Funnily enough, I sort of had the opposite journey, Swift seems horribly complicated and over-specified. The Dart compiler knows if I do a null-check, it can treat following references as safe, instead of if let x = x etc.
And it just hasn't lived up to any of the initial starry-eyed hype I had for it and expectations for a wider Swift ecosystem...
Like, here, it's great it runs on Raspberry Pi Pico but...it can't render UI unless someone writes a new UI library from scratch. And whoever does that inevitably will have to copy SwiftUI directly or alienate the people who actually know Swift.
> And whoever does that inevitably will have to copy SwiftUI directly or alienate the people who actually know Swift.
I may be wrong, but I don’t think this is actually true. SwiftUI isn’t universally loved in the community, largely thanks to its rough edges and how it struggles with that last 10% of polish in projects written with it. While copying SwiftUI wouldn’t necessarily be wrong per se, you’d actually probably get more rapid buy-in by copying “boring” old imperative UIKit, as that’s what most writers of Swift are using.
s/SwiftUI/UIKit, doesn't matter. (though I'm happy to have someone else point out SwiftUI has...some growing pains...it would have felt unfair for me to do so)
Point is its DOA for UI on non-Apple platforms.
Either you:
- attract current Swift programmers and keep up with $APPLE_UI_FRAMEWORK with no assistance.*
- attract no one and make a brand new framework in Swift.
- bind to some other UI framework, alienate current Swift programmers*, get all the people who have been dying to write Swift yet didn't build a UI framework for 7 years
There's just no universe where using Swift to write UI on non-Apple platforms makes sense to any constituency. You have to be simultaneously obsessed with Swift and don't care about platform-first UI, or want to maintain two separate UI frontends.
I hate being that absolutist, but that is what 16 years of experience on Apple platforms has taught me.
I'm surprised by the # of people arguing there's a possibility something else coudl work. I guess what I'd say is, if there is, where is it? Swift was on Linux eons ago, I believe before I started at Google so 7+ years.
* People used to try this to do iOS/OSX cross-platform, those projects are dead. One could argue this is just because Catalyst exists. Sure, but, let's just say you need at least N / 2 engineers, where N = the number of engineers Apple has on an arbitrary UI framework, in order to keep up with it. That's a lot.
** I've been in Apple dev since iPhone OS 2.0, and while in theory this could work, in practice it doesn't. Ex. Qt won't appeal to Apple programmers. It's antithetical to community values.
> Swift seems horribly complicated and over-specified. The Dart compiler knows if I do a null-check, it can treat following references as safe, instead of if let x = x etc.
`if let x = x` is an alternative to a null check, it's not as if now you need both a null check and an `if let`. What makes an `if let` more complicated?
Small change made in swift means an unwrap can just be
`if let x {
}`
Which while small, is great if you do it all the time.
Swift though made me question how much I used optionals, now I get annoyed if they're over used, it usually means someone down the line has not made a decision, they propagate through the program and eventually someone wonders why they didn't get a value.
I like the Result type and the Flexibility of the Enums a lot for this
You return an optional with no reason, it colours all subsequent calls.
When i was younger, i thought of them as an extension of bool, yes/no/maybe, but now, was it cancelled? Did an error occur? Did it just timed out? How many cpu cycles am i wasting throughout my code because every thing down the line checks it for null? It snowballs
It's not the same op though. One is a check, the other creates an in-context copy that's unwrapped from the optional. The compiler of course optimizes this out but it's not the same.
They have different use cases.
if x != nil is to do some work without needing x, because you'd still have to unwrap it if you used it.
That's what if let y = x is for, it creates a safe lexical context to use the unwrapped value.
And then you have guard let y = x which is a stronger guarantee that no code further down in the context is executed if x is nil. This helps to write happy-path code with early returns, avoiding pyramids of doom and encouraging writing code where all the assumptions are listed up front with logic handling exceptional cases before finally arriving at the logic that works under all the assumptions.
Dart feels like a step backwards after seeing the benefits these provide.
But there’s also no harm in combining them. I agree with the OP, TypeScript does what they describe Dart doing and I find it much simpler than the way Swift does it. It’s never confusing.
Disagree. I’ve seen people write things like the following:
if x != nil { x!.foo() }
and then later the x != nil condition is changed to no longer check for nil, or the x!.foo() is moved out of that lexical scope elsewhere, or copypastad, and then the force unwrap crashed. I’ve seen similar yet more subtle bugs with soft unwraps instead of forced: x?.foo()
Not possible if you use if let y = x . If you copypasta the inner scope with the different variable name then it won’t compile.
