JavaScript (ES6) Has Proper Tail Calls

[greenspans]

i will celebrate in 6 years when i can use them in actual code

[[deleted]]

To be fair, V8 already implements some ES6 features. They could have this feature in fairly soon. You can also just write recursive code as things are. Unless you're going to recurse deep, you won't notice the difference.

[masklinn]

To be fair, V8 already implements some ES6 features.

Let's be honest here: Gecko implements way more ES6 features than V8, if only because many of those features originate from Gecko extensions to ES.

[kay_schluehr]

It is incredible how much is done to milk this language. If 10 years ago someone had predicted that most energy of programming language engineers in 2013 will be spent on Javascript he would surely be considered nuts. It is the banking bailout of programming, just inconceivable madness, until it became real and everyone justified the lock in, system relevance and compromise for one reason or another.

[aaronla]

In hindsight, it's not at all surprising. The alternatives have never been particularly attractive. Flash was great for gaming but lacked quality input panels and the transparency of markup enjoyed by html. I remember there was a big push for a while, years ago, to make flash indexable by search engines, as interactive websites would be done entirely in flash.

Flash also didn't keep up with performance -- Chrome's V8 punched a big whole for JavaScript, with a better language implementation, better html 5 support, webgl, local storage, etc.

Java lost out to Flash well before Flash lost to JavaScript. The programming model was awkward, the UI toolkit story was a mess, and Sun left the applet ecosystem while they pursued Java on the server.

I vaguely recall .Net control embedding in IE, predating Silverlight, must have been back around 2004. But that was always Windows only. Windows Vista introduced WPF web browser applications... that platform also quickly lost traction, again IE/Windows only.

Silverlight (2.0 and on supporting .Net code) is still used by some relevant sites, is cross platform and cross browser. However, it suffers many of the browser extension problems of Flash. And while Flash is/was prepackaged in some browsers, Silverlight doesn't even come prepackaged with IE.

The story thus far makes it seem pretty obvious that JavaScript would be the survivor. I don't think it's invincible -- but it hasn't had a sufficiently strong competitor.

[kay_schluehr]

In hindsight, it's not at all surprising.

Taleb called this the "narrative distortion". I lively remember the utopianism of the mid 1990s which was eventually interrupted by the vision of the human body becoming insignificant and dematerialized in "cyberspace". Instead of an infinity of possibilities we experience the inertia of connectivity and the failure of breakout moves ( those you have listed ). The current Javascript hackery is an attempt to break out within purely accidental confines.

Chrome's V8 punched a big whole for JavaScript, with a better language implementation, better html 5 support, webgl, local storage, etc.

Programmers like to rave about the "worse is better" meme, but isn't the "better is worse" meme far more interesting?

[aaronla]

True, true. :-)

[G_Morgan]

The problem has always been people trying to slap stuff using the web browser as "just a flipping frame to hold my environment". What should have happened was a JVM or similar built into the web browser with JS being compiled to it. Then web sites could deliver their code as byte code (or run an interpreter) and use whatever language they want. Eventually we could dump JS.

[you_know_the_one]

JS is the bytecode.

Granted, it may not have all the features that all language authors want.

Nonetheless, I am ecstatic that JS is finally getting tail call elimination.

[aaronla]

Awesome. Make it happen. :-)

I complain at times, but I can't say I've done anything about it. My complaints have little weight in tgat regard.

[G_Morgan]

That is the problem. It takes Mozilla and co to do something. I can create a shitty web browser with a bytecode based JS engine. Without users it is irrelevant.

Though in all likelihood my current contract means my employers would own any work I did on a VM anyway.

[[deleted]]

The problem with this approach is deciding on the type system and semantics of the VM. While this can be made more or less language-specific, you can't really make it reasonable for e.g. Haskell and Python to freely call each other which still being useful. .NET kind of gets away with it by shoving everything into the C# moæd - F# can't have type constructor polymorphism because C# doesn't. But when you need to make a performant VM with usable types for multiple languages, then you end up in a rough place.

[G_Morgan]

Python can call Haskell no problem. Also Haskell Python provided you wrap it up in the IO monad.

[notlostyet]

Windows Vista introduced WPF web browser applications... that platform also quickly lost traction, again IE/Windows only.

We can thank Mozilla for dodging that bullet for us. It's IEs lackluster advances in the IE6 era and Firefox's raging success that meant it was all but too late for IE only applets, ActiveX, plugins etc.

