, we can write a "hello, world" program:
main :: IO () main = putStrLn "Hello, World!"
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
y are IO actions, then
(x >> y) is the action that
x, dropping the result, then performs
y and returns its
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
x >>= f is the action that first performs the action
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.
You might see do-notation all over the place in real Haskell programs. In do-notation, our example program might look like:
main = do putStrLn "Hello, what is your name?" name <- getLine putStrLn ("Hello, " ++ name ++ "!")
This is in fact entirely equivalent to the above form, and is
translated into it by the Haskell compiler. So whenever you see a do
block, you can just imagine a chain of applications of
and some lambdas when appropriate to capture the results of actions. An action on its own on a line in a do-block will be executed, and a line of the form
v <- x will cause the action
x to be run, and the result bound to the variable
A common mistake is to put something other than an action in the place of
x, usually some other value. If you want to make a variable binding inside a do-block which doesn't involve running an action, then you can use a line of the form
let a = b, which, like an ordinary let-expression will define
a to be the same as
b, but the definition scopes over the remainder of the do-block.
Note that there is no function:
unsafe :: IO a -> a
as this would defeat the referential transparency of Haskell --
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 more on monads in general, see my Monads as containers article, or my Monads as computation article which give two different ways to look at what monads abstract.
Footnote: if you need comprehensive tutorial on using IO monad, look at the Haskell I/O inside: Down the Rabbit's Hole
An explanation of the basic Monad functions, with examples, can be found in the reference guide A tour of the Haskell Monad functions, by Henk-Jan van Tuyl.