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Facebook Launches Flow, Static Type Checker for JavaScript

code.prod.facebook.com

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Re: Facebook Launches Flow, Static Type Checker for JavaScript

#51

Post by stiff »

drderidder wrote: post #47
stiff wrote: post #40

Earlier quoted context omitted.

JavaScript is a bit special in this respect, I think. Because of the weird type coercion rules, and how it treats null, undefined and because of the presence of NaN many common coding mistakes end up producing mysterious errors pointing somewhere far away from the place in the code that actually produced the problem in the first place. Basically JavaScript continues propagating null/undefined/NaN in many situations w…

Yeah, that can be confusing. On the other hand, if you write unit / functional tests then testing for invalid inputs one of the first things you'll probably do. Your comment makes me think a fuzzer to test all those falsey values could be useful.

A lot of JavaScript is UI code though, so unit tests might not be possible, and a suite of comprehensive functional tests with something like Selenium gets dog slow very fast, in my experience.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#52

Post by kmavm »

inglor wrote: post #46
DigitalSea wrote: post #39

What a great tool. Facebook are absolutely killing with the last year or so with all of their open source contributions and releases. First HHVM, Haxl, React.js (amongst other things) and now Flow, this is fantastic. I am really liking how companies like Facebook & Google are concentrating their efforts on the web language of the future: Javascript. The support for JSX alone is a MASSIVE feature (expected given React…

It's hard for me to give appreciation but while Google have traditionally been tech leaders in web technologies: Facebook are doing really awesome stuff lately. It's not just React and this - it's also FLUX, HHVM, Hack, Haxl (Their Haskell libraries), contributing to writing a spec for PHP and other ventures. I'm interested in who is the driving force behind this open source change in Facebook, I don't recall faceboo…

Thanks for the kind words. For context, I'm an HHVM alum who has been at Facebook for almost six years now (wow time flies).

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.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#54

Post by ufo »

slashnull wrote: post #31

At last! 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 Ei…

You can't really use sum types as they are found in Haskell in Javascript. With Haskell option types you do a pattern matching and create a new binding for the non-null value but in Javascript you don't do that - you keep using the same object that you tested against null.

    case mx of Just x -> f(x)
vs

    if (x != null){ f(x) }
What you can do in a Javascript-like language is use union and intersection types. However, they can get a bit complicated (specially if you allow unions of non-primitive types) and the extra flexibility can confuse the type inference a bit so I can understand them restricting things to the common case of handling null.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#56

Post by slashnull »

inglor wrote: post #44
slashnull wrote: post #31

At last! 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 Ei…

Strongly typed JS is actually pretty hard - probably not by Haskell and Scala standards - but if you take promises for example the signature of `then` is: Promise -> ((A -> (Promise | B)),(E -> (Promise | C))) -> Promise 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 ne…

I don't want to say what I'm about to say, but...

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.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#57

Post by inglor »

kmavm wrote: post #52
inglor wrote: post #46

Earlier quoted context omitted.

It's hard for me to give appreciation but while Google have traditionally been tech leaders in web technologies: Facebook are doing really awesome stuff lately. It's not just React and this - it's also FLUX, HHVM, Hack, Haxl (Their Haskell libraries), contributing to writing a spec for PHP and other ventures. I'm interested in who is the driving force behind this open source change in Facebook, I don't recall faceboo…

Thanks for the kind words. For context, I'm an HHVM alum who has been at Facebook for almost six years now (wow time flies). 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…

Thanks for answering, it's always good to get answers straight from the source.

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.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#58

Post by ufo »

slashnull wrote: post #34
inglor wrote: post #29

Earlier quoted context omitted.

This is a form of cross-compilation kind of like how TypeScript is cross compilation. It does require a build step to produce JavaScript - you will not be able to enjoy fiddles as easily and so on. 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.

If only to remove syntactically invalid type annotations, I guess. Strong typing is not incompatible with the absence of run-time typing information, due to the magic of type erasure.

Type erasure is only safe if the whole program is statically typed though - you are still going to need to use runtime checks if you want to interface with dynamic code.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#59

Post by masklinn »

inglor wrote: post #45
drderidder wrote: post #41

Earlier quoted context omitted.

Really? It appears to be much more of a static analysis tool than a cross-compiled psuedo-syntax. Here's what it outputs: 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

EDIT: looks like it supports type annotations similar to closure compiler's (in docs) https://github.com/facebook/flow/blob/master/tests/docblock/... 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 Ty…

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

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.

Re: Facebook Launches Flow, Static Type Checker for JavaScript

#60

Post by slashnull »

ufo wrote: post #58
slashnull wrote: post #34

Earlier quoted context omitted.

If only to remove syntactically invalid type annotations, I guess. Strong typing is not incompatible with the absence of run-time typing information, due to the magic of type erasure.

Type erasure is only safe if the whole program is statically typed though - you are still going to need to use runtime checks if you want to interface with dynamic code.

Well yeah nobody talked about removing JS's duck typing algo.
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