Hi Daniel,

I'm glad to be part of the discussion :)

On Aug 28, 2012 8:53 PM, "Daniel Gackle" <danielgackle@gmail.com> wrote:

Hi Red,

I was hoping you'd chime in. I'll see your scenario 3 and raise you a

3a and a 3b:

Scenario 3a: A non-Parenscript function that calls a mv-returning

             Parenscript function but only needs its first return value;

Scenario 3b: A non-Parenscript function that calls a mv-returning

             Parenscript function and needs all its return values.

3a works fine as long as the MV implementation is careful to use a

normal JS return to pass the first return value back to the caller.

That's true both of what PS does today and of the global-var proposal.

As for 3b (the scenario, not the hacker!), seems to me this can't work

at all and there's no need to support it. If you're a non-PS function

then by definition you can't use PS's MULTIPLE-VALUE-BIND to access

the additional return values because the MV construct only exists in

PS. I suppose if you really wanted to you could manually write JS to

do whatever PS does to supply those values to the caller, but then

you're not really a non-PS function anymore, so much as a

manually-compiled PS function.

Daniel

I wasn't clear in my original post.  I'm not concerned with the non-Parenscript function's ability to receive multiple values.  I'm concerned that the non-Parenscript function will interfere with the multiple value return.

If a non-Parenscript function calls a Parenscript function that messes with the global variables, the non-Parenscript function could end up returning stuff unintentionally.  This is because the non-Parenscript function will not manipulate the global variables to clean up the unused multiple values that are returned.

Here's some code to illustrait the point:

(defun ps-foo ()

  (multiple-value-bind (a b) (bar)

     (+ a b)))

(defun ps-fn-returns-mv ()

  (values 1 2))

function barWorks() {

  return psFnReturnsMv();

}

function barBreaks() {

  psFnReturnsMv(); // global variables for MV returning get set up and linger

  // return a single value: 42

  return 42;

}

Let's assume the two Parenscript functions translate into something like this:

var RETURN_VALUES = null;

var MV_CALL = false;

... other global variables related to multiple values

function psFoo() {

  // global variable stuff

  MV_CALL = true;

  RETURN_VALUES = null;

  

  var a = bar();

  MV_CALL = false;

  var b = RETURN_VALUES ? RETURN_VALUES[0] : undefined;

  return a + b;

}

function psFnReturnsMv() {

  if (MV_CALL)

    RETURN_VALUES = [ 2 ];

  return 1;

}

When bar = barWorks, foo will return 1 + 2, as intended.  When bar = barBreaks, foo will incorrectly return 42 + 2 because the global variables were not properly cleaned up.

I hope this makes sense.

- Red