Generator use-cases

[Brendan Eich-2]

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@... 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 native) stack frames, which would  
have to be saved, and then restoring the saved stack frames later.  
They're simple, for better and for worse.

In my opinion, and to borrow from Tallyrand (or whoever said it  
first), if ES4 has iterators but not generators, it would be  
something worse than a crime -- it would be a mistake.

/be
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[Kris Zyp-4]

> Generators, besides supporting one level of coroutine suspending and
> (re-)calling, are the cheapest way to implement an iterator.

I think Neil's inspiring demonstration of pseudo-threading with generators
is also worthy of inclusion in your list of generator use cases:
http://www.neilmix.com/2007/02/07/threading-in-javascript-17/
It is quite interesting in that single level coroutines can be combined to
create multi level coroutines in a form that provides explicit control of
creating composable flow or maintaining atomicity. Generators are a pretty
powerful and valuable construct, IMO. I certainly hope they stay in ES4 as
well.
Kris

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[Igor Bukanov-2]

On 29/03/2008, Kris Zyp <kris@...> wrote:
> I think Neil's inspiring demonstration of pseudo-threading with generators
>  is also worthy of inclusion in your list of generator use cases:
>  http://www.neilmix.com/2007/02/07/threading-in-javascript-17/

That code can be written without generators. In general whenever the
code in the examples from the blog does yield, one can replace that
with "return function() { the rest of code from the function}". But
this would require to replace imperative loops from the examples by
recursive functions.

This shows that if one programs in a functional style, then generators
are not that useful. But they are valuable if the code uses explicit
loops etc. Plus with generators one can assume certain time and space
complexity bounds in ES4 which is not the case for functional code in
the view of deferred tail call proposal.

Regards, Igor
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[Brendan Eich-2]

On Mar 29, 2008, at 7:40 AM, Igor Bukanov wrote:

> On 29/03/2008, Kris Zyp <kris@...> wrote:
>> I think Neil's inspiring demonstration of pseudo-threading with  
>> generators
>>  is also worthy of inclusion in your list of generator use cases:
>>  http://www.neilmix.com/2007/02/07/threading-in-javascript-17/
>
> That code can be written without generators. In general whenever the
> code in the examples from the blog does yield, one can replace that
> with "return function() { the rest of code from the function}". But
> this would require to replace imperative loops from the examples by
> recursive functions.

This particular continuation-passing style would also require proper  
tail calls

http://wiki.ecmascript.org/doku.php?id=proposals:proper_tail_calls

to be normative, but (news flash) proper tail calls are out of ES4 as  
of yesterday's Ecma TC39 meeting, by general (regretful, in Mozilla's  
case) agreement.

> This shows that if one programs in a functional style, then generators
> are not that useful. But they are valuable if the code uses explicit
> loops etc. Plus with generators one can assume certain time and space
> complexity bounds in ES4 which is not the case for functional code in
> the view of deferred tail call proposal.

I'm glad you mentioned tail calls. I'd word it more strongly, as  
above: tail calls are required if CPS is the preferred style to use  
in lieu of generators. Of course my mail showed a non-CPS expansion  
of generators into functions and objects, which does not require tail  
calls. But it's verbose, sub-optimal for implementations, and error  
prone compared to generators.

/be

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[Kris Zyp-4]

>> I think Neil's inspiring demonstration of pseudo-threading with
>> generators
>>  is also worthy of inclusion in your list of generator use cases:
>>  http://www.neilmix.com/2007/02/07/threading-in-javascript-17/
>
> That code can be written without generators. In general whenever the
> code in the examples from the blog does yield, one can replace that
> with "return function() { the rest of code from the function}". But
> this would require to replace imperative loops from the examples by
> recursive functions.

Of course, but with any flow control (loops, branches, etc.), CPS in JS can
quickly turn into a ugly and painful exercise. Generators don't provide any
fundamentally higher level of computability than lambda gave us, but they
are powerful construct with value. My point was simply to add to the list of
use cases for generators and throw in my vote for them. I didn't mean to
detract from Brendan's list.
Thanks,
Kris

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[Jason Orendorff]

On Fri, Mar 28, 2008 at 10:32 PM, Brendan Eich <brendan@...> wrote:
> This is a long-ish case for including generators in ES4 as proposed.
> [...]

