Good job Facebook.

I can certainly see being stuck with Javascript (just like we're stuck with the x86 instruction set even if simpler alternatives exist), but I'm not sure it's something I rejoice about. Javascript is like anti-Batman: a language we all deserve, but not one we need.

While its no Haskell, it certainly isn't much more verbose than other dynamic languages anymore. And a typesystem like TypeScript or Flow pretty much eliminates the rest of the gotchas.

Off the top of my head, there are two embarrassing holes: bigger integers and parallelism. Can't think of anything else at the moment (macros maybe, but they're a double-edged sword wrt tooling). Wonder if anything else is missing?

Yes. But the existing warts do not go away (and neither will they ever, due to the need for backwards compatibility).

I'd go so far as to consider it a feature.

This is a long and complicated answer. I doubt I'll do it justice in a few paragraphs, but I'll try.

In one word: simplicity. Primarily, not having parallelism is far simpler than having it. Yes, some models of parallelism are simpler than others but at the end of the day I think we can all agree none of these models are as simple as not having parallelism. As per "net negative": yes, ofcourse, not having parallelism is a "negative". However, "net negative" is interestingly more linked to the applied domain rather than the actual concept. Simply put, the domains that JavaScript is used in don't heavily rely on parallelism (or rather, require the optimization of parallelism).

Thinking about this now, its sort of similar in nature to why people like garbage collection. There is an inherent negative to using a garbage collector, however, (I think we can both agree) in certain (most) domains it turns out to be a net positive. Why? For the same reason, simplicity.

I mean, I could go on, but I'm trying to be as concise as possible. Hopefully, that was a useful. I'm happy to elaborate if you have more specific questions about this.

* s/simplicity/developer efficiency + implicit safety guarantees/g -- Since "simplicity" is quite vague;

With Erlang you can't share memory [0], but in exchange you get sequential code (no callbacks or yields), true parallelism, and even distribution over a cluster. With node.js all the asynchronous calls live in the same memory space, but code is written with nested callbacks (or yields), and of course you get no parallelism.

[0] There are shared dictionaries for when it is really absolutely necessary.

When people sat down and decided how workers are to act in browsers - Actors are what they had in mind.

This is why Web Workers don't have access to the window scope, use explicit message passing etc.

Correct me please, but from what I understand web workers are not a part of the JavaScript language. The same way AJAX isn't a part of the JavaScript language. I would consider those (AJAX/WebWorkers) more like "system calls" to the browser. And like any system call, it has privileges the application doesn't.

- The JavaScript language is specified under the ECMAScript specification.

- Web workers are indeed not a part of the ECMAScript specification which considers them "host objects".

- WebWorkers and other browser APIs (timers, ajax etc) are called the DOM API. The DOM (document object model) is how JS interacts with the web page and what capabilities it exposes. (document.getElementById isn't any more JavaScript than web workers).

Imagine your on your favourite social network that used a JS based encryption in a p2p chat to your friend. On that same page advertisers are pushing content to you. That content could be a malicious JS file which can eavesdrop on your conversation, all the while you think its encrypted.

I think we all agree. JS has very ugly things, but it isn't going anywhere for the forseeeable future.

If we're going to use for at least a few more years, I'd applaud anyone making better tools and rejoice when I see better frameworks and easier to use libraries.

Now by the time alternatives to javascript become viable, we might have made javascript something way better than what we already have now. It could survive a long long time and with people actively making it evolve, and it could be very enjoyable. Who knows.

If you factor in all these improvements, and the fact that it runs brilliantly on the server, it's a vastly different situation than just a few years ago.

You're looking at it from a technical perspective, and from a technical perspective JS is an awful language.

It may be a turd. But it's the only sandboxed-by-default, zero-install, reasonably-fast, free, preinstalled-on-every-machine turd that we have.

For example, yes callbacks were very messy but very soon we will have generators. And, yes, the scoping was nasty but soon we will have the 'let' keyword. I cannot remember where I read it, but I do also remember seeing a talk about some proposals to extend "use strict" to allow people to fix some of the type-casting behaviours made infamous by wat, too.

My point isn't that everything is fixed and we can stop complaining about the bad parts. My point is that it is very impressive how those in charge are handling the evolution of the language.

I think it's worth taking a bet on a language which improves so much every year.

Then deal with the 5-10 years or so legacy browsers hang around with significant market share.

JS was anointed long ago, and if we knew then what we knew now, maybe it wouldn't have been so? I remember DHTML and how silly it was and how it was just a toy and BOOM! Ajax.

So here we are. Every browser supports it, none could agree to change, so we deal with what we got. Better tools are better than nothing.

I would consider C++ to be way more verbose than Javascript. Especially with ES6 coming (and things like Flow / Typescript allowing you to use ES6 today).

Weird scoping? What are you talking about here? It's not the same as other languages. That does not make it weird. When you understand how it works, it's not a problem. Use it to your advantage.

Not nice to optimize for? Javascript is fast enough for most tasks, provided you use best practices. You can even build AAA games with it nowadays, through ASM.js. I'd like to learn more about what you mean exactly when you say it's not nice to optimize for, if you have the time.

