[parenscript-devel] 'use strict' and 'with' in loop

[Daniel Gackle]

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 at 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_function_scope/Strict_mode
>
> 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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>
>
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