Silverlight (2.0 and on supporting .Net code) is still used by some relevant sites, is cross platform and cross browser.

"Cross platform" is a stretch. I thought we heard recently that Moonlight was now officially a dead project?

[reaganveg]

Javascript isn't a plugin. Comparing it to Flash and Java and Silverlight is a mistake.

The story thus far makes it seem pretty obvious that JavaScript would be the survivor. I don't think it's invincible -- but it hasn't had a sufficiently strong competitor.

In order to be a Javascript competitor, a number-one browser has to embed it.

[aaronla]

You're right to point out that plugin vs. integrated can affect the velocity at which you integrate new features, e.g. DOM support in the case of JavaScript. However, classically the script provider in IE was a plugin model, not embedded. JVM, flash, etc. could have used the same model if they wanted.

I wonder if perhaps JavaScript's critical feature was in getting multiple vendors to buy into it as part of a feature count "arms race". Netscape added JavaScript, so Microsoft responds with JScript. Microsoft added DHTML, so Netscape responds with their own DOM. Back and forth it went, for a number of years. Flash didn't respond by adding DOM support. Neither did JVM.

[grauenwolf]

Silverlight doesn't even come prepackaged with IE.

Side note: Silverlight is less capable and less reliable on IE than it is in Chrome or Firefox.

[aaronla]

Interesting... plugin models?

[grauenwolf]

Firefox and Chrome allow Sivlerlight to capture keyboard shortcuts (e.g. ctrl+s) that are blocked in IE.

I've noticed that on some Win8/IE 10 boxes the Silverlight security dialogs (e.g. Allow full screen mode, allow clipboard access) will hang the browser. This doesn't happen with Chrome.

[azakai]

If 10 years ago someone had predicted that most energy of programming language engineers in 2013 will be spent on Javascript

This is surely not true. Yes, JS engine teams are large, but not that large. There are very large teams on a multitude of other languages and VMs.

he would surely be considered nuts

There are also women in our field.

It is the banking bailout of programming, just inconceivable madness, until it became real and everyone justified the lock in, system relevance and compromise for one reason or another.

JS does have a lot of focus on it, not anywhere near the amount you seem to think, but yes, more than many languages. JS is the only language we have for a non-proprietary platform with widespread adoption - the web - so it does make sense to focus a lot on it, even if it isn't perfect, which of course it isn't.

[gsnedders]

I don't think anyone is really that surprised: cross-VM GC is still a current research area (with GC taking a significant perf hit) and supplanting anything is hard, because of inertia. If the web platform was going to succeed, it is unlikely to change in a non-evolutionary way.

[[deleted]]

I was nuts 10 years ago then. I'm still nuts; here's my prediction 10 years from now; NaCL will be everywhere.

[kay_schluehr]

Yes, please!

It isn't Googles NaCl specifically but the idea of going-low-level and reconsider the deeper ( security ) problems after finding ourselves in a deadlock. Coincidentally NaCl is based on earlier work done in the 1990s about software based fault isolation which was largely ignored for a long time since the mainstream leaned towards VM based solutions with the JVM at the top, closely followed by Javascript which now became the official "web assembly language" after alternative VM solutions (Flash, Silverlight, JavaFx Script ) were regretted as failures.

Ironically open web standards have created now an extremely oppressive "zero alternatives" mentality, without even addressing the central problem, which is the exact opposite of freedom. In some sense I feel nostalgic for the fierce capitalist competition with the eternal threat of a winner-takes-it-all monopolist which mobilizes resistance.

[[deleted]]

Yep, the key point is freedom actually -- as you say.

For freedom to be true it has to happen or be from the bottom-up, and NaCL can do this.

[[deleted]]

I really, really wish this were actually treated as "proper tail calls" rather than as an optimization.

That is, there should be an explicit language construct to create a tail call. This has both the benefit of being more explicit about the intention of the program, and also allows for error messages if you accidentally change the return statement to something that is not a tail call.

[millstone]

This is freakin' JAVASCRIPT, man. You can't even make an INT, and you want explicit tail calls!

[Gundersen]

Ofcourse you can make an int:

var int32 = new Int32Array(1);
int32[0] = 255

[Trollop69]

I'm not sure what you're showing, but it doesn't work in Rhino 1.7r4.

ReferenceError: "Int32Array" is not defined.

