How to return to a line of code and restore the call-stack?

Question

I asked a question earlier, but I don't think I was clear enough about the sort of answers I was hoping for, so let me provide a more concrete example:

class Program
{
    private static bool State;

    static void Main(string[] args)
    {
        State = false;
        Console.WriteLine(And());
        Console.ReadLine();
    }

    static bool And()
    {
        return Or() && C();
    }

    static bool Or()
    {
        return A() || AB();
    }

    static bool C()
    {
        return State;
    }

    static bool A()
    {
        return true;
    }

    static bool AB()
    {
        State = true;
        return true;
    }
}

The flow of this program looks like:

1. And() gets called
2. And() calls Or()
3. Or() calls A()
4. A() returns true
5. Flow returns to Or(), which returns true (lazy evaluation)
6. Flow returns to And(), And() calls C()
7. C() returns false
8. Flow returns to And(), which returns false

Now if Or() did not perform lazy evaluation (I change || to |), the program will return true. However, I don't want AB() to executed unless the result of the entire parse fails (And() returns false).

So what I'd like to do, is somewhere in the Or() function, save the current state on a stack (static variable) so that if And() returns false, I can pop an item off the stack and try the alternative.

How would I accomplish this in C#?

Answer 1

The problem has nothing to do with the || operator. The problem is that you forgot to backtrack on a failed Parse. If Parse returns false, you need to restore i and any other state variables to their original value so the next parser can give it a try. C# is not a goal-seeking language. If you want backtracking you will have to do it yourself. (Or switch to a language with backtracking, like Prolog.)

Answer 2

It strikes me as being fairly trivial- just rearrange the calls:

static void Main(string[] args)
{
    State = false;
    Console.WriteLine(Or());
    Console.ReadLine();
}

static bool Or()
{
    return A() && C() || AB() && C();
}

Or am I missing something? Maybe C() has side effects such that it should not be called twice?

EDIT: Now I understand what you are trying to do. Do yourself a favor. Listen to the suggestions you've gotten, get a copy of GPPG, or (probably much simpler) ANTLR, with ANTLRWorks. It is so, so much easier and less error prone than attempting to roll a parser by hand, and you still get C# at the end.