[parenscript-devel] Multiple value calls
Vladimir Sedach
vsedach at gmail.com
Thu Nov 26 06:44:09 UTC 2009
Just pushed a version of Daniel's code that avoids the problems you
brought up Red.
I have to say again how awesome it is that something like multiple
values can be implemented so easily in JavaScript. Extreme late
binding really is better than any kind of bolted-on metaprogramming
facilities.
Now what remains to be done is VALUES as a setf place.
Thanks,
Vladimir
2009/11/25 Red Daly <reddaly at gmail.com>:
> On Thu, Nov 5, 2009 at 10:43 AM, Daniel Gackle <danielgackle at gmail.com> wrote:
>> < Why do you say some of the time? Under what cases would it break? >
>>
>> Red will think of something :)
>>
>
> Well, I think this implementation is quite close and would work with
> beefier semantic analysis.
>
>> < I think you should push this code as a patch; let's see what happens. >
>>
>> I'll do that, perhaps once Scott has our code working with the implicit
>> return stuff, to minimize upgrade inconvenience.
>>
>> < The ability to attach arbitrary properties to function objects finally
>> comes in useful. >
>>
>> This is actually the one feature of JS that I miss in CL: the ability to
>> attach arbitrary properties to pretty much anything. It is a godsend for
>> exploratory programming since it eliminates the gruntwork of packing things
>> into defined structures (objects, arrays, whatever) which you then have to
>> destructure to get at them. It's an extreme of loosey-gooseyness that must
>> give the static bondage people nightmares (a feature, not a bug). I wish
>> someone would figure out a way to hack the same ability into CL (we've
>> already done so in a limited, good-enough-for-our-app way). It's interesting
>> that symbol-plists just aren't that useful in this regard.
>>
>> Daniel
>>
>>
>> On Wed, Nov 4, 2009 at 10:16 PM, Vladimir Sedach <vsedach at gmail.com> wrote:
>>>
>>> > Ok, here's a variation that abandons global MV altogether and stores it
>>> > instead as a property on the caller.
>>>
>>> Oh, wow. The ability to attach arbitrary properties to function
>>> objects finally comes in useful. Very clever!
>>>
>>> > It passes all the previously mentioned
>>> > cases and works at least some of the time with anonymous functions.
>>>
>>> Why do you say some of the time? Under what cases would it break?
>>>
>>> > (defpsmacro values (main &rest additional)
>>> > (with-ps-gensyms (mv)
>>> > `(let ((,mv (list , at additional)))
>>> > (when (defined (@ (@ (@ arguments :callee) :caller) :mv))
>>> > (setf (@ (@ (@ arguments :callee) :caller) :mv) ,mv))
>>> > (return ,main))))
>>> >
>>> > (defpsmacro multiple-value-bind (vars expr &body body)
>>> > (with-ps-gensyms (mv prev)
>>> > `(let ((,prev (@ (@ arguments :callee) :mv)))
>>> > (try
>>> > (progn
>>> > (setf (@ (@ arguments :callee) :mv) t)
>>> > (let ((,(car vars) ,expr)
>>> > (,mv (if (objectp (@ (@ arguments :callee) :mv))
>>> > (@ (@ arguments :callee) :mv)
>>> > (make-array ,(1- (length vars))))))
>>> > (destructuring-bind ,(cdr vars) ,mv
>>> > , at body)))
>>> > (:finally (if (undefined ,prev)
>>> > (delete (@ (@ arguments :callee) :mv))
>>> > (setf (@ (@ arguments :callee) :mv) ,prev)))))))
>
>
>
> I think this breaks in the case where EXPR in the above macro has more
> than one form:
>
> (multiple-value-bind (a b)
> (progn
> (returns-mv)
> (doesnt))
> (alert a)
> (alert b))
>
> Where (defun returns-mv() (values 1 2)) and (defun doesnt () (return 3)).
>
> (As an aside, the evaluation order of the MV arguments is not serial:
> (values (foo) (bar)) evaluates (bar) first. This is a simple bug that
> the ONCE-ONLY macro should solve)
>
> This is so close, and I think attaching MV information to the caller
> is a viable solution, since (1) MV returns only make sense for
> parenscript functions, not functions written by others, and (2) with
> just a little bit more semantic analysis we should be able to derive
> the form that will be responsible for returning the values bound by
> multiple-value-bind. So good work! This solution will stand for now.
>
> Note, however, that this solution requires every implicit return (e.g.
