[parenscript-devel] Bug: let within an outer let's init-form

[Andy Peterson]

Vladmir,

Thanks for the detailed response.  I can see you are right.  Now I am
totally confused why I got the error a month ago in google chrome as I can
not recreate it. I swear I tested my original example. I remember writing
an function a and b and testing. On top of that, the original macro I wrote
now seems to work within a let.

Anyway, this is the original macro for a "functional-pseudo-chain":
(defpsmacro fpchain (object &rest chains)
  "generate a pseudo-chain of method calls of given object
and return the object. for use of js prototypes that do not
allow chaining like j-query."
  (ps-once-only (object)
    (append '(progn)
            (mapcar (lambda (x) `(chain ,object ,x))
                    chains)
            `(,object))))

I use this primarily for the html5 canvas context object which requires a
lot of method calls in a row but does not allow chaining.
Similar to ps:chain except for the fact it does not chain properties
 (fpchain foo bar (baz)) does not work.

thanks again,

andy peterson


On Thu, Nov 29, 2012 at 12:31 AM, Vladimir Sedach <vsedach at gmail.com> wrote:

> Believe it or not, that's actually valid JavaScript code:
>
> PS> (ps (let ((x (let ((y 12))
>                    (+ 1 2)
>                    y)))
>       (1+ x)))
> "(function () {
>     var y;
>     var x = (y = 12, (1 + 2, y));
>     return x + 1;
> })();"
> PS> (cl-js:run-js *)
> 13
>
> In your example, a() and b() were expressions, so the inner let
> generated a sequence of expressions by using the comma operator. If
> for example you replace b() by a statement:
>
> PS> (ps (let ((x (let ((y 12))
>                 (dolist (x '(1 2 3))
>                   (* x x))
>                 y)))
>        (1+ x)))
> "(function () {
>     var y;
>     var x4 = (y = 12, ((function () {
>         for (var x = null, _js_arrvar6 = [1, 2, 3], _js_idx5 = 0;
> _js_idx5 < _js_arrvar6.length; _js_idx5 += 1) {
>             x = _js_arrvar6[_js_idx5];
>             x * x;
>         };
>     })(), y));
>     return x4 + 1;
> })();"
> PS> (cl-js:run-js *)
> 13
>
> This is ugly, and I would love examples of how to generate better code
> for nested LETs.
>
> Happy hacking,
> Vladimir
>
>
> On Wed, Oct 24, 2012 at 5:50 AM, Andy Peterson <andy.arvid at gmail.com>
> wrote:
> > If you use a let within the init-form of an outer let, the result is
> invalid
> > javascript code.
> >
> > Here is a simplified example:
> >
> > (ps (let ((x (let ((y (a)))
> >        (b)
> >                y)))
> >       (1+ x)))
> >
> > ==>
> >
> > "(function () {
> >     var y;
> >     var x = (y = a(), (b(), y));
> >     return x + 1;
> > })();"
> >
> > Not that I would normally write such code, but the inner "let" was
> generated
> > by a macro.
> > And many macros use "let" with gensyms.  In my case, the macro used
> > "ps-once-only".
> >
> > Andy Peterson
> >
> > _______________________________________________
> > parenscript-devel mailing list
> > parenscript-devel at common-lisp.net
> > http://lists.common-lisp.net/cgi-bin/mailman/listinfo/parenscript-devel
> >
>
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