What's the value of IO Monad?

Question

When I'm writing code in the form of IO Monad, I wonder what's real value of it.For example I have a function as def something(in: In): IO[Out]; my function is a pure function that **returns an impure recepie(just another function) which could be composed with other receipes, and finally triggered.

Is postponing the side-effects the best I get? In other words, what's the difference in the value between a function with side-effects, and a pure function which return an impure function? In my eyes both approches could be testable, trackable, debugable (the IO version could be harder, as stacktraces are not as helpful as simple stack-calls).

Answer 1

In addition to what Jacques said about the side effects being explicit in your type system, you also get referentially transparent side effects. This means you can store effects or groups of effects in variables and lists and end up with the exact same result. The cats State documentation has a good example of this for the State monad.

One thing this enables that isn't immediately apparent how useful it is, is the order you specify your effects can be different from the order they execute, and some you specify may not end up executing at all.

Let's say you have a request that connects to a list of servers, does some processing, and returns the first successful result. The request contains all the information needed to do the processing step, so you specify that effect first. The list of servers is read from config, so that's handled somewhere else, perhaps memoized. You can specify all the effects for connecting to the servers in one place, but then later choose to run them one at a time, or in parallel, or with retries.

In other words, it makes it a lot easier to separate the concerns of what gets executed from what order it gets executed.

Answer 2

The primary benefit is you get the IO explicitly in the type of the function. So you know that any function without IO in the signature is side-effect free. The pure parts of the program is guaranteed to be pure and side-effect free by the type system.