Allow for shorter dot syntax to access enum values

[rami-a]

Admin comment: this is being implemented, feature specification, dart-lang/sdk#57036, experiment flag enum-shorthands.

---

When using enums in Dart, it can become tedious to have to specify the full enum name every time. Since Dart has the ability to infer the type, it would be nice to allow the use of shorter dot syntax in a similar manner to Swift

The current way to use enums:

enum CompassPoint {
  north,
  south,
  east,
  west,
}

if (myValue == CompassPoint.north) {
  // do something
}

The proposed alternative:

enum CompassPoint {
  north,
  south,
  east,
  west,
}

if (myValue == .north) {
  // do something
}

[johnsonmh]

This would be especially nice in collections:

const supportedDirections = <CompassPoint>{.north, .east, .west};
bool isSupported = supportedDirections.containsAll({.north, .east});

It's worth noting too that we would only allow it in places where we can infer the enum type.

So

final north = .north; // Invalid.
final CompassPoint north = .north; // Valid.
final north = CompassPoint.north; // Valid.

[kasperpeulen]

In Swift this feature works not only for enums but also for static properties of classes. See also:

munificent/ui-as-code#7

class Fruit {
    static var apple = Fruit(name: "apple");
    static var banana = Fruit(name: "banana");
    
    var name: String;
    
    init(name: String) {
        self.name = name;
    }
}

func printFruit(fruit: Fruit) {
    print(fruit.name);
}

// .banana is here inferred as Fruit.banana
printFruit(fruit: .banana);

[lrhn]

How would the resolution work?

If I write .north, then the compiler has to look for all enums that are available (say, any where the name of the enum resolves to the enum class), and if it finds exactly one such which has a north element, use that.
If there is more than one enum class in scope with a north element, it's a compile-time error. If there is zero, it is a compile-time error.

If we have a context type, we can use that as a conflict resolution: <CompassPoint>[.north, .south] would prefer CompassPoint.north, CompassPoint,south over any other enum with a north or south element.
We won't always have a context type, the example if (myValue == .north) { does not.

Alternatively, we could only allow the short syntax when there is a useful context type.
For the equality, you will have to write CompassPoint.north (unless we introduce something like "context type hints" because we know that if one operand of an == operator is a CompassPoint enum type, and enums don't override Object.==, then the other is probably also a CompassPoint, but that's a different can of worms).
Then we could extend the behavior to any static constant value of the type it's embedded in.
That is, if you have Foo x = .bar; then we check whether Foo has a static constant variable named bar of type Foo, and if so, we use it. That way, a user-written enum class gets the same affordances as a language enum.

I guess we can do that for the non-context type version too, effectively treating any self-typed static constant variable as a potential target for .id.

(Even more alternatively, we can omit the . and just write north. If that name is not in scope, and it's not defined on the interface of this. then we do "magical constant lookup" for enum or enum-like constant declarations in scope.
That's a little more dangerous because it might happen by accident.

[eernstg]

One approach that could be used to avoid writing CompassPoint several times is a local import (#267).

[kasperpeulen]

How would the resolution work?

@lrhn You may want to study how it works in Swift. I think their implementation is fine.

[johnsonmh]

@lrhn

Alternatively, we could only allow the short syntax when there is a useful context type.

If we're taking votes, I vote this ☝️

Regarding the case with if (myValue == .north) {, if myValue is dynamic, then I agree, this should not compile. However; myValue would often already be typed, if it is typed, it should work fine. For example:

void _handleCompassPoint(CompassPoint myValue) {
  if (myValue == .north) {
    // do something
  }   
}

For the equality, you will have to write CompassPoint.north

I don't know enough about this, but I don't see why this would need to be the case if we're going with the "useful context type" only route?

Right now we can do:

final direction = CompassPoint.north;
print(direction == CompassPoint.south); // False.
print(direction == CompassPoint.north); // True.
print("foo" == CompassPoint.north); // False.

If we know that direction is CompassPoint, can we not translate direction == .south to direction == CompassPoint.south? Or is that not how this works?

Even more alternatively, we can omit the . and just write north

I don't personally prefer this approach because we risk collisions with existing in scope variable names. If someone has var foo = 5; and enum Bar { foo, }, and they already have a line foo == 5, we won't know if they mean Bar.foo == 5 or 5 == 5.

[lrhn]

The problem with context types is that operator== has an argument type of Object. That gives no useful context type.

We'd have to special case equality with an enum type, so if one operand has an enum type and the other is a shorthand, the shorthand is for an enum value of the other operand's type. That's quite possible, it just doesn't follow from using context types. We have to do something extra for that.

[lrhn]

We can generalize the concept of "enum value" to any value or factory.

If you use .foo with a context type of T, then check whether the class/mixin declaration of T declares a static foo getter with a type that is a subtype of T. If so, use that as the value.
If you do an invocation on .foo, that is .foo<...>(...), then check if the declaration of T declares a constructor or static function with a return type which is a subtype of T. If so, invoke that. For constructors, the context type may even apply type arguments.

It still only works when there is a context type. Otherwise, you have to write the name to give context.

[ReinBentdal]

To omit the . would make sense for widgets with constructors.

