Re: Is multi-level function return possible?

On 3/11/14, 3:15 PM, Kaz Kylheku wrote:
  >
  > Recursion means there is possibly more than one activation of the environment
  > which contains the target label of the goto. Which activation is chosen as the
  > jump target? Does the goto return to the same recursion level in which
  > the closure was created or does it just find the innermost activation of
  > that label?
  >
  > If the closure jsut goes to the innermost block which has the label,
  > then the target resolution is in fact dynamically scoped, even though
  > the label is lexical. (Because the target block is not lexically captured
  > along with the closure, but chosen just from whatever is visible at the
  > time the goto takes place.)


No, the label/goto scoping is strictly lexical. What happens is you
pop all the frames until you reach the frame of the correct lexically
enclosing routine with the defined label and then set PC to the
address of the instruction the label points to.


Here's a convoluted example using C syntax (but Pascal's goto and
nested function semantics):


typedef int(*int_f)(int);


int fun(int_f g) {
                    return g(1) + g(0);
}


int top(int v, int_f f) {
                    int sub(int x) {
                                    if (x == 0)
                                                    goto lab1;
                                    return x+1;
                    }
                    int HOfun(int_f argfun1) {
                                    top(v-1, argfun1);
                    }
                    if (v > 1)
                                    return HOfun(sub);
                    return fun(f);
lab1:
                    return -1;
}


When top is called with v=2 and f=some random function, it
will call HOfun with argfun1=sub. HOfun calls top again and
passing its argfun1. Now top calls fun with the passed in
function, which sub (but from the previous call to top). When
fun calls g (which is really sub) with x=0, the stack has
these activation frames:


top // original call
HOfun
top // call from HOfun
fun
sub


Since sub's static link points to the frame of the original call to
top, four frames will be popped & PC set to the instruction named by
lab1. BTW, label declarations have to precede function/procedure
declarations so that a one pass compiler can identify the correct
enclosing scope!

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