Macros in JavaScript | Hacker News Reader

Macros in JavaScript

fitzgeraldnick.com

18 pointsmnemonik7 comments

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Parenscript already provides macros. You can write in parenscript and output a .js file.

Example usage (taken from the mailing list):

1. standard with-foo macro

(with-elements-by-ids (mydiv (other "otherdiv")) (setf mydiv.inner-h-t-m-l other.inner-h-t-m-l))

2. multiple-value return in javascript

(defun divmod (a b) (return (values (/ a b) (% a b))))

(receive (div mod) (divmod 10 3) (debug (+ "div: " div ", mod: " mod)))

Since JS has true closures and a flexible object model (someone wrote a CLOS-y layer for it recently) there is no need to take the JSR way and try and push for a community-accepted broken macro system. Besides, as someone (Jason Dominus?) posted to the perl list, lisp macro's are regular and do not break, unlike every other proposal that uses infix syntax.

There are infix-sytnax macro systems that work. See metalua, for example.

Yes, if you really want the power of macros inside a browser ParenScript is the only real choice. I mentioned ParenScript at the end of the post. A ParenScript and Hunchentoot project is definitely on my TODO list for whenever I next have time off of school and work.

I'm sure Douglas Crockford would agree with you and say that JavaScript already has too many nasty and broken features (ie the DOM and "with") but I was just having some fun on my day off from work.

As others have pointed out, the expander can't even survive expansion, and using regexes for code parsing is a poor choice. Maybe, I haven't emphasized it enough, but I was just having some fun and imagining what it would be like to code with real macros in JS. I'm not advocating anyone actually use this code in any of their projects, nor that ECMAscript include simple template style macros in the spec.

That said, I wouldn't mind if someone built real lispy macros into a JavaScript implementation by forking Rhino or Spider Monkey. I'm convinced that this is possible despite the verbose syntax. Check this out: http://javascript.crockford.com/tdop/tdop.html

Proposals for how to support macros in JavaScript are very interesting. I am working on Mascara (http://www.mascaraengine.com/) which is basically a type-verification engine and macro expander for JavaScript. However it only supports "hardcoded"-macros in the form of transformations from ECMAScript Harmony (or whatever the next version of ECMAScript is going to be called) into plain old JavaScript.

I would love the hear ideas about how to support user-defined macros.

I notice that the submitted article uses simple string-substitution macros (like the C preprocessor). I think that is too limited and have too many pitfalls. I would prefer lisp-style macros (macros operating on the syntax tree), although I realize it is more complex in a language like JavaScript with a larger grammar.

The most powerful approach would be to allow the user to extend the language grammar however they like. But that is also complicated for the user, because you has to know the grammar of the language pretty well to avoid introducing ambiguities.

A more limited approach would be to allow macro-invocation with a predefined syntax, e.g. macroname ( arguments ) { statements } - the macro could then define transformations for the arguments and statements. This could support much of what you could want without having to mess with the actual language grammar. It would not be able to support the -> and ~> ideas in the post though, so perhaps this is to limited.

These are C-style "#define" macros, not lisp-style macros. More here: [ http://news.ycombinator.com/item?id=644956 ].

Incidentally, running his macro processor on his macro processing code breaks it. Parsing code with regexps is very, very flaky.

Consider http://code.google.com/p/google-jstemplate/

"Simple and robust browser side template processing for Ajax based web applications"

While this isn't a macro processor, it actually solves a related problem of management and separation between data (javascript, json) and presentation (html)

Here are the slides: http://google-jstemplate.googlecode.com/svn/slides/jstemplat...

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