Understanding null safety in Dart

https://dart.dev/null-safety/understanding-null-safety

by timsneath • 6 years ago

96 89 6 years ago

Null safety is absolutely essential for me. It's 2020, why are we still having NullPointerExceptions? However I'm curious about the next frontier of safety. We've slowly added a lot of nice features around safe use of values and safe memory. I wonder what's next.

Personally I've been thinking about safe use of arrays. Arrays are such an important data structure and yet they're horribly unsafe. Every time I do index math I get that same nervousness that I used to get accessing fields in nullable data structures. I'd love for a way to verify array indices that doesn't involve too much dependent typing.

foolfoolz• 6 years ago

collections apis have solved this. i haven’t used an array in many years

riffraff• 6 years ago

But some things at the lower level still need indexes, don't they? Think of in place sort routines.

gameswithgo• 6 years ago

map filter fold etc.

bigyikes• 6 years ago

I think twice about writing for loops. Functional style APIs make these kinds of mistakes way less common

catalogia• 6 years ago

Personally I'm a fan of list comprehension, particularly as implemented by rackets for/list forms. I certainly don't use them for everything, sometimes a plain old map fits the circumstance better, but I find myself using for/list fairly often. And I don't think it's error prone in the same way that a traditional C-style for loop would be.

https://docs.racket-lang.org/reference/for.html?q=for%2Flist...

    > (for/list ([i '(1 2 3)]
                 [j "abc"]
                 #:when (odd? i)
                 [k #(#t #f)])
        (list i j k))
    
    '((1 #\a #t) (1 #\a #f) (3 #\c #t) (3 #\c #f))

baddox• 6 years ago

In my view the next nice thing to have is algebraic data types with compile-type exhaustiveness checks. I believe several languages implement null safety using a simple algebraic data type that can be “Nothing” or “Some(‘a)” where “a” is a type. The compiler can enforce that you can never access the “a” without checking for both cases: “Nothing” and “Some(‘a)”.

That extends naturally to any number of types. For example, a type representing a network request can be “Loading” or “Loaded(‘data)” or “Error(‘error)” and you can’t just blindly access the data without also handling loading states and error states.

I absolutely love this when doing frontend work in ReasonML, and I miss it in JavaScript. TypeScript definitely has this functionality but I’ve never felt that it was natural to use.

blandflakes3• 6 years ago

I'd agree; if I had to guess what's most likely to become mainstream soon, it would be union types with matching (like Either or Result types). Strange that Go did the lazy thing and made the type signature a tuple, requiring you to match yourself...

satvikpendem2• 6 years ago

Dart has compile time ADTs with the freezed package. I like Dart since it has code generation features so you can write or import custom language features, much like JS with Babel.

https://pub.dev/packages/freezed

johnfn2• 6 years ago

I think the next frontier is things like Rust. I've griped about Rust in the past but I do think that the borrow checker often feels like I'm coding in the future - it understands my code on a much deeper level than any other statically typed language I've used.

I definitely agree with your feelings around array indices. I feel the same nervousness every time I do a slice or count backwards from the end of an array. I'm not really sure what the fix would be here, until I can just tell the computer what I want it to do, though...

RivieraKid5• 6 years ago

But the advantage of that doesn't apply in garbage collected languages, right?

staticassertion3• 6 years ago

I think the fix is dependent types, right? If the compiler can reason about the index and the size of the vector, that's one way.

For example;

let x = 5; let y = vec![0; y * 2];

for i in 0..x * 3 { y[x] }

That shouldn't compile.

hardwaregeek3• 6 years ago

Yeah I've been writing a lot of Rust for my compiler and it's so much fun. But it still bugs me that indexing panics. Returning an Option would make a lot more sense...

What I've been wondering for my own language is whether I could design something like Rust's borrow checker but for ranges of values. That way I can guarantee valid indexing.

RivieraKid5• 6 years ago

Statically typed metaprogramming. Ceylon is the only language that has that. No idea whether it's useful in real world though.

Also, one feature I would love to see implemented in a modern language is this:

    numbers..toString() // Equivalent to numbers.map(x => x.toString())

dunefox7• 6 years ago

numbers..toString() // Equivalent to numbers.map(x => x.toString())

You mean something like

    string.(1:5)

in Julia?

cygx• 6 years ago

Also, one feature I would love to see implemented in a modern language [...]

