[parenscript-devel] ps-symbols (was Re: Conflict with % between Parenscript and CSS-Lite)
tc at travislists.com
Sun Dec 7 07:15:20 UTC 2008
Vladimir Sedach wrote:
> The gory details is that there was a patch pushed previously that
> introduced some changes to the way Parenscript handled symbols, which
> stripped out package information from macro names. I think this is
> the third time someone has tried to introduce "Parenscript symbols."
That would have been me (on 2008-04-05).
> There really is nothing wrong (and a lot of things otherwise not
> possible) with using Common Lisp symbols as-is.
I've given quite a bit of consideration to this issue, and I continue to
believe that there is something wrong with this, conceptually, as a
matter of practice, and with regard to our ability to make future design
improvements to the system. Perhaps you can convince me otherwise, but
please consider the problems with this approach. Here are my concerns:
Conceptually, I see the Parenscript system as a compiler. I see PS code
as data to the compiler, not as CL code. As data, not code, I think
there should be some basic barriers between PS and the lisp system.
There are substantial semantic differences between PS/JS symbols and
lisp symbols, and I think it suboptimal to have to import/use a ton of
PS/JS symbols into every lisp package where I want to use PS.
As a user of the PS compiler, I would expect that simply calling the PS
compiler interface with syntactically correct PS code should produce
correct JS output. But it doesn't unless I import/use all the possible
PS specials. Simple example:
cl-user> (require 'parenscript)
cl-user> (ps:ps (new foo))
"foo;" ;; wrong
That should at least raise an error to the user that things aren't going
to happen as expected.
While I understand that using CL symbols does permit some interesting
flexibility, at best it seems like a hack and causes leakage and
inconsistency in the PS abstractions. An longer example:
;; some library unrelated to PS/JS
(defpackage #:foolib (:use #:cl) (:export #:new))
(defpackage #:myjslib (:use #:cl #:ps) (:export #:bar))
(ps (defun bar () (do-some-js-magic)))
(defpackage #:myapp (:use #:cl #:foolib #:ps))
;; symbol conflict, resolve in favor of FOOLIB:NEW
(ps (new foo))
=> "foo;" ;; I think this is wrong. When deciding how to resolve the
;; NEW symbol conflict, I'm thinking about lisp, not PS.
(ps (ps:new foo))
=> "new foo;" ;; you might think this is acceptable, but then what
=> "bar();" ;; correct perhaps, but has completely different
;; semantics than above
=> "bar();" ;; following from the way we (didn't) handle the NEW
;; symbol, shouldn't this raise an error? And if we ever
;; namespaces right, this would almost certainly get in
;; our way.
I could go on with examples -- there are other cases I feel that violate
the principle of least surprise, but my basic point is that if we want a
sane package system for PS we really need to treat it as distinct from
the CL package system and implement one ourselves.
The reason for this is simple really. The PS/JS code has a different
'context' than the code in the lisp system. The context for CL code is
defined by the CL package system and lexical scoping rules -- the CL
compiler. The context for the JS output of PS, and hence for the PS
code itself, is defined by how the output JS files are loaded into the
browser. If I compile:
(output-ps-to-browser ;; made-up lisp function
(ps (defvar a))
(ps (let ((a 1))
Then the output will be subtly broken. The variable 'a' should be
treated by PS as special/global in BAR as well as FOO. There should be
a way to communicate to PS that both instances of JS output are going to
be evaluated together -- that they should be treated in the same
context. CL packages are a lousy way to do this. The CL package
approach encourages the user (once they run head-on into these surprises
a few times) to write all of the PS code for their application from a
single CL package. This discourages useful ways of emitting JS via PS
from various places in one's application and libraries.
I believe this is why ps-symbols keep getting introduced into the
library -- using the CL package system for PS causes subtle and
surprising behavior. There are definitely improvements that we can make
to the ps-symbol approach, but I believe it is a better basis from which
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