npt

% Escape Function

Npt documentation.

Reference: ANSI Common Lisp npt
Prev: 2. Hold Variable
Next: 4. Registering Functions

3.1 Escape Function

An escape function is a function from which an escape may take place. In the previous chapter, the escape function is a function that may cause error. But escape does not mean only error.

An escape is an instruction that interrupts the current execution and forces the stack frame to terminate. Five factors exist for escaping

In other words, a major change in the flow of execution and a jump to another stack frame is called an escape.
Escape is like setjmp/longjmp in C or try/catch in C++.
However, npt does not use these operators, it just exits the escape function.

An escape function is not referring to some technique, but rather a rule for creating functions in C.
Common Lisp behavior is achieved by following the rules described below.

3.2 Creating an Escape Function

Here is an example of how to create an escape function.

int test(void)
{
    lisp_format8_(NULL, "Start~%", NULL);
    lisp_funcall8_(NULL, "TEST-THROW", NULL);
    lisp_format8_(NULL, "End~%", NULL);

    return 0;
}

The name of Escape functions end with an underscore _, so the lisp_format8_ and lisp_funcall8_ in the example statement are escape functions.
The test function ignores all the return values of the escape function, but it should be handled correctly. The following is a rewrite of the correct escape function.

int test_(void)
{
    if (lisp_format8_(NULL, "Start~%", NULL))
        return 1;
    if (lisp_funcall8_(NULL, "TEST-THROW", NULL))
        return 1;
    if (lisp_format8_(NULL, "End~%", NULL))
        return 1;

    return 0;
}

By executing return 1 when each of the escape functions returned a value, the correct escape function test_ was processed.

Suppose that the test-throw function is defined as follows.

(defun test-throw ()
  (throw 'hello 999))

When test_ executes lisp_funcall8_ and calls test-throw, the lisp_funcall8_ function returns 1 because throw is executed if the corresponding catch is present.

Then the test_ function is executed as it is and the return 1 is executed, so the next format statement is not executed and the test_ function ends.

This is how the escape is achieved.

3.3 Escape with lisp_push_control

Suppose that lisp_push_control is used in the usual function test.

int test(void)
{
    addr control, v;

    lisp_push_control(&control);
    v = Lisp_hold();
    lisp_format8_(NULL, "Start~%", NULL);
    lisp_funcall8_(v, "TEST-THROW", NULL);
    lisp_format8_(NULL, "End: ~A~%", v, NULL);
    lisp_pop_control_(control);

    return 0;
}

When rewriting the escape function, statements surrounded by push / pop’ should always be popped to free the stack frame rather than return 1 immediately if an escape occurs.

Here’s an example of a fix.

int test_(void)
{
    addr control, v;

    lisp_push_control(&control);
    v = Lisp_hold();
    if (lisp_format8_(NULL, "Start~%", NULL))
        goto escape;
    if (lisp_funcall8_(v, "TEST-THROW", NULL))
        goto escape;
    if (lisp_format8_(NULL, "End: ~A~%", v, NULL))
        goto escape;
escape:
    return lisp_pop_control_(control);
}

Although goto is easy to understand in a simple case like the example, it becomes difficult to understand in a complex syntax.

So, we used the following rewrite in the development of npt.

static int test_call_(void)
{
    addr v;

    v = Lisp_hold();
    if (lisp_format8_(NULL, "Start~%", NULL))
        return 1;
    if (lisp_funcall8_(v, "TEST-THROW", NULL))
        return 1;
    if (lisp_format8_(NULL, "End: ~A~%", v, NULL))
        return 1;

    return 0;
}

int test_(void)
{
    addr control;

    lisp_push_control(&control);
    (void)test_call_();
    return lisp_pop_control_(control);
}

The example sentence makes extensive use of the following syntax to determine escape.

if (...)
    return 1;

Because this syntax is used in so many places, we created the following macro

#define Return(x) {if (x) return 1;}

The function test_call_ can be rewritten with this macro as follows.

static int test_call_(void)
{
    addr v;

    v = Lisp_hold();
    Return(lisp_format8_(NULL, "Start~%", NULL));
    Return(lisp_funcall8_(v, "TEST-THROW", NULL));
    Return(lisp_format8_(NULL, "End: ~A~%", v, NULL));

    return 0;
}

If you plan to create a lot of escape functions, you may want to consider using this simple macro, as it is very easy to use.

In summary, an escape function is one that imposes the following rule.

3.4 Rewrite the factorial example.

In the previous chapter 2. Hold Variable, we created a function to output a factorial.
However, for the sake of explanation, we did not deal with error handling, so we ignored all return values of the escape function.

The following is a rewrite of the correct escape function.

static int fact_(addr x, addr value)
{
    addr control, y;

    if (! lisp_plus_p(value)) {
        lisp_fixnum(x, 1);
        return 0;
    }

    lisp_push_control(&control);
    y = Lisp_hold();
    if (lisp_funcall8_(y, "1-", value, NULL))
        goto escape;
    if (fact_(y, y))
        goto escape;
    if (lisp_funcall8_(x, "*", value, y, NULL))
        goto escape;
escape:
    return lisp_pop_control_(control);
}

int main_lisp(void *ignore)
{
    addr control, x, y;

    lisp_push_control(&control);
    x = Lisp_hold();
    y = Lisp_hold();
    lisp_fixnum(y, 123);
    if (fact_(x, y))
        goto escape;
    if (lisp_format8_(NULL, "fact: ~A! = ~A~%", y, x, NULL))
        goto escape;
escape:
    return lisp_pop_control_(control);
}

The fact_ function in the example sentence will be complete without any further modification.

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