In Raku (the language formerly known as Perl6), that is spelled

    numbers>>.toString()

or using proper idioms

    @numbers>>.Str

mindvirus2• 6 years ago

I'm very happy about this - lack of null safety and implicit downcasts are two of the biggest problems with Dart/Flutter, which I otherwise really enjoy working in. I've gotten in the habit of adding "assert(x != null)" in all of my methods.

cbolton• 6 years ago

I've gotten in the habit of adding "assert(x != null)" in all of my methods

Is this not useless most of the time, since you'll usually get an exception (with stack trace) when the value is used?

mindvirus2• 6 years ago

Yeah, but at least this catches it early - i.e. if a branch/if-statement uses the field, it might not always be hit when testing things out. Unit tests _should_ catch these things, but so should the type system, and for my side projects I'm kinda fast and loose with testing for better or for worse.

jitl2• 6 years ago

Does Dart2 offer any compelling features as a language that aren’t also present in Kotlin, Typescript, Swift, or Haxe? I know many users are interested in Flutter - but I’m curious more about the language itself. Whenever I see Flutter examples I’m disappointed I can’t use it with Kotlin (which already has many specialized backends) or some other language with more widespread support outside the Google ecosystem.

hamstercat• 6 years ago

I come from a TypeScript background, so I can compare it to that. The biggest difference to me was the soundness of the type system, where you're ensured (from a runtime perspective) that objects are correctly typed.

This cuts both way: it was great not having to think about typings all the time around input/output, but it was also cumbersome when implementing the internal implementation. I'm a bit biased because I've come to think in term of structural typing much more. Typing in Dart reminded me more of my time with C#.

Null safety was a huge missing piece though, glad to see it being added.

smichel17• 6 years ago

Is there any language that does both? I've really enjoyed js/ts's structural typing for prototyping, but there are certain places I miss nominal typing, particularly with input validation (and other "primitive type, plus restrictions") scenarios.

For example, an integer that's used to index into an array, so it must be positive. Or a string that's a valid street address. You can maintain that info by packing it into a unique structure (ie, wrap in an object, using a unique key), but that's awkward to access. Or you can always pass around the parent object (eg, House), which has a unique structure, but then you're introducing unnecessarily tight coupling which is a disaster to maintain.

RivieraKid5• 6 years ago

I find Dart comparable or maybe even slightly better than Kotlin, not sure why exactly, some things I like:

- Overall the language is simpler and more regular, it's easier to read it.

- Async / await is way simpler and less mentally taxing than Kotlin's coroutines.

loic-sharma• 6 years ago

Could you say more about your experience of Kotlin's coroutines vs async / await? I'd love to learn more about that! :)

timsneath• 6 years ago

One of the big attractions for Dart is that it compiles to native ARM code on iOS and Android, compiles to Intel code for (today's) desktop platforms, and transpiles to JavaScript for the web. There are other high-level languages that support _some_ combination of these targets, but often there are significant differences between implementations (e.g. not all are of production-quality, differences in memory management or threading approaches, inconsistencies in implementation or APIs).

markdog123• 6 years ago

To be picky, and for those reading, Dart no longer uses the same "threading" approaches. Dart 1 used Isolates for the vm and web. Dart 2 uses Isolates for vm, aot, but web workers for the web. Of course with Dart 1 it compiled to web workers, but had the same Isolate abstraction.

Unfortunately, the Dart web worker implementation isn't very convenient and I think some work could be done there to make it nicer to use. It's a very important use case for today's devices.

satvikpendem2• 6 years ago

It compiles to JS, ARM, and x86, for web, mobile and desktop support respectively.

Also, Dart has code generation so you can write your own language features, like the freezed package which implements algebraic data types even though Dart doesn't have them natively.

markdog123• 6 years ago

Yeah, I'm not a huge fan of the current code gen methods. They're pretty hackish and ugly. I understand it's prob the only option right now. Need partial classes: https://github.com/kevmoo/language/blob/5a1e560bb0871f07f682...

eat_veggies• 6 years ago

Does freezed feel nice in day-to-day use, or is it more of an incomplete/leaky hack?

I'm planning on writing a library in Dart that implements a custom TCP protocol, and I think ADTs would be the best fit for capturing all the valid states and error conditions. I really don't want to have a bunch of nullable fields or use the visitor pattern :/

I've been tracking this issue [0] and hopefully with the release of null safety, the team will have the time to implement ADTs!