> a tail call) to execute the multiple-value machinery. It's probably
> not a big deal except for certain sections of code, and hey, it's
> Javascript anyway.
>
> Red
>
>
>>>
>>> I think you should push this code as a patch; let's see what happens.
>>>
>>> Vladimir
>>>
>>> > Specifically, it passes the case in Red's email that broke my previous
>>> > attempt. That is, given ADD-TO-RESULT as defined in that earlier email,
>>> >
>>> > (defun foo ()
>>> > (multiple-value-bind (a b) (add-to-result (lambda (x) (values 1 10))
>>> > 2)
>>> > (return (list a b))))
>>> >
>>> > foo() now correctly evaluates to [3,undefined] instead of [3,10].
>>> >
>>> > Can you guys come up with a new case to break it?
>>> >
>>> > Daniel
>>> >
>>> >
>>> >
>>> > On Sun, Nov 1, 2009 at 1:41 PM, Red Daly <reddaly at gmail.com> wrote:
>>> >>
>>> >>
>>> >> On Sun, Nov 1, 2009 at 9:50 AM, Daniel Gackle <danielgackle at gmail.com>
>>> >> wrote:
>>> >>>
>>> >>> It might help to use a PS special variable and make
>>> >>> MULTIPLE-VALUE-BIND
>>> >>> responsible for cleanup.
>>> >>>
>>> >>> (ps (defvar *mv* undefined))
>>> >>>
>>> >>> (defpsmacro values (main &rest additional)
>>> >>> (with-ps-gensyms (mv)
>>> >>> `(let ((,mv (list , at additional)))
>>> >>> (when (defined *mv*)
>>> >>> (setf *mv* ,mv))
>>> >>> (return ,main))))
>>> >>>
>>> >>> (defpsmacro multiple-value-bind (vars expr &body body)
>>> >>> `(let ((*mv* '()))
>>> >>> (let ((,(car vars) ,expr))
>>> >>> (destructuring-bind ,(cdr vars) *mv*
>>> >>> , at body))))
>>> >>>
>>> >>> This works in the obvious cases. I'm not sure it handles Red's
>>> >>> scenarios.
>>> >>> Red, can you supply an example where this breaks?
>>> >>
>>> >>
>>> >> The main case I am concerned about is dealing with non-parenscript
>>> >> functions that will not manipulate the mv state. Something like the
>>> >> following would break the above code:
>>> >>
>>> >> (defun foo ()
>>> >> (let ((my-callback (lambda (x) (values 1 10)))
>>> >> (multiple-value-bind (a b)
>>> >> (add-to-result my-callback 2)))
>>> >>
>>> >> =>
>>> >>
>>> >> // imagine this is a non-Parenscript function that we cannot manipulate
>>> >> with the
>>> >> // Parenscript compiler
>>> >> function addToResult(callback, x) {
>>> >> // returns the result of adding x to the result of calling the
>>> >> callback
>>> >> return callback() + x;
>>> >> }
>>> >>
>>> >> function foo () {
>>> >> var myCallback = function (x) {
>>> >> if (MV !== undefined)
>>> >> MV = [10];
>>> >> return 1;
>>> >> };
>>> >>
>>> >> var old_MV = MV; // begin let
>>> >> MV = null;
>>> >> var a = addToResult(myCallback, 2);
>>> >> var b = MV ? MV[0] : null;
>>> >>
>>> >> // now b === 10 but it should be nil
>>> >> // this is because addToResult did not reset MV
>>> >>
>>> >> MV = old_MV; // end let
>>> >>
>>> >> }
>>> >>
>>> >> This is why I think you might need to identify the callee in another
>>> >> special variable, and for emitted functions reset the MV_CALLEE
>>> >> variable
>>> >> before returning (and maybe in other places?).
>>> >>
>>> >>
>>> >> Red
>>> >>
>>> >>
>>> >>
>>> >>>
>>> >>> Daniel
>>> >>>
>>> >>> p.s. I took the easy way out of making VALUES always prepend RETURN,
>>> >>> but
>>> >>> once Vladimir bestows implicit RETURN upon us we can take that out ;)
>>> >>>
>>> >>>
>>> >>> On Sat, Oct 31, 2009 at 9:53 PM, Red Daly <reddaly at gmail.com> wrote:
>>> >>>>
>>> >>>> I apologize for sending the first half of this email in error
>>> >>>> earlier:
>>> >>>>
>>> >>>> On Sat, Oct 31, 2009 at 8:35 PM, Red Daly <reddaly at gmail.com> wrote:
>>> >>>>>
>>> >>>>> Hi Parenscripters,
>>> >>>>>
>>> >>>>> As far as I can tell, multiple-value function calls are a unique
>>> >>>>> feature of lisp. I would like the ability to perform multiple-value
>>> >>>>> calls
>>> >>>>> in Parenscript but I don't know if a sane solution exists or not.
