This is a long-ish case for including generators in ES4 as proposed.
I offered, to several Ecma colleagues, to mail pointers to examples
of how useful the Python-inspired generators in JS1.7 and JS1.8 in
Firefox, and proposed for inclusion in the ES4 standard, are in real-
world code. But I figured that folks on es4-discuss@mozilla.org might
like to see these few examples too.
I ported Peter Norvig's Sudoku solver:
http://norvig.com/sudoku.html
from Python to JS1.8:
https://bugzilla.mozilla.org/attachment.cgi?id=266577
This code uses not only generators and array comprehensions (e.g. the
cross function), but also generator expressions -- which are sugar
for generator functions immediately applied, and therefore lazy,
unlike array comprehensions. This laziness is important to avoid
using exponential amounts of memory.
My Mozilla colleague Igor Bukanov rewrote this code in more
straightforward JS-functional-programming style in JS1.8 (so using
expression closures, e.g. function add(x,y) x + y; but not
generators) -- see here:
https://bugzilla.mozilla.org/attachment.cgi?id=266938
Opinions vary on which version is better, but the generator-based one
is significantly shorter, and also faster in SpiderMonkey. And the
main thing is that it lets the code focus on the essentials of the
search algorithm and minimize the bookkeeping, which Peter's Python
code did very well (Python has lighter syntax, unburdened by the C
heritage, but JS can't disown curly braces and parens; other than
that the JS and Python versions are close).
Another example is a static analysis script (one of many by Dave
Mandelin) for Mozilla's TreeHydra GCC plugin (developed by Taras
Glek and Dave):
https://bugzilla.mozilla.org/attachment.cgi?id=311698
Notice the yield usage, and also the array comprehensions returned,
e.g., by rectify_attribute_args.
Rewriting these to use iterators, or ES3-style functional
programming, adds a lot of source boilerplate that again obscures the
essential code, and tends to perform not as well to boot. To pick one
example, here is rewrite from the flatten_chain generator:
function flatten_chain(chain_head) {
for (let o = chain_head; o; o = TREE_CHAIN(o)) {
yield o;
}
}
to this roughly equivalent iterator coded using pure ES3:
function flatten_chain(chain_head) {
return {
next: function () {
var o = chain_head;
if (o) {
chain_head = TREE_CHAIN(chain_head);
return o;
}
throw StopIteration;
}
}
}
Note that this rewrite loses integrity since next is mutable, which
ES3 can't control (the ES4 methods, also in JS1.7 and higher versions
in Firefox 2-3, are DontDelete and ReadOnly). And of course it leaves
out the rest of the generator suite, send/throw/close, which come for
free in the ES4 Generator class instantiated by a call to a function
containing yield.
Here is the expansion of that array-comprehension-returning
flatten_chain caller, rectify_attribute_args, that I mentioned above,
from:
function rectify_attribute_args(tree) {
return [ TREE_STRING_POINTER1(TREE_VALUE(a)) for (a in
flatten_chain(tree)) ];
}
to this ES3 code:
function rectify_attribute_args(tree) {
var r = [];
var i = flatten_chain(tree);
for (;;) {
var a;
try {
a = i.next();
} catch (e if e instanceof StopIteration) {
break;
}
r.push(TREE_STRING_POINTER1(TREE_VALUE(a)));
}
return r;
}
Again I omitted the finally clause to call i.close and other bits of
the general generator mechanism. Of course one could specialize the
termination technique and other details to re-optimize, but why
should this be necessary?
The Python-based syntax is subject to criticism for changing the
meaning of a function once yield is used in the body of the function,
but we are hitching wagons to Python and reusing community brain-
print and design experience (also giving feedback to simplify future
versions of Python based on our experience, specifically by
eliminating the GeneratorExit exception).
And as far as I know from the experience in Firefox 2 and 3, we've
had no problems with the potential confusion caused by this extension
to function syntax -- it has been painless.
The ES4 iteration protocol is proposed here:
http://wiki.ecmascript.org/doku.php?
id=proposals:iterators_and_generators
The iteration protocol underlies for-in and for-each-in constructs in
loops and comprehensions. It hides iterator-specific implementation
details such as StopIteration, while providing uniform looping syntax
that can be customized to improve (I would say restore) the utility
of the built-in JS for-in syntax.
Generators, besides supporting one level of coroutine suspending and
(re-)calling, are the cheapest way to implement an iterator. Unlike
general coroutines, they do not break functional abstraction by
jumping over multiple (possibly