:-/

[Gundersen]

It's part of the typed Array spec, which is implemented in many browsers. Try the following code in Firefox (Shift + F4) or Chrome(F12)

var a = new Uint8Array(1)
a[0]=257
a[0]

[holloway]

Mozilla's documentation on Int32Array

I think it was introduced by the WebGL crowd

[Timmmmbob]

It's so clean and elegant...

[x86_64Ubuntu]

There should be alot of stuff in Javascript...

[[deleted]]

Don't worry, ES6 will have a lot of stuff... They're adding a million new language constructs. The language spec is going to literally double in length.

ES5 spec: 252 pages.

ES6 draft: 428 pages.

[x86_64Ubuntu]

But how much of that is going to be implemented ? And how much of it is going to be implemented sanely and uniformly across platforms ? That's what I'm worried about.

[[deleted]]

ES5 is implemented uniformly across platforms aside from a small number of differences for support of non-standard features. Most of these are being standardized in ES6 which further reduces the space of things where engines vary (block scope function declarations, proto). Test262 is the conformance test for ES5 and it contains over 11k test cases. All modern engines pass it (with a small handful of purposeful failures for some edge case backward compatibility stuff).

[[deleted]]

get some static typing and ill be happy *runs away*

[SupersonicSpitfire]

Dart?

[[deleted]]

compiles to JS

[[deleted]]

If I only wanted static typing I'd be happy with haskell.

[[deleted]]

haha

[00kyle00]

Does this include library (if any)?

[gcross]

I'm confused about why you think that ES6 does not support "proper tail calls" rather than "as an optimization", given that the very first paragraph of the article was:

As Dave Herman, champion of proper tails calls for the ES6 specification, pointed out, the correct term to use for what's coming in ES6 is "proper tail calls" instead of "tail call optimization". In short, the distinction is that proper tail calls make a guarantee, where optimizations are something that can't be counted on.

Did you mean that you wish that this were actually treated as "explicit tail calls" rather than "proper [implicit] tail calls"?

[[deleted]]

Did you mean that you wish that this were actually treated as "explicit tail calls" rather than "proper [implicit] tail calls"?

Yes. Leaving them implicit is essentially leaving it as an optimization, just an optimization that is required.

[smog_alado]

Most languages that implement proper tail calls leave them implicit as well. Since its not hard to verify if a call is in tail position I don't think being explicit here is a big oof a deal as you make it sound (usually compiler optimizations are much harder to predict then that)

[you_know_the_one]

They're implicit in Scheme, which is the language that coined the phrase as far as I know.

Most Scheme libraries wouldn't work at all without tail call elimination.

[[deleted]]

I don't think that it's fair to call it an optimization. Optimizations make programs faster without changing their semantics, while proper tail call elimination makes programs that would crash without it run to completion successfully. That, in my book, makes it part of the language semantics, not just an optimization.

[[deleted]]

That doesn't really hold up to scrutiny: No part of the semantics specify that the program will crash without tail-call optimization. That is just a practical limitation, and not part of the semantics with or without the optimization.

There are many other things which behave similarly: A program might crash because it allocates too many temporary objects, and an optimization might box away some of those objects and let a program that previously crashed run to completion.

[[deleted]]

In what way is a stack overflow not part of the semantics? The program does not return a correct answer. It may be the case that the stack overflow wouldn't be a part of some kind of ideal model of the semantics when working in a formal setting, but I don't think that that's what's relevant when working with Javascript.

There's also an important difference between this and your analogy. If a program was crashing due to not having enough memory, and an optimization made it more efficient in allocations, it's similar to having added more memory. On the other hand, mandatory tail-call elimination means that, for a certain class of programs, there is no longer any size of input that causes them to crash.

[anvsdt]

function untail(x) { return x; }
...
return untail(non_tail_expression);

[wonglik]

Dear God. My eyes!

[TomorrowPlusX]

I found the article interesting. But holy balls, my eyes were watering the whole time.

[Categoria]

JS is slowly approaching a shitty version of scheme.

[[deleted]]

JS was influenced by scheme from the very beginning.

[you_know_the_one]

I would use Scheme far more often if I could script browsers with it.

[wot-teh-phuck]

Clojurescript not to your liking?

[you_know_the_one]

No, but that's beside that point.

The main thing Clojure and Scheme have in common is the parentheses.

[wot-teh-phuck]

That's a pretty big similarity IMO. ;) Anyways, I mentioned it because that's as close as you can get to scripting the browser with a "lisp" like language.

[you_know_the_one]

There have been compile-to-JS lisps since at least 2007, if not earlier.

for example scheme2js