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.<div>
<br><br><div class="gmail_quote">On Sat, Nov 13, 2010 at 2:11 PM, Vladimir Sedach <span dir="ltr"><<a href="mailto:vsedach@gmail.com">vsedach@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
I just pushed a patch that tries to do the right thing with both<br>
lexical and dynamic-extent BLOCK (including implicit BLOCK forms) and<br>
RETURN-FROM. It's also supposed to provide backwards-compatibility<br>
with the old-style RETURN behavior (although that does issue a<br>
warning).<br>
<br>
The big thing is that right now in most of the interesting cases it<br>
does the control jump, but does not return a value. That will be fixed<br>
in future patches.<br>
<br>
I haven't really tested it, so try it out and let me know what breaks.<br>
<br>
Vladimir<br>
<br>
2010/8/18 Daniel Gackle <<a href="mailto:danielgackle@gmail.com">danielgackle@gmail.com</a>>:<br>
<div><div></div><div class="h5">> I like your suggestion of emitting TRY/CATCH only in the cases where<br>
> it's necessary, i.e. only when trying to escape out of more than one<br>
> level of function nesting, seems like a good way to go. Then you're<br>
> only paying for the ugliness when you need it. It's in keeping with<br>
> PS's philosophy of staying close to what one would write by hand.<br>
><br>
> On Wed, Aug 18, 2010 at 6:12 AM, Red Daly <<a href="mailto:reddaly@gmail.com">reddaly@gmail.com</a>> wrote:<br>
>><br>
>> I added RETURN-FROM and BLOCK without too much effort using the<br>
>> implicit return functionality and try/catch. In my view this is the<br>
>> most reasonable way to implement this in the general case, since<br>
>> BLOCK/RETURN-FROM require non-local exit much in the same way that<br>
>> lisp's TRY/CATCH do.<br>
>><br>
>> The alternative to this approach is to exit from each function in the<br>
>> call stack via a Javascript `return' statement. Unfortunately, the<br>
>> call stack can contain many functions code over which the Parenscript<br>
>> compiler exerts little control, requiring throw as the control<br>
>> transfer mechanism. Thus, in the general case of unknown code on the<br>
>> call stack, there is no means to exit without a throw. I do not view<br>
>> throwing as an ugly solution at all, since try/catch was designed for<br>
>> non-local exits of all sorts.<br>
>><br>
>> Nonetheless, using try/catch to implement Parenscript features<br>
>> deserves some attention. Programs will need to ensure that they do<br>
>> not use try/catch in a way that interferes with the Parenscript<br>
>> convention. Generally, try/catch blocks should only catch specific<br>
>> exceptions and re-throw PS's exceptions. I'm happy to also implement<br>
>> a safe TRY/CATCH wrapper that re-throws Parenscript errors and catches<br>
>> everything else, too. However, we may want to make an official<br>
>> interface change to try/catch if any lisp-style non-local exit code<br>
>> becomes part of the language.<br>
>><br>
>> I present an example of why try/catch is unavoidable inline below:<br>
>><br>
>> On Fri, Apr 9, 2010 at 3:42 PM, Vladimir Sedach <<a href="mailto:vsedach@gmail.com">vsedach@gmail.com</a>> wrote:<br>
>> > Makes sense to me. I'll add this to my todo list (which I'll publish<br>
>> > in an email as soon as I'm done my current work on the PS compiler).<br>
>> ><br>
>> > Vladimir<br>
>> ><br>
>> > 2010/4/9 Daniel Gackle <<a href="mailto:danielgackle@gmail.com">danielgackle@gmail.com</a>>:<br>
>> >> I just pushed a patch (authored by Scott) to implement JS's LABEL and<br>
>> >> BREAK<br>
>> >> in PS. (Note that this patch deprecates LABELED-FOR since you can get<br>
>> >> the<br>
>> >> same effect by combining LABEL and FOR. Was anybody using LABELED-FOR?)<br>
>> >> Here's an example:<br>
>> >> (label scope<br>
>> >> (foo)<br>
>> >> (when (bar)<br>
>> >> (break scope))<br>
>> >> (blee))<br>
>> >> =><br>
>> >> scope: {<br>
>> >> foo();<br>
>> >> if (bar()) {<br>
>> >> break scope;<br>
>> >> };<br>
>> >> blee();<br>
>> >> };<br>