I get that Javascript has its quirks. But so do most languages. What's awesome is that JS is easy to get started with, but can be used to build complex apps (especially with things like Flow / TS). And it works everywhere. And it has an amazing ecosystem of client side and side libraries.

Rather damning with faint praise there.

> Weird scoping? What are you talking about here? It's not the same as other languages. That does not make it weird.

Yes it does. In the '80s this was an open research area, but a consensus was reached in favour of lexical scoping for a reason.

> I get that Javascript has its quirks. But so do most languages.

False equivalence. Python (to pick an example I'm familiar with) has some quirks, sure, but it's a million times nicer to program in than Javascript, and it has all the other advantages you list (it's easy to get started with, suitable for complex apps, cross-platform and so on). I'm sure the same could be said for Ruby or OCaml or hundreds of other languages. If it were as easy to run these in the browser, I don't think we'd see anyone choosing Javascript - it really is a worse language than so many alternatives.

(I mean, by the standards of a single-application scripting language that was written in three days, Javascript is very good - we wouldn't expect such a language to be the equal of a carefully designed general-purpose programming language)

- Rule #1: C-like syntax

- Rule #2: Dynamic typing with optional static types

- Rule #3: Performance

- Rule #4: Tools

- Rule #5: Kitchen Sink

- Rule #6: Multi-Platform

This tool provides #2 for Javascript, the NBL.

Makes much more sense, but I am going to forgive him.

Oooh, there's many more computers around you than you think ;-) Nearly every modern piece of electronics. Many don't run browsers. For instance your washing machine has an OS, but it probably doesn't run Javascript. Same goes for your car, washing machine, dishwasher, microwave, central heating system, stereo/hifi, etc. And of course the more obvious "non-PC computers" like routers, printers, scanners, TV (might actually run JS if it's "smart" and has a browser), computer monitor and who knows what else. Coffee machine. The "fancier" it is (for a rather low barrier of "fancy"), the more likely it is to have a chip in it that runs some firmware/OS something, equal in power of what people ran as desktop personal computers a few decades ago.

No,it's not fine, it's a horrible language,with a few good features that saves it from being a catastrophy.hence "Good Parts".

Or we wouldnt be here talking about Flow,Typescript or others if the language was "fine". JS was clearly not designed for what we are making out of it today.

But since there is no way around Javascript in webdev,good or bad,it doesnt even matter.It exists.TC39 isnt going to fix types,so types are fixed in userland.hence "Flow".

Also we already have Strict mode; I imagine in the future it will get more and more uncompromising, so the JS subset we'll be actually using will be just fine.

It used to be that building a competent web app was something you could do in a number of different languages, these days you really need to know JS or one of it's skins and it's made developing for the web much less friendly to people that don't do it every day.

   JS the language of the future? Why? It has probably the worst gotchas of any language I've coded in

I'm interested in who is the driving force behind this open source change in Facebook, I don't recall facebook in behaving these way 4 years ago.

Can anyone find anything on a policy change that happened? They really turned around.

From my point of view, most of what has changed is resources and the immediacy of our survival-level concerns. Four years ago Google had declared nuclear war on us, we had far fewer users, we were not profitable, there were constant fires to put out with basic production operations stuff we've gotten better at, and we were enormously more under-staffed. I was working on HHVM already, but it was in a million little pieces spread across Drew's, Jason's, and my desks. The tools we're open sourcing over the last two years mostly did not yet exist, and if they existed, it was in some primordial form. We also have gotten much, much better imho at being good stewards of our open source projects; HHVM's predecessor system, the HipHop compiler, was also open source, but we people were spread way too thin to be able to respond to bug reports, pull requests, get FB's latest code into public hands, build binary packages for popular distros, etc. on a timely basis. Huge props are due to all of the technical people on our open source teams.

It would be awesome if you blogged about this. It would be interesting to read _how_ you got better at being good stewards of your open source projects and what made you open source these. I think it's a challenge a lot of people face.

I don't think there's been any policy changes. I think the policy has always been that we're open to open sourcing stuff, so long as it is useful to others (ie, not just a code drop that nobody can use) and someone signs up for the work and there isn't something important that's being dropped in the process.

The difference I think is people, energy, and momentum.

We've been able to find people (many already at the company) genuinely interested in some of the less glamorous parts of building a scalable program to open source things - things like sync processes and pull request management and UIs for ACLs and CLAs and so forth.

We've had people who, often due to the availability of these tools, have developed an internal energy to want to put in the extra effort to make their project ready to be open sourced (like making sure all dependencies are available already, or scoping out or stubbing out things that are Facebook-specific (our asset management flow, for example) while still keeping the software useful, and so forth.

The momentum has also made the idea of open sourcing code more top-of-mind to people, which helps to get people to rewrite their changes to accommodate a nascent open sourcing effort on a piece of code, or to get more discretionary time to investigate or work on making something open source. Or even just moral support from your team and colleagues.

And I think these efforts will pay off in spades for Facebook, in terms of attracting top talent and in terms of the expertise that results from having the some of the best dev tools available being developed in-house.

Not to say that Google's OSS hasn't done much of the same, but AFAIK many of their projects fail on at least a couple of these points.

My guess is that Facebook was just much smaller 4 years ago, over that period its staff grew very quickly. And even today it has an order of magnitude less employees than say Google, Apple, Microsoft, etc.

