Hi Peter,
Returning the closure from a function works the same in CL and parenscript and does not generate the 'with'
Yes, because functions create new bindings for their args, which of course suggests that PS might be able to use:
(function (i) { ... })(i)
instead of
with ({ i : i }) { ... }.
For example, evaluating the following in a PS hacked to remove the 'with' trick produces the desired '(10 20 30):
(let ((closures (loop :for i :from 1 :to 3 :collect ((lambda (i) (lambda () (* i 10))) i)))) (loop :for fn :in closures :collect (funcall fn)))
Would that trick work correctly everywhere that the 'with' trick does? I'm not sure if one might run into trouble with statements vs. expressions in some cases, but the PS implementation has solved most of those issues, so I'd be surprised if lambdas couldn't do it. You'd arguably take a hit in readability, but might gain something in performance, since 'with' is slow (or used to be). It's also worth noting that since the 'with' trick is there only for closures that capture loop vars, it needn't be the whole loop body that gets wrapped in an extra lambda (the way it currently gets wrapped in 'with'), but only the creation of the closure itself — which is certainly just an expression.
Hopefully Vladimir will eventually chime in on this (he's been busy working in LA lately, but we expect he'll get back to open source at some point.)
There's another problem with strict mode and PS: strict mode banishes arguments.callee, which PS currently relies on for its multiple-return-value implementation.
I'm glad you raised this question. It seems like PS ought to be able to emit strict code on demand, if not by default. One of PS's goals has always been to produce efficient JS, and one of the reasons for strict mode is to support optimization. Moreover, the annoyance of having to declare strict mode in JS scripts or functions is alleviated by having a compiler like PS do it for you. A nice touch would be to leverage the DECLARE syntax to get this at the function level.
Daniel
On Sat, Mar 2, 2013 at 1:47 AM, Peter Wood p.r.wood@gmail.com wrote:
Hi Daniel
Thanks for the nice explanation. It's a cool trick (although personally I would prefer it if parenscript followed CL's scoping). You can get the desired '(10 20 30) behaviour in CL (I'm using SBCL) like this:
(let ((closures (loop :for i :from 1 :to 3 :collect (let* ((j i) (cl (lambda () (* j 10)))) cl))))
(loop :for fn :in closures :collect (funcall fn)))
==> (10 20 30)
However, when I tested this in a parenscript hacked not to generate the 'with ({i : i})' it didn't work (the generated js returns '[30, 30, 30]'. It _does_ work in the vanilla parenscript, but also generates the offending 'with ({j . null})'. Returning the closure from a function works the same in CL and parenscript and does not generate the with object stuff.
(defun foo () (labels ((make-cl (i) (lambda () (* i 10))))
(let ((closures (loop :for i :from 1 :to 3 :collect (make-cl i)))) (loop :for fn :in closures :collect (funcall fn)))))(foo) ==> (10 20 30)
foo() ==> [10, 20, 30]
So, apart from using do, there is also a labels 'workaround' when I want to 'use strict'; Whatever you guys decide, thanks again for parenscript. It's excellent and fun as well!
Regards, Peter
On 2 March 2013 02:08, Daniel Gackle danielgackle@gmail.com wrote:
Hi Peter,
Welcome! The 'with' trick (which confused me when I first saw it as well) occurs when a loop contains a closure that captures the loop iteration variable:
(loop :for i :from 1 :to 3 :collect (lambda () (* i 10)))
The question is what value of i each lambda should use when it's called. The 'with' trick establishes a new scope with a new binding for 'i' inside the loop body and puts the closure inside that scope. Thus, if you do this:
(let ((closures (loop :for i :from 1 :to 3 :collect (lambda () (* i 10))))) (loop :for fn :in closures :collect (funcall fn)))
... you get '(10 20 30), because each closure remembers the value that i had when it was created. Without the 'with' trick, you'd get '(40 40 40), because the closures all share the loop's original binding for i, and that held 40 by the time the loop terminated.
The fact that it breaks Strict mode, though, means that either PS's implementation should change, at least to offer the option of not using it, or drop the trick altogether. I have a feeling the latter would be simplest. For one thing, Common Lisp, which is PS's touchstone, doesn't have this scoping behavior. In CCL I get '(40 40 40) for the above expression. And DOTIMES is the same:
(let ((list nil)) (dotimes (i 3) (push (lambda () (* i 10)) list)) (mapcar #'funcall (reverse list)))
=> (30 30 30)
So this is a case of plus royaliste que le roi that could arguably just be abandoned. If not, though, a special variable for Strict Mode would be a good idea. Vladimir?
Daniel
On Fri, Mar 1, 2013 at 12:39 PM, Peter Wood p.r.wood@gmail.com wrote:
Hi
It's my first post, so first of all, thanks to everyone who works on parenscript. It is a lifesaver.
If I 'use strict'; in the start of my scripts, they fail in some of the loops because parenscript is generating a 'with' (which is not allowed in strict mode). There is quite a nice explanation of why it isn't allowed here:
https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_...
It is easy to change my code to use 'do' instead, and for now, that's what I've done, but it's a shame not to be able to use parenscript's very nice loop for writing javascript loops. I haven't looked at parenscript's code before this evening, but I think this is the relevant spot (in src/special-operators.lisp)
(defun compile-loop-body ... (aif (sort (remove-duplicates *loop-scope-lexicals-captured*) #'string< :key #'symbol-name) `(ps-js:block (ps-js:with ,(compile-expression `(create ,@(loop for x in it collect x collect (when (member x loop-vars) x)))) ,compiled-body)) compiled-body)))
Here is an example of some lisp and the js which it generates:
(ps:ps (defun foo () (loop for i from 1 to 5 append (loop for j from 1 to 5 collect (list i j))))) ==> "function foo() { return (function () { var append9 = []; for (var i = 1; i <= 5; i += 1) { with ({ i : i }) { ^^^^^^^^^^ append9 = append9.concat((function () { var collect10 = []; for (var j = 1; j <= 5; j += 1) { collect10['push']([i, j]); }; return collect10; })()); }; }; return append9; })(); };"
What is the point of even having the 'with ({ i : i })' in there ?? I have tried removing the form starting (ps-js:with ... ) and the code which is then generated runs fine and has no 'with', but of course it is probably breaking something else. I don't understand why it's there.
Regards, Peter
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