Hans van Thiel wrote:
However, some functions in Haskell may have side effects, like printing something on the screen, updating a database, or producing a random number. These functions are called 'actions' in Haskell.
Not really true (disregarding things like unsafePerformIO). I haven't been following this thread, so I don't know if anybody else has suggested this, but perhaps it would be helpful to distinguish between evaluating expressions and performing actions. Evaluation is simply graph reduction, which is Haskell's only method of computation. There are no side-effects when you evaluate an expression (again, disregarding unsafePerformIO and company), even if that expression evaluates to an IO action. To perform an action is to cause the side-effect which that action represents. *You* never perform an action in Haskell. The runtime does. All you do is say how to evaluate those actions. Essentially, what the IO monad does is give you a DSL for constructing (by evaluation) effectful, imperative programs at runtime. The runtime will cause your program to evaluate the next action, then perform it, then cause your program to evaluate the next action, then perform it, and so on. At no point is the purity of your program broken by this.
However, there is a mechanism (sometimes) to compose functions using an extra type m a, m b, m c etc. instead of types a, b, c... This does not solve the problem concerning side effects, but it does provide a sort of 'Chinese boxes' to contain them within these type constructors m. Moreover, in the case of the type designation 'IO ...', you can't get anything out of the box. So now, at least, you've got a clean interface between the parts of your program which do not involve side effects, and the 'actions'.
This is another way to think of it. It is how I first thought of IO before I discovered the way I described above. It's a pretty good metaphor, and I don't think it's harmful, so you can stick with it for now if you would prefer.
If I guess correctly, then the general statement 'monads are for actions' is wrong. It should be something like, 'monadic composition is a useful method of generalization, which, by the way, allows you to isolate side effects in a controlled manner'.
Right on. Monads are extremely useful for things that don't involve side-effects. Hope this helps, - Jake