Larger, more established companies have more opportunity to open source things on this scale.

"...on the web language of the past, which we are unfortunately stuck with for the foreseeable future: JavaScript" might have been a better summation of the current (rather dismal) state of affairs with regard to web scripting.

That said, I am curious what solutions this solves that isn't already solved by enforcing good code coverage. Full disclaimer, the largest js projects I've worked on were in the tens of thousands of lines, not hundreds of thousands, but type checking just seemed completely unnecessary provided a good coding guide and test coverage were maintained and enforced.

It solves not having to write a bunch of tests that can trivially be caught by a program and never having to update or maintain those tests. It hits every code path automatically, you don't have to think of cases to try to hit edge cases.

- It type checks JSX - It supports some ES6 features others don't (like destructuring) as the build step - It has union types (TS will get those soon, already in master) - It does a lot more inference and a lot more assumptions. It assumes you won't multiply a string by a number for example (although technically '10' * 5 is legal in JS). So it's opinionated in that it enforces checks that rule out code that is legal but not likely intended in JS (an opinin I agree with).

Yes - it is possible to write tests and get good coverage and not use those tools - however static analysis can be very valuable in finding errors early. While I'm not sure I'd bother with explicit typing in this in smaller projects from the examples and unit tests it looks like it could find errors people would not easily notice otherwise. (Think JSHint, on steroids).

That said, only time will tell how good the implementation will really get in understanding your code.

you're not wrong, but out of curiosity, I just genuinely wonder how often modern programmers really hit type errors on smaller projects (obviously not FB size). I don't think I've ever had a type cast bug in my js code, provided we don't include accidental nulls in that statement. So in my experience, if I were ever told that I now had to always use annotations, I would feel like I was losing flexibility in the language for little gain. And I feel like the reason I never hit type issues, is because I write full test coverage, which in turn make it painfully obvious what is and is not expected in each method.

For the reasons above, I love the idea of this tool dynamically checking all my code paths, and looking for things that are likely mistakes or result in null exceptions.

But I'm afraid to recommend this to my boss, for fear that for now on, everything must be maximum static... everything annotated, no union types, etc.

Just my 2 cents.

Static typing also serve as a documentation, and enables "intellisense-coding" without having to google documentation every 2 minutes. Take this function signature from pythons standard library: subprocess.Popen(args, bufsize=-1, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=())

Without documentation, can you answer: what is env? what is stdin? what values are allowed in creationflags? is it obvious that startupinfo should be a STARTUPINFO object?

Compare to a C# equivalent, FileStream Open(string path, FileMode mode, FileAccess access, FileShare share). Here it is obvious that FileAccess can only have one out of 3 available options in the enum since the autocomplete will only give you those three options.

Interstingly it's almost always the same kind of type error- accessing nested maps/arrays and forgetting to go deep enough before I call some method on the elements in that collection.

I can't remember one example of type error where I intended to call method on MyClassA and ended up trying to call it on MyClassB. But that may be the effect of my coding style (I'm not big fan of class hierarchies in js).

BTW it's funny how many times late binding was reinvented in j2ee ecosystem :)

However, whilst I never hit on basic type errors such as passing a string when I need an int - I regularly come across object type errors. I'll pass an object with a project_id property to a method that actually expects an id property. Not sure if flow can handle these - if it could it would be pretty amazing.

Today I got the order of parameters to a callback wrong ("error" was the last rather than the first parameter). The documentation didn't say anything about parameters, it only said that the callback would be called with a result. I had to look into the source code to discover the details. I am not sure if Flow would be able to catch this kind of bug but if so, it would certainly save me a lot of time.

In a dynamically-typed codebase, it's near impossible to have rare type-error occurrence because you're off-loading the type system into into the programmers head (which is subject to human fallibility - even when countered with excellent unit testing!)

Issues with null are much more common.

I frequently see methods incorrectly overidden. For example, the ancestor method effectively has a variadic argument list, but the descendant doesn't have as sophisticated argument handling and doesn't do the appropriate superFunc.apply(this, Array.prototype.slice.call(arguments, 0)) etc.

The (supposed) need to have "good code coverage" is itself a problem I'd like solved.

This tool claims it can do advanced inference. If it were possible to hook it up to an editor or IDE to analyse existing untyped js in this way it could be invaluable.

As others have said, I love the approach of inference giving static type benefits, for free. If you get tooling support, at no extra work, why not adopt it?

But static types can be a hard sell for JS programmers. What sort of reception has it gotten inside facebook?

Typescript (from MS) sounds a lot like what you describe - it doesn't change Javascript, it just adds types to it (and makes some ES6 features available), and it puts a lot of work into playing nice with the wider JS ecosystem (e.g. via the definitelytyped project, which integrates type definitions for most popular "third-party" javascript libraries).

> That said, I am curious what solutions this solves that isn't already solved by enforcing good code coverage. Full disclaimer, the largest js projects I've worked on were in the tens of thousands of lines, not hundreds of thousands, but type checking just seemed completely unnecessary provided a good coding guide and test coverage were maintained and enforced.

Perfect testing can do everything a type system can. But a type system can do it with less programmer effort, much lower maintenance overhead, and in a standard form that makes it easier to maintain. So you can do the same thing but cheaper - or, more realistically for how software is developed in most companies, you can get better reliability for the same engineering budget.