(ofc these are trivial representative examples, in reality these were lexical scopes with many lines and more complicated code)
It’s not just about how the code looks when you write it, it’s the paths that can be taken by future maintenance programmers.
I‘ve complained about the large surface area of swift syntax, stdlib etc in the past, but I don’t personally find this specific thing confusing.
Changing the type of something based on a conditional check is not great, because if I wanted a non-optional I would request it. Sometimes a nil check is literally just a nil check. What if I want to set the value back to nil if it is not-nil? Why should I have to wrap things back in an optional if I want to pass it along as such?
The reason why languages promote variable types based on control flow is because developers en masse actually expect that to happen, e.g. facing the code like
Dog? maybeDog;
if (maybeDog != null) {
maybeDog.bark();
}
If compiler says "sorry, maybeDog might be null", developer (rightfully so) usually responds "but I have just checked and I know it is not, why do you bother me?". So languages chose to accommodate this.
> What if I want to set the value back to nil if it is not-nil?
You can. The type of the variable does not actually change. You could say that the information about more precise type is simply propagated to the uses which are guarded by the control flow. The following will compile just fine:
See, I don't think languages should accommodate this, because I see it as an ugly solution. It's nice that it works in a few cases but then it very quickly breaks down: a developer finds that their null check is enough to strip an optional, but a test against zero doesn't convert their signed integer to unsigned. Checking that a collection has elements in it doesn't magically turn it into a "NonEmptyCollection" with guaranteed first and last elements. I'm all for compilers getting smarter to help do what programmers expect, but when they can't do a very good job I don't find the tradeoff to be very appealing. Basically, I think pattern matching is a much better solution this problem, rather than repurposing syntax that technically means something else (even though 90% of the time people who reach for it mean to do the other thing).
Also, fwiw, I was mostly talking about things like the last example you gave. I guess it would be possible that in circumstances where T is invalid but T? would be valid, the language actually silently undos the refinement to make that code work. However, I am not sure this is actually a positive, and it doesn't help with the ambiguous cases anyways.
This isn't the same thing as "magically" changing the type.
What does the syntax technically mean?
What refinement is undone?
I think you're a bit too wedded to the idea that there's a type conversion going on or some dark magic or something behind the scenes that's "done" then "undone" like a mechanism. There isn't. It's just static analysis. Same as:
let x: Animal;
x = Dog()
if (x is Dog) {
x.bark()
}
The Zen koan you want to ponder on your end is, why do you want to A) eliminate polymorphism from OOP B) remove the safety of a compiler error if the code is changed to x = Cat()?
I don’t like that either. x is Dog is a boolean expression. Presumably I can write
let isDog = x is Dog;
And the value whether it is a dog or not goes into that new variable. The fact that I can’t then immediately go
if (isDog) {
x.bark()
}
shows the deficiencies of this static analysis (even if you could do some simple value tracking to make this work, it doesn’t really address the real issue I have with it, which I described above: why can’t I do this refinement for other things?) The conceptual model is one of “we special cased 2-3 cases that we can detect”, which I don’t find very elegant, especially considering that other languages seem to have better solutions that express the operation in what I feel is a better way.
(The equivalent Swift code would be something like this:
let x = Animal()
if let dog = x as? Dog {
dog.bark()
}
I see this as much superior. One, because x is still available in the scope of I want an Animal and not a Dog for whatever reason. I understand that most of the time you don’t do this but to have it available and not have to do a special case for it is nice. The second thing is that I just like the syntax, since it gives you an opportunity to give a more specific name in that branch as part of the assignment. Third, it composes, because none of the operations are special save for the “if let” binding. This code is also valid:
let x = Animal()
let dog /* : Dog? */ = x as? Dog
if let dog /* = dog */ {
}
The static analysis for undoing polymorphism is the exact same as the one for binding to optionals, because the idiomatic way to cast results in an optional.)
Who said you can't? :) This actually works in Dart:
Dog? dog;
bool isDog = dog is Dog;
if (isDog) {
dog.bark();
}
i.e. boolean variables which serve as witnesses for type judgements are integrated into promotion machinery.
I do agree with you that
1. In general there is no limit to developers' expectations with respect to promotion, but there is a limit to what compiler can reasonably achieve. At some point rules become too complex for developers to understand and rely upon.
2. Creating local variables when you need to refine the type is conceptually simpler and more explicit, both for language designers, language developers and language users.
I don't hate excessive typing, especially where it helps to keep things simple and readable - so I would not be against a language that does not do any control flow based variable type promotion. Alas many developers don't share this view of the world and are vehemently opposed to typing anything they believe compiler can already infer.
