Objective-C ParseKit return value

问题:

In flex/lex/bison/yacc (all of which I just started reading about), you can set "$$" to be equal to some value ($1,$2,$3) and that's the value that gets returned. At least I think that's how it works.

In ParseKit, you are given a stack so I imagine that the ($1,$2,$3) would be the first three values on the stack for example. But then I think what you would want to do is pop those values off the stack and put your return value on the stack. I see that the stack comes with a push method. Do you have to pop the incoming values first before pushing something on?

Thanks

回答1:

Developer of ParseKit here. I would say: it depends. A few thoughts:

1. Yes, it is often useful/desirable to store objects/values on the assembly's stack by calling -[PKAssembly push:] on the assembly sent to your parser delegate callbacks. Later callbacks will find these values on the assembly's stack and may want to take action when they are found.

2. Another option: if your callback methods are building some result object, you usually want to store it as the -[PKAssembly target] property of the assembly passed into your callback method. So you have two places where you can store values: the assembly's target or the assembly's stack. The target is the 'correct' place for this, but often the stack is also convenient. Either is fine, but i would say: store temp values on the stack, but store the ultimate object you are building as the target. But again, you can do either.

3. Yes, your callbacks should often want to pop values off the stack first, but that is not required. Think of if this way: Your delegate callback method receives a PKAssembly object as a parameter. And usually your callback method will inspect the assembly's stack and take action depending on what it finds there. Usually, in your callback, you'll want to pop the values you find there, if you are taking action on them. Basically: your callback should pop the values it is interested in/taking action on, because in some sense your callback was the intended recipient of those items/information.