>> >> I was astonished to discover recently that JS has supported this<br>
>> >> ability all<br>
>> >> along in the form of labeled statements and labeled breaks. I'd always<br>
>> >> assumed that to get explicit returns from an arbitrary scope, you'd<br>
>> >> have to<br>
>> >> resort to the ugly hack of muscling TRY/CATCH to do it, thinking that<br>
>> >> this<br>
>> >> was the closest JS counterpart.<br>
>> >> (See <a href="http://news.ycombinator.com/item?id=793092" target="_blank">http://news.ycombinator.com/item?id=793092</a> for a thread in which<br>
>> >> several people believe this.) But it appears we were all wrong.<br>
>> >> What's not clear yet is how far this can be taken. Can you use it<br>
>> >> inside a<br>
>> >> nested local function to return immediately from the top-level<br>
>> >> function?<br>
>> >> That is one thing I've wanted for a long time.<br>
>> >> In the ideal case, LABEL/BREAK could be used as a base for implementing<br>
>> >> a<br>
>> >> proper BLOCK and RETURN-FROM in PS, something which we'd long believed<br>
>> >> to be<br>
>> >> impossible. One challenge is that in CL, RETURN-FROM can take a value,<br>
>> >> which<br>
>> >> becomes the value of BLOCK. In other words, BLOCK in CL is an<br>
>> >> expression<br>
>> >> while LABEL in JS is not. It seems, though, that most of this challenge<br>
>> >> has<br>
>> >> already been conquered with the development of implicit return in PS.<br>
>> >> The<br>
>> >> only thing you'd need to add is detecting when BLOCK is being used as<br>
>> >> an<br>
>> >> expression, declaring a gensymed variable and assigning whatever is<br>
>> >> happening inside BLOCK to that variable (much like implicit return<br>
>> >> already<br>
>> >> does with e.g. CASE), then put the variable in the expression position<br>
>> >> that<br>
>> >> BLOCK was in. It seems like this ought to work. It would also make<br>
>> >> things<br>
>> >> like this possible in PS:<br>
>> >> (1+ (case foo<br>
>> >> (:eleven 11)<br>
>> >> (:twelve 12)))<br>
>> >> Vladimir (and everybody), is the above clear? What do you think of it?<br>
>><br>
>> As stated above, I think try/catch is the way to go. There is no<br>
>> other way to exit a stack of functions in the general case otherwise.<br>
>><br>
>> For example, I can write a Javascript function that calls its argument<br>
>> infinity times and never returns.<br>
>><br>
>> function mapForever(fn) {<br>
>> while(true) fn();<br>
>> }<br>
>><br>
>> Now consider some parenscript:<br>
>><br>
>> (block non-local<br>
>> (map-forever<br>
>> (lambda ()<br>
>> (return-from non-local "we got out!"))))<br>
>><br>
>> To extricate itself from map-forever, there is no alternative but JS's<br>
>> throw statement.<br>
>><br>
>> Even if we had the ability to alter every function in the system, it<br>
>> would be necessary to inspect nearly every function call's return<br>
>> values to properly unwind the stack to the appropriate BLOCK.<br>
>><br>
>> Having said all that, there are cases when try/catch is not necessary<br>
>> for BLOCK/RETURN-FROM, as you have described. BLOCK should emit code<br>
>> according to the contexts in which RETURN-FROM appears. If there is a<br>
>> RETURN-FROM inside the same function, BLOCK can use a label for a<br>
>> local exit. If RETURN-FROM appears inside a lambda, try/catch is<br>
>> necessary (except in cases where you want to optimize this away by<br>
>> inspecting how that lambda gets passed around). If there are no<br>
>> return-froms, just emit a PROGN.<br>
>><br>
>> My solution does not do the local optimization, but it does refrain<br>
>> from putting try/catches around code with no return-froms.<br>
>><br>
>><br>
>><br>
>> Red<br>
>><br>
>> >> Daniel<br>
>> >> _______________________________________________<br>
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>> >><br>
>> >><br>
>> ><br>
>> > _______________________________________________<br>
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>> ><br>
>><br>
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</div></div></blockquote></div><br></div>