[0] https://github.com/dart-lang/language/issues/349

agumonkey• 6 years ago

Forgot what but I remember feeling quite inspired by some of the abstractions .. seemed just slightly innovative yet solving a problem too.

hopia• 6 years ago

Dart feels slightly less versatile than Typescript but has type soundness.

Both feel highly verbose to me, but maybe that's just cos I'm used to something with quite minimal syntactic overhead.

Ultimately, they're very similar as languages so it wouldn't take long for a Typescript developer to picn up Dart or vice versa.

chubot• 6 years ago

I used flow typing in MyPy and it's awesome. TypeScript has the same thing (although I would be curious if their rules are significantly different).

I think you can't practically have null safety without flow typing, at least in an imperative language, otherwise you end up with casts everywhere. MyPy has null safety, though it takes some effort to turn it on for most codebases.

millimeterman• 6 years ago

Not at all - that kind of dataflow analysis is basically just a hack to patch over the fundamentally broken design of having everything be nullable. It's totally unnecessary in a language with proper algebraic data types where there's no all-pervasive concept of "nullability". The only imperative language I know of that has this is Rust, but it originates from ML languages like OCaml and Haskell.

It's one of those features where once you get used to it, it's hard to fathom how anyone programs without it. It's such a simple yet expressive feature that's incredibly useful when writing normal, ordinary code. Nothing fancy or theoretical.

blandflakes3• 6 years ago

I'm mostly agreeing with both you and the sibling - flow-sensitive typing is great. I think it's awesome in Kotlin, and removes the Java-style casting or manual unwrapping inside of if guards you always end up with.

As sibling said, ML-style languages get away with this a different way - chained "container" operations in Scala, for example, are "right-biased", so the only get called if the result of the preceding operation was a success or not null.

I've heard it called "Railway Oriented Programming": https://fsharpforfunandprofit.com/rop/

Abishek_Muthian2• 6 years ago

Not a Dart programmer, but looking at it for flutter. I see from Jetbrains Developer Survey[1] that Dart has grown to 9% from 6% last year[2] which I think is because of flutter, which has grown its share to 39% in cross-platform development.

But doesn't it seem reasonable to use TypeScript for flutter now? If you are not a Googler, Where do you use Dart (outside flutter) in production for a purpose where it is better than TypeScript?

[1]https://www.jetbrains.com/lp/devecosystem-2020/

[2]https://www.jetbrains.com/lp/devecosystem-2019/

latchkey• 6 years ago

I use Dart for my Flutter ios/android app and Typescript for my backend for the app. I don't think I'd even consider reversing that.

Why? Libraries.

sebe• 6 years ago

But doesn't it seem reasonable to use TypeScript for flutter now?

Flutter started as an experiment from chrome team, so their first language was JavaScript, when they move to dart they decided to write the whole framework layer in dart, the engine is in C/C++, so it seems there is no going back to a JS world.

https://flutter.dev/docs/resources/technical-overview#layer-...

With the Dart, they got a VM, GC and other tools and a language team that could grow Dart to suit flutter. They also got stateful hotreload, a Dart idea, which was something that the Flutter team didn't know they wanted.

syspec• 6 years ago

I say this as a huge IntelliJ fan, ask anyone who knows me. My friends joke that I must be collecting commission checks, for as much as I nudge people toward IntelliJs awesome IDEs.

Take this survey with a grain of salt, because the results come from IntelliJ users, and IntelliJs most important and popular product is IntelliJ IDEA, a Java IDE (and other things)

So the results will skew in that direction for that userbase.

Abishek_Muthian2• 6 years ago

I understand what you are suggesting, considering IntelliJ products are tightly integrated with Android ecosystem there are chances that the survey results are skewed towards flutter development.

Then again, none of the responses here are outside flutter development!

chgibb• 6 years ago

Using Typescript for Flutter is the intent behind this project https://github.com/chgibb/hydro-sdk

Though, note I am the author.

filleduchaos• 6 years ago

For Flutter's needs (performant-enough JIT as well as stable AOT compilation for a variety of targets), TypeScript's (own) tooling is pretty much nonexistent.

