谁能解释这个简单服务器代码所基于的评估模型？

Question

I can across http://therning.org/magnus/archives/698 while looking for some examples of networking with Haskell. It's probably about as simple an example as there is.

While I can use this as a template, and it works, I don't fully understand the semantics and how the code is evaluated. I can see that it's laziness that prevents the forever $ do block from being constantly evaluated, but what forces it to be evaluated at all?

Answer 1

Laziness has nothing to do here. And there's nothing "preventing" the subject forever $ do -block from anything either. It is infinitely evaluated, in fact you can see it as analogue to while (true) of common imperative languages. The implementation of this server revolves around the accept serv_sock function call, on which the program waits for a connection thus synchronously blocking the forever loop execution until it receives the connection, does what it's supposed to with it and then loops again.

It all may seem very much like a standard imperative program, the difference though is that instead telling the computer what to do with every expression, you are composing instructions from other instructions. Thus you can see the main function definition as a single composite instruction.

What triggers the evaluation is the fact that you assign this instruction with a name main and a type signature IO () (which got inferred in your case). By this you tell the Haskell compiler to treat this instruction as an entry point of your program, and thus to begin the evaluation from it when the program is launched.