The Google Closure Compiler (https://developers.google.com/closure/compiler/) has been around for years and years.

It does everything you mentioned (100% optional type annotations, type inference), plus more (dead code removal, inlining, compiler-time constants).

+ it's not incremental hence unusably slow, requires Java. No, Closure is not the same as Flow even from a brief look at both.

> code intelligence, which aids code maintenance, navigation, transformation, and optimization

I suspect that for facebook, this could be just as important, if not more so, than code quality benefits.

I specifically didn't send this tool to the team I work on, because my team lead is a sql / java / c# guy who loves static languages, and only touches the front end with a stick if he has to, and then only some basic jQuery or angular.

I've sold him on jasmine and requiring front end test coverage, recently. But right before I hit the send button I realized that if I sent him this tool, I'd never be allowed to use dynamic typing or non annotated functions/arguments in js ever again.

Hence my question : /

(ref this absolutely fascinating paper

http://bibliography.selflanguage.org/_static/implementation....

and this piece of V8 dox quoting the aforementioned paper

https://developers.google.com/v8/design)

It seems that adding a type system to a dynamic language has little real drawbacks compared to designing language and type system at the same time, for both performance and type safety considerations.

This all seem extremely cool.

I went straight from hacking Scala and Haskell as a hobbyist to doing (mostly) front-end JS job, and I've always found that my code, and a lot of good libraries I read, naturally emulate something close to Hindley-Milner typing, by using objects as tuples/records and arrays as (hopefully well-typed) lists, as well as the natural flexibility of objects as a poor substitute for Either types.

I'm definitely pleased to see that the designers of this library have also realized that strongly-typed javascript was just a few annotations and a type inference algorithm away.

I'm just wondering why are nullable types inmplemented as such and not as a natural consequence of full sum types, which are inexplicably absent.

Promise<A,E> -> ((A -> (Promise<B,F> | B)),(E -> (Promise<C,G> | C))) -> Promise<B|C,E|G|C>

That is - a promise's then - takes the promise (as this) and executes either a `.then` fulfillment handler or a catch handler.

If the `fulfill` handler executes the value is unwrapped and either a new value, or a Promise over a new value and its own type of error is returned.

Now, if the `reject` handler is executed the error is unwrapped and either a new value, or a promise over a new value or a new error is returned.

This is quite simple and easy to use because it behaves like try/catch in the dynamic type system of JS with recursive unwrapping - however it is challenging to reason about when you're starting to type code and you want to actually have correct type information with promises.

Static languages generally approach these problems with pattern matching on the type - in JS that's not common nor is it feasible at runtime - you just expect a value of a certain type. When I implemented promises in another language (swift) this was a lot of fun to work through and not very trivial - if their compiler cna do this I'd be very impressed.

Promises are just one example.

Anyway - this looks cool. I definitely agree that full sum types would've made more sense - having explicit nullables is usually a smell (like in C#).

I don't care about the type signature of promises. I actually don't care about the type of anything which has a complicated type; I think it's just incredibly winning that I will now be able to describe the shape of the raw data circulating in my code.

I've always found really annoying, in Haskell, to see incredibly complex type idioms emerging to allow stuff that doesn't really deserve it. And yes, I'm talking about monad transformers.

Don't get me wrong, I think that Haskell and the typing techniques and idioms it has fostered are a tremendous achievement, but right now, I'm more focused on bringing my web code, which right now is unfortunately a jungle of implicitly-typed garbage, closer to a safe and predictable better-typed form.

I tried to do that in Haskell in the back-end, but everytime I tried, I lost mind-boggling amounts of time dealing with the monadic stack of the framework I tried.

As the saying goes, I'm not clever enough to use dynamic typing, and bugs happen. Unfortunately, I'm also not clever enough to use real strong typing, and nothing compiles, let alone gets done.

Hence why I'm immensely thankful to see facebook embracing gradual typing in a way that lets me leverage my knowledge of algebraic typing.

It is not always clear how to factor code which does not have this rigor into effect-typed form. It can be extraordinarily difficult to recognize what effects are being carried out in which parts of code at first—especially if you haven't been forced into the discipline early. Thus, I find it unsurprising that you feel the translation effort is challenging.

But, coming from the other angle—building things with well-defined effect typing from day zero and composing pieces atop one another to reach your final complexity goal—works exceedingly well. Better, it forms a design which can be translated to untyped settings and retain its nice composition properties.

Which is to say not much more than: there is some logic to all that madness and once you're on the "other side" it's hard to judge these "incredibly complex type idioms" as anything other than useful and nearly necessary for sane code reasoning.

I miss transformer stacks a lot when using other languages.

I think it all boils down to considerations of idealism vs. realism, in the end, and yes, I have to admit, I managed to get a sort of big picture understanding of Yesod and its ORM, and it's definitely a cool design. Can't say I know as much about Happstack, but the hello worlds were smaller. And the authors didn't have to invent two dependency management tools to get it to compile... /off-topic

I actually used something akin to monads to write a quick and dirty parser combinator library... In PHP! It was really fun, but I have to admit that it got really hard to keep track of what was function, what was return, what was supposed to be passed along to the next function and so on without any real typing. For the first time I... I wanted monads.

