[parenscript-devel] A simpler way to do multiple value return? (Was: PS test failures in 7be9b45)
Vladimir Sedach
vsedach at gmail.com
Fri Aug 31 01:12:45 UTC 2012
I played around with several approaches to multiple values, and Red is
correct that they all need to be stack aware. However, it turns out
you don't need callee.caller for that, just first-class functions and
closures. If you don't care about changing the calling convention,
you don't even need that.
The basic idea is that a form expecting multiple values creates a
mutable array to store those values, and passes it down the stack
(this is a common pattern in C code). If you don't care about function
calling convention, you can just pass around a mutable array for
multiple values as an implicit part of the argument list.
Parenscript cares, so we need to pass that data out-of-band. We can do
this by associating the function object about to be called with the
multiple value array:
(multiple-value-bind (x y) (foo)
...body...)
prev_mv = foo.mv; // don't clobber things up the stack
var values = [];
foo.mv = values;
x = foo();
if (values.length > 0) {
y = values[0];
}
foo.mv = prev_mv;
...body...
Note that we just need some way to associate foo with the array. You
can do that with a global table instead of setting a property on the
function object.
Functions that return multiple values look like:
(defun foo (x y z)
(values x y z))
function foo (x, y, z) {
var values = arguments.callee.mv;
if (values) {
values[0] = y;
values[1] = z;
}
return x;
}
Note that you don't need arguments.callee for a function to have a
reference to itself:
var foo = (function () {
var self = function foo () {
self.blah = whatever;
...body...
};
return self;
})();
That is very ugly though.
multiple-value "pass-through" only happens in the case when there is
an expression like "(return (some-multi-valued-function))" in the
code. Since Parenscript now instruments all returns (this was not the
case when the original multiple value mechanism was worked out), we
can pass multiple values along like so:
(defun bar ()
(foo)
(foo))
function bar () {
foo(); // first invocation, don't care about multiple values
foo.mv = arguments.callee.mv;
var result = foo();
delete foo.mv;
return result;
}
As you can see we only give the array to functions in instances where
they can potentially return multiple values.
A problem arises for recursive functions (and any function objects
that can appear multiple times in the stack):
(defun foo (x)
(if (= x 1)
(values 1 2)
(1+ (foo (1- x)))))
foo(2) will now return multiple values, even though it shouldn't.
In general, this can happen if foo calls any function x calls... a
function that eventually calls foo again.
This wouldn't happen if the values array was passed as an argument.
One way I see to solve this problem is to add some code to any
function that can potentially return multiple values:
function foo (x) {
var values = arguments.callee.mv;
delete arguments.callee.mv;
if (x === 1) {
if (values) values[0] = 2;
return 1;
} else {
return 1 + foo(x - 1); // not expecting values
}
}
Obviously the above code will need things like unwind-protect and
gensyms, etc., but does anyone see anything that's wrong with the
above proposal?
Vladimir
On Wed, Aug 29, 2012 at 1:33 PM, Daniel Gackle <danielgackle at gmail.com> wrote:
> Ah yes clearer. This is similar to Vladimir's case from upthread:
>
> (defun foo ()
> (blah)
> (some-random-js-function))
>
> ... except that my suggestion that the compiler figure out when BLAH
> isn't in a return position and do something like:
>
> function foo() {
> blah();
> RETURN_VALUES = null;
> return someRandomJsFunction();
> };
>
> ... won't work in your case, i.e. when FOO is not a PS function.
>
> But I wonder whether perfect interop from JS back into PS isn't an
> overly ambitious a thing to promise. When language A and language B
> have different calling conventions and you call B from A, it's normal
> to have to follow some protocol to manually bridge the gap between
> them. In this case the protocol might be: you must either return the
> call to BLAH or clear RETURN_VALUES. Yeah this would be a pain and
> easy to forget, but it is arguably a reasonable card for PS to have to
> play here.
>
> That being said, it's not surprising that a simple global variable
> wouldn't do the trick in every case. I hope we can come up with
> something that does.
>
> But let's not forget that the current implementation is even more
> broken. It doesn't do the right thing even if BLAH is in a return
> position - so *none* of the cases we're talking about actually work
> right now.
>
> Daniel
>
>
> On Wed, Aug 29, 2012 at 10:16 AM, Red Daly <reddaly at gmail.com> wrote:
>>
>> Hi Daniel,
>>
>> I'm glad to be part of the discussion :)
>>
>> On Aug 28, 2012 8:53 PM, "Daniel Gackle" <danielgackle at 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
>>
>
>
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