[parenscript-devel] Patch: added JS label/break to PS
Vladimir Sedach
vsedach at gmail.com
Mon Nov 15 00:44:22 UTC 2010
Dammit, I was counting on being the lazy one.
There's 6 different situations that block and return-from can be involved in:
1. implicit nil block in iteration forms (do/dolist etc.) in lexical extent:
(ps (dolist (x '(1 2 3)) (when (= x 1) (return))))
=>
for (var x = null, _js_arrvar2 = [1, 2, 3], _js_idx1 = 0; _js_idx1 <
_js_arrvar2.length; _js_idx1 += 1) {
x = _js_arrvar2[_js_idx1];
if (x === 1) {
break;
};
};
2. explicit nil or named block in lexical extent (we have to assign a
name to the nil block):
(ps (block nil (return) (+ 1 2)))
=>
nilblock: {
break nilblock;
1 + 2;
};
3. implicit named block established by defun, flet, and labels with
lexical extent:
(defun foo () (return-from foo))
=>
function foo() {
return null;
};
4. implicit named block established by defun, flet, and labels with
dynamic extent:
(defun foo () ((lambda () (return-from foo))))
=>
function foo() {
try {
return (function () {
throw { 'ps-block-tag' : 'foo', 'ps-return-value' : null };
})();
} catch (err) {
if (err && 'foo' === err['ps-block-tag']) {
err['ps-return-value'];
} else {
throw err;
};
};
};
5. explicit named block with dynamic extent:
(block nil ((lambda () (return))) (+ 1 2))
=>
nilblock: {
try {
(function () {
throw { 'ps-block-tag' : 'nilblock', 'ps-return-value' : null };
})();
1 + 2;
} catch (err) {
if (err && 'nilblock' === err['ps-block-tag']) {
err['ps-return-value'];
} else {
throw err;
};
};
};
6. implicit nil block in iteration forms with dynamic extent return
(ps (dolist (x '(1 2 3)) ((lambda (x) (when (= x 1) (return))) x)))
=>
Which is currently not implemented
Vladimir
2010/11/13 Daniel Gackle <danielgackle at gmail.com>:
> Sorry for being lazy, but can you post an example or two? This is a feature
> I will definitely try out. One of the unwanted weaknesses of my code on the
> JS side is the inability to get out of a top level function from inside a
> lambda.
>
> On Sat, Nov 13, 2010 at 2:11 PM, Vladimir Sedach <vsedach at gmail.com> wrote:
>>
>> I just pushed a patch that tries to do the right thing with both
>> lexical and dynamic-extent BLOCK (including implicit BLOCK forms) and
>> RETURN-FROM. It's also supposed to provide backwards-compatibility
>> with the old-style RETURN behavior (although that does issue a
>> warning).
>>
>> The big thing is that right now in most of the interesting cases it
>> does the control jump, but does not return a value. That will be fixed
>> in future patches.
>>
>> I haven't really tested it, so try it out and let me know what breaks.
>>
>> Vladimir
>>
>> 2010/8/18 Daniel Gackle <danielgackle at gmail.com>:
>> > I like your suggestion of emitting TRY/CATCH only in the cases where
>> > it's necessary, i.e. only when trying to escape out of more than one
>> > level of function nesting, seems like a good way to go. Then you're
>> > only paying for the ugliness when you need it. It's in keeping with
>> > PS's philosophy of staying close to what one would write by hand.
>> >
>> > On Wed, Aug 18, 2010 at 6:12 AM, Red Daly <reddaly at gmail.com> wrote:
>> >>
>> >> I added RETURN-FROM and BLOCK without too much effort using the
>> >> implicit return functionality and try/catch. In my view this is the
>> >> most reasonable way to implement this in the general case, since
>> >> BLOCK/RETURN-FROM require non-local exit much in the same way that
>> >> lisp's TRY/CATCH do.
>> >>
>> >> The alternative to this approach is to exit from each function in the
>> >> call stack via a Javascript `return' statement. Unfortunately, the
>> >> call stack can contain many functions code over which the Parenscript
>> >> compiler exerts little control, requiring throw as the control
>> >> transfer mechanism. Thus, in the general case of unknown code on the
>> >> call stack, there is no means to exit without a throw. I do not view
>> >> throwing as an ugly solution at all, since try/catch was designed for
>> >> non-local exits of all sorts.
>> >>
>> >> Nonetheless, using try/catch to implement Parenscript features
>> >> deserves some attention. Programs will need to ensure that they do
>> >> not use try/catch in a way that interferes with the Parenscript
>> >> convention. Generally, try/catch blocks should only catch specific
>> >> exceptions and re-throw PS's exceptions. I'm happy to also implement
>> >> a safe TRY/CATCH wrapper that re-throws Parenscript errors and catches
>> >> everything else, too. However, we may want to make an official
>> >> interface change to try/catch if any lisp-style non-local exit code
>> >> becomes part of the language.
>> >>
>> >> I present an example of why try/catch is unavoidable inline below:
>> >>
>> >> On Fri, Apr 9, 2010 at 3:42 PM, Vladimir Sedach <vsedach at gmail.com>
>> >> wrote:
>> >> > Makes sense to me. I'll add this to my todo list (which I'll publish
>> >> > in an email as soon as I'm done my current work on the PS compiler).