But still, I have 99 problems and I'd say 97 of them are undefined, nulls, erroneous typecasts, unexpected layers of wrapping, and so on. I wrote bad code, my teammates did, and here we are. I want to get rid of those so that I can, at last, have real problems.

Then I'll port my REST server in Haskell ; )

I would say that types are really difficult to do in your head. It's tedious and error-prone. That said, the advantages are high so it's valuable, e.g. your parser combinators in PHP.

Parser combinators are actually a great example of where monad transformers shine. You can see them as nothing more than a stack of `State` atop `Maybe` which dramatically simplifies the presentation and why they work. Better, you can rip out `Maybe` and replace it with `[]` to get non-deterministic parsers "for free"—all of the code remains the same.

I actually happened to write up about this recently:

    https://gist.github.com/tel/df3fa3df530f593646a0

Let me know when you start porting to Haskell. That'll be an exciting time.

If I'm part of a 5 developer team working on a code base, or I'm using a library someone made - I want the functions I'm calling to be very explicit about what they take and return and I want the functions I provide others to be very clear on what they take and return.

The problem is that even something people use every day like a Promise or an event handler creates very complex types (like the example above). I think any viable solution that expects to be type safe needs to be able to express that.

If your code does not expose any callbacks, or anything async I'd say it's simply not very typical JS code.

The alternative of course is to be _less_ safe about our type. We could say that we treat a promise as:

Promise<A> -> A -> Promise<B> -> Promise<B>

(just a `bind` from Haskell) - this would let us maintain _some_ type safety which is better than nothing.

And yet, as someone who's been working in Scala for nearly 5 years now, more and more of these abstractions are starting to seem "worth it". My first Scala code was quite imperative, mixing random effects left and right. But eventually I started handling async calls explicitly - or perhaps I should say, I became fluent enough in the language that I could make an explicit distinction between sync and async calls without in being too cumbersome - and then I reaped the rewards, with more reliable, more performant, more maintainable code. And then I did the same thing with error handling, replacing surprise exceptions with an explicit Either (stacking this inside the Futures), and again I found my code became clearer, easier to reason about.

I've just finished factoring out database access into a Free monad based construct, and for the first time in my life I can test database access in a smarter way than just creating a (possibly in-memory) testing database and hoping it does the same things a real database would do. The monad tools are good enough to make it easy - as easy as "magic" Spring AOP, but explicit and ordinary. I've written a library for dealing with monad stacks that I'm sure would have horrified myself of three years ago (https://github.com/m50d/scalaz-transfigure), but I've come here through small incremental steps that have made sense at every stage (it helps that I'm a big believer in Agile). If I'd been dropped in it with a language like Haskell where everything has to be monadic from day 1, I think I'd've given up. I still wouldn't use monads for I/O (at least, not yet) - the advantages don't seem worth the overhead. But I'm glad I'm working in a language where these things are possible, and where I can gradually adopt them in my own time.

Let's say a promise is a thing which can either succeed or fail eventually. If it fails, it gives a type e, if it succeeds a type a

    Promise e a

    then :: Promise e a -> (a -> Promise e b) -> Promise e b

    1. No differentiation of error types
    2. No explicit annotation of the ability to 
       return constant/non-promise values
    3. No tied-in error handling

    mapError :: (e -> e') -> Promise e a -> Promise e' a

    pure :: a -> Promise e a

    catch :: Promise e a -> (e -> Promise e' a) -> Promise e a

So, each of these smaller types is now very nice to work with. They are equivalent in power to the fully-loaded `then` you gave, but their use is much more compartmentalized. This is how you avoid frightful types.

Additionally, AFAICT Promise-like things aren't actually used very much in Haskell in practice; at least not in the code I tend to see/write. MVar is probably the nearest direct equivalent, modulo error handling (which would probably just be handled using an Either in an MVar). Given that the runtime is M:N threaded, there doesn't seem to be much need for Promise and its ilk, since you can just do ordinary function composition and HoFs (with channels for communication). If you want something fancy you just use the marvellous "async" library.

Promise A E -> (A -> A') -> (E -> E') -> Promise A' E'

That would eliminate some strange corner cases and make it easier to explain the function. The special cases could just get their own functions. Real algebraic data types would probably eliminate the need for E entirely and make it even simpler.

Promise A E -> (A -> Promise A') -> (E -> Promise E') -> Promise A' E'

, with implicit boxing of bare types and thrown exceptions, as well as flattening of superfluous Promise wrappers. The `flatMap`iness of it is what really makes it interesting, in my opinion.

    case mx of Just x -> f(x)

    if (x != null){ f(x) }

  function nothing() {
    return {
      match: function(cases) { return cases.nothing(); }
    };
  }

  function just(x) {
    return {
      match: function(cases) { return cases.just(x); }
    };
  }

  just(1).match({
    just: function(x) { return x + 1; }
    nothing: function() { return 0; }
  });

Haskell-style sum types are a generic type with multiple values describing the alternatives.

Since the goal of Flow is to typecheck existing JS semantics, the addition of wrapper types and objects to support such sum types makes very little sense while the addition of "anonymous" union types makes a lot of sense. Dialyzer took the exact same path (except even more so as its type unions can contain values) as it tried to encode Erlang's existing semantics.