Of course, the Flutter team could build the toolchains they need themselves. Or they could just...use the up-to-par language they're already using instead of rewriting things for no real reason.

ElliotH• 6 years ago

I'm really hoping this will make AngularDart a lot nicer to write. Inputs might have a null value and it's very easy to forget to account for it, will be great to catch that at compile time.

yagodragon• 6 years ago

I believe Dart is a more exciting language than Go. It's the most well designed scripting language out there, that can also be AOT compiled into native binaries. It's a better JS and a better Java at the same time. It would be also a killer language for backends and simple websites. Just imagine a better node.js built around Dart's excellent tooling.

What made Go more successful than Dart was a mix of decisions, luck, and momentum. On an alternate reality, Dart would swap places with Go. Dartium VM would have replaced Javascript on the Web and everyone would have been happy. Go would be seen as an obscure language with missing features that doesn't really do anything better than java/C#/C++

dunefox7• 6 years ago

I believe Dart is a more exciting language than Go.

Almost every language is, that's not difficult.

leonidasv• 6 years ago

It's the most well designed scripting language out there

You misspelled Kotlin.

freeqaz• 6 years ago

Kotlin is statically typed only runs on the JVM though, I thought?

dunefox7• 6 years ago

Strangely, both of you managed to misspell Julia.

titzer• 6 years ago

I believe Dart is a more exciting language than Go.

For certain types of tasks, you want to choose a language that is not moving fast and breaking things. Such a language tends to be less "exciting" than others.

dunefox7• 6 years ago

Look at Common Lisp which fits this description much more than Go and is still a million times more interesting and powerful. Go just really is a below average language.

hootbootscoot2• 6 years ago

Funny, I find Dart to be horrid and verbose, and oddly daunting, mostly because of the expected massive surface area your code has to traverse, and these odd "let's pretend async is sync" erm "features" that the JS community decided was OK Koolaid to drink... I promise you a future that you can async await. Spare me. I want to know the truth, not a lie. Did that request return a 200 or what?

I don't want a better node.js nor any node.js at all, really. Node.js called and it wants it's browser back. It's an event loop, for crying out loud, and has no business trying to pretend that it understands operating system processes and threads and such...

Dart reminds me of the worst of large corporate committee-ware. Go is at least a decent and respectable programming language, even if it IS garbage-collected...

I suppose I will get flagged for having strong expressed emotions about Dart. I dislike working with it, immensely, and this was even before Flutter... let's please avoid discussing Android Studio, chip heat dissipation, and global warming... Clearly the Goog has no influence on the fact that you need 6 cores running at 1.whatever gigahertz to open Farty Birds or whatever is hip. Flag away. Sorry, "null safety"... is that like "if (cat ==== 'undefined') {break exception omg try catch}?"

.... What does Dart do better than it's nearest competitor? What does Dart compete with again?

msla• 6 years ago

Can you list your reasons to dislike Dart in a format without the vitriol?

earthboundkid2• 6 years ago

Dart is a better ES5, but a worse ES6. If you compare Dart to the JavaScript that existed when it was created, it’s great. If you compare it to modern JS, it’s crap. Modern JS is really, really good. The main problem with it is that the ecosystem is only just so-so. People still complaining about JS in 2020 are mostly just showing their own ignorance of what the language is now.

wolco• 6 years ago

The lack of a standard lib puts pressure on the ecosystem to provide simple functionality like leftpad for every project. Managing those is diffcult.

jolux2• 6 years ago

Dart is statically typed. I still miss the type system in JavaScript, and as much as I like TypeScript after I get it working, it has too many configuration options.

fauigerzigerk• 6 years ago

So that was your proclamation. Now let's hear your arguments...

skocznymroczny• 6 years ago

Dart is one of my favourite languages right now. I know it gets a lot of criticism for not reinventing the wheel and being heavily OOP based, but it's simple enough, standard library works well, IDEs work great. Also it actually has some unique features which I now miss from other OOP languages, such as named constructors - new Angle.fromRadians(...), new Angle.fromDegrees(...) - feels much cleaner and more self-documenting than the static factory method other languages have you use.

Optional typing I never really cared about, because even in languages with var I find myself writing the types whenever possible.

olodus• 6 years ago

Looks to be very close to the solution Zig also went for. I like the solution and it seems to fit both languages very well. I should look a bit further into Dart some time I think.

Understanding null safety in Dart

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