Here's a brief case:  generators let you factor a complex loop,
dividing the value-producing part from the value-consuming part.
Neither part has to be transformed in a non-obvious way.  With
iterators alone, you do this by rewriting the value-producing part as
an Iterator class.  This obfuscates the iterator implementation.

It's hardly news that abstracting from sequences is useful.  Various
languages have had iterators, streams, and lazy lists as core,
essential features (with syntactic support) for decades now.  For
stateful languages, like Python and C#, iterators and generators fill
this gap better than anything else I've seen.

Adobe's position paper commented about generators not being that
useful, because they're not "deep", like full coroutines.  Well, some
people get excited about the coroutine-like uses for generators.  For
me, generators are just the language support you need to use iterators
in your code as the powerful abstraction they ought to be.

-j
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[Brendan Eich-2]

On Mar 30, 2008, at 10:58 AM, Jason Orendorff wrote:

> On Fri, Mar 28, 2008 at 10:32 PM, Brendan Eich  
> <brendan@...> wrote:
>> This is a long-ish case for including generators in ES4 as proposed.
>> [...]
>
> Here's a brief case:  generators let you factor a complex loop,
> dividing the value-producing part from the value-consuming part.
> Neither part has to be transformed in a non-obvious way.  With
> iterators alone, you do this by rewriting the value-producing part as
> an Iterator class.  This obfuscates the iterator implementation.

Thanks, this is nice and short. I wanted to give example links and  
excerpts for those new to generators, but what you wrote here is a  
great summary and intro.

BTW, no single nominal Iterator class or interface is required, of  
course -- all you need is an object 'like IteratorType'.

> It's hardly news that abstracting from sequences is useful.  Various
> languages have had iterators, streams, and lazy lists as core,
> essential features (with syntactic support) for decades now.  For
> stateful languages, like Python and C#, iterators and generators fill
> this gap better than anything else I've seen.

Agreed.

> Adobe's position paper commented about generators not being that
> useful, because they're not "deep", like full coroutines.

Not to worry, Adobe is on board:

http://spreadsheets.google.com/pub?key=pFIHldY_CkszsFxMkQOReAQ&gid=2

(I hope this is publicly readable -- it should be).

/be

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[Neil Mix]

On Mar 30, 2008, at 12:58 PM, Jason Orendorff wrote:
> Here's a brief case:  generators let you factor a complex loop,
> dividing the value-producing part from the value-consuming part.
> Neither part has to be transformed in a non-obvious way.  With
> iterators alone, you do this by rewriting the value-producing part as
> an Iterator class.  This obfuscates the iterator implementation.

Here's a concrete example of this: fetching and aggregating rows from  
a database.  You want to fetch rows from large queries in bulk (say  
10000 at a time) for perf reasons, but you want to hide that from your  
consumer which wants to see things one row at a time.

I recently wrote some python code that aggregated and integrated two  
data sources where performance was critical.  I had two SQL queries  
ordered by id (multiple rows per id).  Needed to aggregate each query  
by id and merge the results when the same id was in both results.  
Datasets were way to large to fit in-mem, and it was too slow to let  
the DB do the work spooling to disk.  So I created a generator for  
each query that fetched forward in bulk and aggregated by id, and a  
third generator that pulled records from the two query generators and  
merged as necessary.  The top-level code was then able to iterate  
simply one record at a time by pulling from a generator, oblivious to  
all the buffering, aggregating, and merging going on beneath.  Elegant  
and easy.  Had I not had generators at my disposal I might still be  
writing that script.  ;)

Krys mentioned the "advanced" (some would call it "abusive") mock-
coroutining on my blog.  Setting that aside as perhaps an over-
indulgence in generator goodness, I'm a big fan of generators for very  
practical use-cases like the one I mentioned above.  Especially for a  
language that lacks blocking IO facilities in most implementations,  
plus no concurrency, I'm certain generators would find a host of real-
world practical uses.  It would be a shame if they didn't make it into  
ES4.
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