I missed the part about sum types!

My god this is perfect

It has static type checking with optional type annotations and type inference.

It doesn't have compiler-time constants, dead code removal, inlining, or other optimizations.

But....still really cool.

- more powerful type system (union types, hurray)

- support for JSX

- no windows binaries

- supports more of ES6 stuff

- ...but has no support for modules yet

- no generics (??)

How about performance? and workflow? Didn't yet find this: does it use a normal "write then compile" model like TS or has something like Hack (if I'm not mistaken it has a daemon running in the background, checking the code as you write it).

Wonder why FB decided to roll this on instead of using TS.

- Has other comments have said: TypeScript is getting (already in master branch) union types.

- Support for JSX isn't really a huge deal.

- Windows support will probably come - it's an open source library. I hope it's not the OCAML tooling.

You make some great points about generics and using their own type system instead of TS, especially since TS has investments from both Microsoft and Google (with AtScript which supersets it).

They state they use a model like Hack - and the repo also looks this way but I'm also curious, it looks like a very peculiar choice.

http://flowtype.org/docs/classes.html#polymorphic-classes

http://flowtype.org/docs/functions.html#polymorphic-function...

For workflow, check out: http://flowtype.org/docs/getting-started.html#_

I see some mentions of import in the tests and the grammar: https://github.com/facebook/flow/search?utf8=%E2%9C%93&q=imp...

Then perhaps you are understating the importance of software correctness. While type systems can be an almost religious topic, the benefits of type checking are real -- a whole class of bugs to disregard, less testing code, and a more maintainable codebase for other developers. Moreover, for languages with ADTs and exhaustive type-checking, you are forced to reason about boundary and error conditions. All of which leads to higher quality software, at the minimal cost of up-front work when designing your types and fixing what the compiler/checker says.

> type related errors are usually easy to find and fix and have rarely if ever been the root cause of our most difficult problems.

Type related errors may be easy to diagnose and fix -- although by definition, in the absence of type-checking, type errors are only so after the fact, eg after causing a crash. Hence the utility of type checkers, especially if it means fewer avoidable errors appearing in production.

> Dynamic type checking and implicit conversion is one of the more powerful features of JavaScript and certainly no less prone to error or counter-productive than type-casting, making variadic functions or class templates are in other languages.

Type checking doesn't diminish the usefulness/convenience of dynamic languages, but IMO lends more weight to the benefits of strongly static languages -- benefits which are negated by the abuse of "features" such as typecasting or variadic functions.

There are other issues as well, JavaScript doesn't check function arity on function calls (the arguments not supplied just get the "undefined" value), control structures do not introduce a new lexical scope etc.

Having optional typing does save you the trouble of overly complicated types for things where you aren't particularly concerned.

I wasn't suggesting using tests; I was suggesting using asserts. Directly in the function that is type sensitive. If that function is ever called with an incorrect type it will throw an exception.

I get the benefit of optional type checking without a compile step in development like with Flow.

Sure it's less certain than what you get from a compiler; that's the tradeoff for not having to compile in development. Since most JavaScript code is not type-sensitive it's only important in code that you are more than likely going to be testing heavily anyways.

We've had plenty of other bugs in the past months that tooks hours to track down but would have been caught in seconds in a sensible language. A lot of them won't be caught by flow either, unfortunately. What I really want is for operations like key lookup to fail instead of just returning me gibberish.

Typecasting is at least explicit. In js every single operation is a timebomb.

In a language without a strong type system you rarely appreciate quite how many of your invariants could be lifted into the type system. When I wrote Python most of my errors didn't seem like type errors (e.g. I remember forgetting to close a connection and so leaking connections), but now that I write Scala I can see how I'd structure my program using types so that that would be a type error (I'd use monads to compose the idea of an operation that uses a connection, and then execute them in one place).

(Python now has an ad-hoc fix for this specific problem in the form of the "with" statement, just as 3.4 adds an ad-hoc fix for the proliferation of different ways of doing async calls. But a good type system is a general solution to both these problems and more).

> Dynamic type checking and implicit conversion is one of the more powerful features of JavaScript

In a strongly typed language you can do this in a controlled way; it can be part of the language, and can apply equally well to user-defined structures. Whereas with Javascript you're stuck with those implicit conversions built into the language, and if you want to convert e.g. an address datatype, you're out of luck.

> certainly no less prone to error or counter-productive than type-casting, making variadic functions or class templates are in other languages.

Then use a language that doesn't have those problems either. There are good languages out there - if Scala isn't for you then how about OCaml or Haskell?

Then again their rationale is very clear and pretty good: You need builds if you're using Facebook's stack anyway (for JSX) so this should not interfere with your current build - which you have to do anyway.

   flow examples/01_HelloWorld/hello.js
   /hello.js:7:5,19: string
   This type is incompatible with hello.js:4:10,13: number

   Found 1 error

Try running an annotated file in the browser - it's simply not valid ECMAScript syntax and no JavaScript runtime will run it 'as is'.

If the code requires transformation by a compiler aware of the language semantics and syntax - it's transpiling in my book. Just like TypeScript is a superset of ECMAScript - so is Flow (in a much lighter, closer to source sense it seems).