>>> >>>>>
>>> >>>>> Can anyone come up with a scheme for returning multiple values that
>>> >>>>> translates well to Javascript? Ideally such a scheme would not
>>> >>>>> introduce
>>> >>>>> much overhead for usual functions that do not use or return multiple
>>> >>>>> values
>>> >>>>> (though perhaps setting some sort of global MV flag might be
>>> >>>>> inexpensive
>>> >>>>> enough). Functions that return multiple values should also only
>>> >>>>> appear to
>>> >>>>> return a single value when called by a function that expects only
>>> >>>>> one return
>>> >>>>> value (including native javascript functions).
>>> >>>>>
>>> >>>>> (defun paren-mv-returner ()
>>> >>>>> (return (values 1 2 3)))
>>> >>>>>
>>> >>>>> =>
>>> >>>>>
>>> >>>>>
>>> >>>>> function parenMvReturner() {
>>> >>>>> /* do some magic with the values 2 and 3 */
>>> >>>>> return 1;
>>> >>>>> }
>>> >>>>>
>>> >>>>> // one implementation might be
>>> >>>>> var mv = undefined;
>>> >>>>>
>>> >>>>> function parenMvReturner() {
>>> >>>>> mv = [2, 3];
>>> >>>>> return 1;
>>> >>>>> }
>>> >>>>>
>>> >>>>> // this scheme needs to adjust the return statement of every
>>> >>>>> function
>>> >>>>> so it might not be sufficient
>>> >>>>> // consider this other function
>>> >>>>>
>>> >>>>> function parenMySingleReturner () {
>>> >>>>> var x = parenMvReturner();
>>> >>>>
>>> >>>> return x;
>>> >>>> }
>>> >>>>
>>> >>>> // parenMySingleReturner() will appear to return multiple values
>>> >>>> unless
>>> >>>> it modifies the mv value itself
>>> >>>>
>>> >>>> // correction:
>>> >>>> function parenMySingleReturner () {
>>> >>>> var x = parenMvReturner();
>>> >>>> mv = null;
>>> >>>> return x;
>>> >>>> }
>>> >>>>
>>> >>>> But it seems like this solution will fall apart for calls to native
>>> >>>> Javascript functions over which we have no control. If we pass a
>>> >>>> multiple-value returning function as an argument to a native
>>> >>>> function, the
>>> >>>> native function will not perform the necessary mv-nulling when it
>>> >>>> returns.
>>> >>>>
>>> >>>> someForeignJavascriptFunction( someMVReturningFunction)
>>> >>>>
>>> >>>> will return whatever the someForeignJavascriptFunction should return,
>>> >>>> but it will also appear to return the other values that
>>> >>>> someMVReturningFunction set in the mv variable, since
>>> >>>> someForeignJavascriptFunction performs no cleanup.
>>> >>>>
>>> >>>> Maybe this limitation can be avoided by having an mv-returning
>>> >>>> function
>>> >>>> A set a global variable "mvFunctionReturner" equal to the function A
>>> >>>> and a
>>> >>>> mv-receiver can check that mvFunctionReturner is set according to the
>>> >>>> function it called expecting multiple values. Does this scheme miss
>>> >>>> any
>>> >>>> cases?
>>> >>>>
>>> >>>> Anyway I have thought a little bit about this and I thought I would
>>> >>>> pass
>>> >>>> it off to the rest of the Parenscripters as a thought experiment.
>>> >>>> Assume
>>> >>>> you can do a lot more semantic analysis than Parenscript currently
>>> >>>> does and
>>> >>>> transform the compiled source however you want. But any compiled
>>> >>>> functions
>>> >>>> must still be able to be treated as normal Javascript functions and
>>> >>>> all and
>>> >>>> only functions that should return multiple values appear to return
>>> >>>> them.
>>> >>>>
>>> >>>> Cheers,
>>> >>>> Red
>>> >>>>
>>> >>>> _______________________________________________
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>>> >>>> parenscript-devel at common-lisp.net
>>> >>>> http://common-lisp.net/cgi-bin/mailman/listinfo/parenscript-devel
>>> >>>>
>>> >>>
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