>> >> >
>> >> > Vladimir
>> >> >
>> >> > 2010/4/9 Daniel Gackle <danielgackle at gmail.com>:
>> >> >> I just pushed a patch (authored by Scott) to implement JS's LABEL
>> >> >> and
>> >> >> BREAK
>> >> >> in PS. (Note that this patch deprecates LABELED-FOR since you can
>> >> >> get
>> >> >> the
>> >> >> same effect by combining LABEL and FOR. Was anybody using
>> >> >> LABELED-FOR?)
>> >> >> Here's an example:
>> >> >> (label scope
>> >> >> (foo)
>> >> >> (when (bar)
>> >> >> (break scope))
>> >> >> (blee))
>> >> >> =>
>> >> >> scope: {
>> >> >> foo();
>> >> >> if (bar()) {
>> >> >> break scope;
>> >> >> };
>> >> >> blee();
>> >> >> };
>> >> >> I was astonished to discover recently that JS has supported this
>> >> >> ability all
>> >> >> along in the form of labeled statements and labeled breaks. I'd
>> >> >> always
>> >> >> assumed that to get explicit returns from an arbitrary scope, you'd
>> >> >> have to
>> >> >> resort to the ugly hack of muscling TRY/CATCH to do it, thinking
>> >> >> that
>> >> >> this
>> >> >> was the closest JS counterpart.
>> >> >> (See http://news.ycombinator.com/item?id=793092 for a thread in
>> >> >> which
>> >> >> several people believe this.) But it appears we were all wrong.
>> >> >> What's not clear yet is how far this can be taken. Can you use it
>> >> >> inside a
>> >> >> nested local function to return immediately from the top-level
>> >> >> function?
>> >> >> That is one thing I've wanted for a long time.
>> >> >> In the ideal case, LABEL/BREAK could be used as a base for
>> >> >> implementing
>> >> >> a
>> >> >> proper BLOCK and RETURN-FROM in PS, something which we'd long
>> >> >> believed
>> >> >> to be
>> >> >> impossible. One challenge is that in CL, RETURN-FROM can take a
>> >> >> value,
>> >> >> which
>> >> >> becomes the value of BLOCK. In other words, BLOCK in CL is an
>> >> >> expression
>> >> >> while LABEL in JS is not. It seems, though, that most of this
>> >> >> challenge
>> >> >> has
>> >> >> already been conquered with the development of implicit return in
>> >> >> PS.
>> >> >> The
>> >> >> only thing you'd need to add is detecting when BLOCK is being used
>> >> >> as
>> >> >> an
>> >> >> expression, declaring a gensymed variable and assigning whatever is
>> >> >> happening inside BLOCK to that variable (much like implicit return
>> >> >> already
>> >> >> does with e.g. CASE), then put the variable in the expression
>> >> >> position
>> >> >> that
>> >> >> BLOCK was in. It seems like this ought to work. It would also make
>> >> >> things
>> >> >> like this possible in PS:
>> >> >> (1+ (case foo
>> >> >> (:eleven 11)
>> >> >> (:twelve 12)))
>> >> >> Vladimir (and everybody), is the above clear? What do you think of
>> >> >> it?
>> >>
>> >> As stated above, I think try/catch is the way to go. There is no
>> >> other way to exit a stack of functions in the general case otherwise.
>> >>
>> >> For example, I can write a Javascript function that calls its argument
>> >> infinity times and never returns.
>> >>
>> >> function mapForever(fn) {
>> >> while(true) fn();
>> >> }
>> >>
>> >> Now consider some parenscript:
>> >>
>> >> (block non-local
>> >> (map-forever
>> >> (lambda ()
>> >> (return-from non-local "we got out!"))))
>> >>
>> >> To extricate itself from map-forever, there is no alternative but JS's
>> >> throw statement.
>> >>
>> >> Even if we had the ability to alter every function in the system, it
>> >> would be necessary to inspect nearly every function call's return
>> >> values to properly unwind the stack to the appropriate BLOCK.
>> >>
>> >> Having said all that, there are cases when try/catch is not necessary
>> >> for BLOCK/RETURN-FROM, as you have described. BLOCK should emit code
>> >> according to the contexts in which RETURN-FROM appears. If there is a
>> >> RETURN-FROM inside the same function, BLOCK can use a label for a
>> >> local exit. If RETURN-FROM appears inside a lambda, try/catch is
>> >> necessary (except in cases where you want to optimize this away by
>> >> inspecting how that lambda gets passed around). If there are no
>> >> return-froms, just emit a PROGN.
>> >>
>> >> My solution does not do the local optimization, but it does refrain
>> >> from putting try/catches around code with no return-froms.
>> >>
>> >>
>> >>
>> >> Red
>> >>
>> >> >> Daniel
>> >> >> _______________________________________________
>> >> >> parenscript-devel mailing list
>> >> >> parenscript-devel at common-lisp.net
>> >> >> http://common-lisp.net/cgi-bin/mailman/listinfo/parenscript-devel
>> >> >>
>> >> >>
>> >> >
>> >> > _______________________________________________
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>> >> > parenscript-devel at common-lisp.net
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>> >> >
>> >>
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>> >
>> >
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>> >
>> >
>>
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