Of course not, but the point was that it's not a different language, it simply adds type annotations. If you strip out the type annotations, it should be a valid JS file.

AFAIK none of these are valid javascript, event assuming ES6:

* class-level variables (resolved as instance variables)

* field visibility

* initializer constructors (constructor without a body automatically assigning to a field)

* enum types

That is, you can't remove type annotations (and `interface` declarations which can probably get a pass) and end up with valid JS, which seems to be what flow yields.

For instance, you could imagine a function calling into some unknown api, where we would need to constrain the type of the variable to the expected one to avoid `any` being applied to it. While the library files do provide typing of external dependencies, this doesn't protect you from bugs in those libraries! The same applies to api's that are explicitly typed as `any`, but which should return a certain type.

Having Flow add a dynamic assertion on the type here would allow the rest of the program to remain type safe, knowing that a violation is guaranteed to halt execution.

While dynamic strong typing is also used at Facebook, we're not quite ready to launch this as an extension to Flow.

    tsc --noImplicitAny hello.tsc

    hello.ts(2,14): error TS7006: Parameter 'x' implicitly has an 'any' type.

[1]: http://flowtype.org

    function length(x) {
      return x.length;
    }

    length(null);

tl;dr: they want to support both of those features, the question is what syntax to use and how to introduce those features into the existing ecosystem.

TypeScript doesn't have that concept, although it's been suggested by the community more than once.

Besides perhaps the extra 'compile' time added to do both translation with TypeScript and then static analysis with Flow. Both tools have their advantages and disadvantages.

May as well throw in a linting tool as well..

Tern.js actually detect types, and may be it would be possible for eslint to incorporate it somehow to detect invalid use of types.

One big ternjs plus for me is the fact that tern.js knows about require.js modules and can look in other require'd files.

Thanks to everyone at Facebook who worked on this. You guys are awesome.

Also: The fact that this is written primarily in OCaml (as opposed to JS) is an excellent example of people choosing the right tool for the job.

JavaScript has a weird ecosystem where it is extremely helpful to have all of your tools in the same language. browser-based IDEs, Node, portability, etc, and just one fewer runtime to juggle.

Same reasons why closure is awkward as a Java program.

See the related discussion on packaging it in the OPAM pull request: https://github.com/ocaml/opam-repository/pull/3083

And on the side note, I bet Facebook did this just to make nerds install OCaml and show them the light :)

I think the key is that these projects actually provide value - they make things faster, more reliable, more scalable - whether that's the code's execution or the people writing the code (or debugging issues, or whatever).

They generally aren't solutions seeking problems - they are responses to problems that exist.

Engineers generally don't build things like this on the weekend - unless they like to structure their time like that, I guess. It may or may not be a full-time job, but the job whatever it is isn't some search for abstract perfection, it is again to solve real problems encountered by others in the company. Often it is a part-time component built as part of trying to solve some more direct goal - like fighting spam, or serving bits, or whatever.

Often it is something the engineers just do - it makes sense to break things up into libraries, or services, or whatever, and they do that, and then that library or service is usable/useful elsewhere, and that's it. Other times they may suggest and motivate it as a goal-in-itself in a team goal setting situation.

I doubt that's a particularly useful answer, but maybe with further questions I can make it more useful to you?

It does raise the interesting question of whether facebook employees are doing this work just to avoid the work that is the "core competency" of the company. Especially given the fact they don't gain a competitive advantage from releasing the work that facebook paid for into the wild. By this I mean, the company benefit would seem to be attract other talent. And the personal benefit for the devs is to get their name out there are something cool and interesting. Certainly, working on the best way to advertise to users is a lot less exciting/sexy than working on static type checking for javascript.

I don't think anyone is "avoiding" any work - there's just tons of work that needs to get done, and people mostly choose to work on that stuff that interests them. One could say that product engineers "avoid" infrastructure work and infrastructure engineers "avoid" product, but I think that would be inaccurate in most cases.

It seems like the vast majority of companies are not far enough out in front of their production issues and requests from the business side that engineers could do this sort of thing. So I guess it's impressive that Facebook (and probably Google) are in that position.

Documentation costs productivity to write, but when there are many people who would be made more productive from it, it makes sense to do it.

I think "slack" is the right way to think about it. It isn't a free-for-all - the business still needs to run - but there's enough space to explore, to rewrite, to document, to polish, and so forth. That's where the magic happens - the unplanned and the unexpected things around that which you thought you were going to do.

(It's a different matter that we would have done this on our free time anyway, because it's so much fun!)

What mechanism ensures this becomes valid javascript? does the code need to be compiled?

The code has a build step - yes. You will not be able to run this code without a step. It could be awesome if they allowed annotations in comments like other tools do - there is a GH issue on that and it looks like it should be possible https://github.com/facebook/flow/blob/master/src/typing/comm...

http://flowtype.org/docs/existing.html#_ http://flowtype.org/docs/running.html#_

The caveats I heard about transpilers often boil down to difficulty of debugging and lack of libraries. But with the amazing browser dev tools we have, debugging potential issues is not that painful. Every language compiling to js provides FFI and/or some escape hatch so you can write javascript manually, for performance tuning or for using 3rd party libs.