Kotlin can't do this, which the language I have more experience with. It's good that Dart does a little better in that regard. And, I think we do agree, I just don't really like the viewpoint of doing this, because I feel like it's not really general enough.
I don't think any of that has to do anything with static analysis deficiencies. The analysis is the same. What Swift requires is for the user to manually help the compiler with a rather verbose and otherwise unnecessary construct.
It’s not a new idea, but I don’t think it’s a good idea. That’s just it. Swift has a construct to do the equivalent and it’s fewer characters to boot when compared to Dart when you’re going from an optional to non-optional type.
But that's exactly what Swift does: it narrows the types. To do that, it uses a separate construct that only exists to make the parser ever so slightly simpler.
The ugly solution is Swift's if let x = x which is literally the same check, but in a more verbose manner.
Yes, compilers should be able to help in this and many other cases, and not just give up and force the programmer to do all the unnecessary manual work
It’s not a check, it’s binding a new variable with a different type (which may or may not have the same name; if it does you can use if let x these days). And solving the other examples I mentioned is generally out of the realm of most programming languages in use today (except maybe TypeScript?) It comes with significant challenges that I’m not sure we are ready to address properly yet.
> it can't render UI unless someone writes a new UI library from scratch
This is true for literally every programming language. You can’t render a UI, in general, without a target compatible UI library.
SwiftUI and UIKit are specific Apple things built on top of CoreAnimation and completely separate from the Swift language. And swift will happily link to C and somewhat C++, so no it’s not all evident why one would copy Apple’s SDK to render UI on a completely separate platform and environment.
> The Dart compiler knows if I do a null-check, it can treat following references as safe, instead of if let x = x etc.
Does Dart bind a new variable 'x' for the if-statement scope or does the if-statement scope refer to the same 'x' from the outer scope?
The reason Swift and other languages bind a new 'x' variable (if let x = x) is because the original 'x' can be assigned a different value elsewhere, like in another thread or a closure. If a closure captures and assigns null to 'x' and there is only one binding of 'x', then that means the 'x' within the if-statement will become null _post null-check_ which is type unsound. I believe TypeScript has this "problem" as well.
1) Dart doesn't have threads, it has isolates (where memory is not shared). So no variable modifications here.
2) Dart will promote a variable as not null after a non-null check only if it cannot be modified elsewhere, ie, the variable is final (can't be modified once assigned), or there's no other reference to it.
To clarify, in the above scenario in dart I can have an input with type MyEnum and do this?
switch myEnum {
case let one:
// react to this here
case let two(twosChild: twosChild):
// call an API passing in a child element of twosChild here
case let three(threesGenericChild: threesGenericChild):
// T: Protocol, and call an extension function threesGenericChild.extension() here?
case let four(foursCallbackChild: foursCallbackChild):
// Pass foursCallbackChild callback into another context, and execute an escaping closure if needed?
}
Last I checked Dart required a nested if/else tree and didn't have compiler-guarantees of switch completeness like the above.
Correct, Dart has enum exhaustiveness checks, associated types, and expressions as of 11-13 months ago: https://cfdevelop.medium.com/dart-switch-expressions-33145c3... (keeping it short and simple because I'm at -2 on the original post, don't want to look like I'm argumentative)
In Dart you use class hierarchies instead, rather than enums (which in Dart are way to define a compile time constant set of values). So the original example becomes:
sealed class MySomething<T> {
}
final class One extends MySomething<Never> {
}
final class Two extends MySomething<Never> {
final Child child;
Two(this.child);
}
final class Three<T> extends MySomething<T> {
final T value;
Three(this.value);
}
final class Four extends MySomething<Never> {
final int Function() callback;
Four(this.callback);
}
And then you can exhaustively switch over values of MySomething:
The declaration of a sealed family is considerably more verbose than Swift - and I really would like[1] us to optimized things for the case when people often declare such families. Macros and a primary constructors will likely provide reprieve from this verbosity.
But importantly it composes well with Dart's OOPy nature to achieve things which are not possible in Swift: you can mix and match pattern matching and classical virtual dispatch as you see fit. This, for example, means you can attach different virtual behavior to every member of the sealed class family.
Funnily enough, I sort of had the opposite journey, Swift seems horribly complicated and over-specified. The Dart compiler knows if I do a null-check, it can treat following references as safe, instead of if let x = x etc.
And it just hasn't lived up to any of the initial starry-eyed hype I had for it and expectations for a wider Swift ecosystem...
Like, here, it's great it runs on Raspberry Pi Pico but...it can't render UI unless someone writes a new UI library from scratch. And whoever does that inevitably will have to copy SwiftUI directly or alienate the people who actually know Swift.