Even if you do write "raw" javascript, some sort of compile step is unavoidable, for running jshint, concatenating, minifying, etc. Why not walk the extra mile and use a better language?

BTW, I'm not saying a tool like this is not super-useful, specially if you already have thousands of lines of js code that you can't get rid of. Congrats to the Facebook team for the release!

0: http://haxe.org/

1: http://purescript.org/

2: http://ceylon-lang.org/

I think Dart seems to be a great language and IE, Firefox and Safari should have implemented it years ago, but they didn't. Now I think TypeScript is a great addition to javascript and I hope they build it into the browsers but I suppose they won't (maybe EcmaScript 7 will have some parts of TypeScript in it, or parts from AtScript from Google).

By the way, you probably still want minification and concateneted files when you create js from other languages. That stops me from using them, I would have many levels of tools between my source code and the production code.

People will tell me that it is a good language if you know how to use it, comparing javascript mastery to C mastery in a sense. I think there lies the problem.

It helps that companies like Google and Facebook have invested a significant amount of research power into designing frameworks and tooling around it. Just from there two companies alone, we have tools like React, Angular, Karma, JSX, Jest, and now Flow. Tooling that involves the browser more include Polymer and Traceur (ES6 to ES5 transpiler).

To contrast this, I have been doing development with Cordova the past week & writing Cordova plugins to fill in missing functionality - the plugin ecosystem with Cordova is horrid, and the documentation is often awful. To compound it, Android developers don't seem to believe in documenting their libraries well.

I will take the JS ecosystem any day when confronted with a choice like that.

My personal preference is to have annotations because it helps future readers and maintainers understand the code better. Instead of looking through the function to see that the variable is in-fact a number, I'd rather just read "@param x {number}". And at that point, one may as well as use closure.

Also, some techniques from Typed Racket (occurrence typing): http://www.ccs.neu.edu/racket/pubs/popl08-thf.pdf

Many other papers have influenced the design in some way or the other. For example, Abadi/Cardelli's theory of objects.

Looks really handy.

[1] - http://flowtype.org/

[2] - https://github.com/facebook/flow

The React.js team has been explicit about the library being GCC advanced mode compatible, so they certainly have awareness of its capabilities. Whether they are using them together internally or they use a different solution for tree shaking et al is another question.

Flow starts with JS and adds a static type system (with attempts to infer types directly from the code). With the exception of the type annotations, Flow is JavaScript. Dart is not JavaScript.

I wonder which will have the most impact: code quality or types as documentation (esp for tooling)?

They are adapting to common idioms, rather than designing it from the ground up. This ad hoc approach is a great way to build useful tools (and startups), but it's also usually a mess. Like NN4. But, they seem to be type experts - plus they're using ocaml. Maybe ad hoc by experts is the way to get these ideas adopted?

Also, if you're code is implicitly statically typed (as checked by Flow) you will likely hit all the right optimizations in the underlying JavaScript VM.

The existing available definitions from the DefinitelyTyped project is a huge productivity booster. Apparently Flow also has similar .d.flow files, but it will probably be a while until they exist for common projects.

After investigating this thought, it looks like this already at the top of the list for future plans for Flow:

http://flowtype.org/docs/coming-soon.html

(1) zero-work: works instantly with existing code and esp third party libraries; and

(2) instant-benefit: provides some compelling benefit in that zero-work case above (of course, it's OK if it provides more benefit if you do more work, adding type annotations etc).

[1] http://flowtype.org/docs/react-example.html#general-annotati...

It'd be quite possible to add the ability to output these annotations to the CoffeeScript compiler itself though. Could be an interesting fork.

I'm surprised I didn't hear more about this before since it was apparently unveiled at the "Flow" conference. Wasn't at the conference and somehow I missed any prior mention of it.

https://news.ycombinator.com/item?id=8625406

The problem was that there were two identical comments, the software killed one as a dupe, and avik deleted the other one. The software tries to fix this very scenario—it normally would have automatically unkilled the remaining member of the pair. But there are some corner cases where that doesn't work, and avik seems to have outsmarted it.

We'll take a look and try to fix the fix.

Edit: I assume it just checks bool/number/string and doesn't care about prototypes?

If I am not mistaken, this tech could be used to build IDEs roughly similar to whats available for Java, couldn't it?

I'm surprised that they don't seem to have launched with a public editor plugin and that the documentation doesn't seem to mention it.

If CoffeeScript is to support these annotations one day - it would require the CoffeeScript compiler to support them itself in order to generate correct annotated JavaScript for Flow.

http://en.wikipedia.org/wiki/Type_system#Static_type-checkin...

Even if that wasn't the case, their design just avoids the issue by not trying to do any typecast, ever.

Things like multi threading which aren't possible in javascript aren't supported, but a lot of the JRE which is used in most code, does come out of the box. Its a huge improvement over regular javascript, anyway. And client + server code can be shared.

You know where else none of that exists? In the machine code on which the JVM rests and to which Java code can be JIT compiled by the JVM before execution.

So, if you can't have it compiling to JS, you never could have it on the JVM, either.

You are writing java and barring some features like multi-threading, you CAN write anything you want. How to translate that to JS is the compiler's job, you need not care.

Edit: what would you say to this - http://www.scala-js.org ?