From

Text(
  'some text',
  style: FontStyle(
    fontWeight: FontWeight.bold
  ),
),

To

Text(
  'some text',
  style: ( // [FontStyle] omitted
    fontWeight: .bold // [FontWeight] omitted
  ),
),

For enums and widgets without a constructor the . makes sense to keep, but for widgets where the . never existed, it makes sense to not add it.

FontWeight.bold -> .bold // class without a constructor
Overflow.visible -> .visible // enum
color: Color(0xFF000000) -> color: (0xFF000000) // class with constructor

From issue #417

_Some pints may have been presented already

Not include subclasses of type

Invalid

padding: .all(10)

This wont work because the type EdgeInsetsGeometry is expected, but the type EdgeInsets which is a subclass is given.

Valid

textAlign: .cener

This will work because TextAlign is expected and TextAlign is given.
The solution for the invalid version would be for flutter to adapt to this constraint.

The ?. issue

Alot of people have pointed out this issue on reddit. The problem is as follows:

bool boldText = true;

textAlign = boldText ? .bold : .normal;

The compiler could interpret this as boldText?.bold.
But as mentioned on reddit: https://www.reddit.com/r/FlutterDev/comments/c3prpu/an_option_to_not_write_expected_code_fontweight/ert1nj1?utm_source=share&utm_medium=web2x
This will probably not be a problem because the compiler cares about spaces.

Other usecases

void weight(FontWeight fontWeight) {
  // do something
}
weight(.bold);

[andrewackerman]

@ReinBentdal

Omitting the period for constructors would lead to a whole slew of ambiguous situations simply because parentheses by themselves are meant to signify a grouping of expressions. Ignoring that, though, I think removing the period will make the intent of the code far less clear. (I'm not even sure I'd agree that this concise syntax should be available for default constructors, only for named constructors and factories.)

And about the ?. issue, like I said in both the reddit post and issue #417, the larger issue is not whether the compiler can use whitespace to tell the difference between ?. and ? .. It's what the compiler should do when there isn't any whitespace at all between the two symbols. Take this for example:

int value = isTrue?1:2;

Notice how there is no space between the ? and the 1. It's ugly, but it's valid Dart code. That means the following also needs to be valid code under the new feature:

textAlign = useBold?.bold:.normal;

And now that there's no space between the ? and the ., how should the compiler interpret the ?.? Is it a null-aware accessor? Is it part of the ternary followed by a type-implicit static accessor? This is an ambiguous situation, so a clear behavior needs to be established.

[ReinBentdal]

A solution could be to introduce a identifyer.

*.bold // example symbol

But then again, that might just bloat the code/ language.

[lukepighetti]

I'd like to see something along these lines

final example = MyButton("Press Me!", onTap: () => print("foo"));

final example2 = MyButton("Press Me!",
    size: .small, theme: .subtle(), onTap: () => print("foo"));

class MyButton {
  MyButton(
    this.text, {
    @required this.onTap,
    this.icon,
    this.size = .medium,
    this.theme = .standard(),
  });

  final VoidCallback onTap;
  final String text;
  final MyButtonSize size;
  final MyButtonTheme theme;
  final IconData icon;
}

enum MyButtonSize { small, medium, large }

class MyButtonTheme {
  MyButtonTheme.primary()
      : borderColor = Colors.transparent,
        fillColor = Colors.purple,
        textColor = Colors.white,
        iconColor = Colors.white;

  MyButtonTheme.standard()
      : borderColor = Colors.transparent,
        fillColor = Colors.grey,
        textColor = Colors.white,
        iconColor = Colors.white;

  MyButtonTheme.subtle()
      : borderColor = Colors.purple,
        fillColor = Colors.transparent,
        textColor = Colors.purple,
        iconColor = Colors.purple;

  final Color borderColor;
  final Color fillColor;
  final Color textColor;
  final Color iconColor;
}

[MarcelGarus]

Exhaustive variants and default values are both concepts applicable in a lot of scenarios, and this feature would help in all of them to make the code more readable. I'd love to be able to use this in Flutter!

return Column(
  mainAxisSize: .max,
  mainAxisAlignment: .end,
  crossAxisAlignment: .start,
  children: <Widget>[
    Text('Hello', textAlign: .justify),
    Row(
      crossAxisAlignment: .baseline,
      textBaseline: .alphabetic,
      children: <Widget>[
        Container(color: Colors.red),
        Align(
          alignment: .bottomCenter,
          child: Container(color: Colors.green),
        ),
      ],
    ),
  ],
);

[munificent]

Replying to @mraleph's comment #1077 (comment) on this issue since this is the canonical one for enum shorthands:

I think this is extremely simple feature to implement - yet it has a very delightful effect, code becomes less repetetive and easier to read (in certain cases).

I agree that it's delightful when it works. Unfortunately, I don't think it's entirely simple to implement. At least two challenges are I know are:

How does it interact with generics and type inference?

You need a top-down inference context to know what .foo means, but we often use bottom-up inference based on argument types. So in something like:

f<T>(T t) {}

f(.foo)

We don't know what .foo means. This probably tractable by saying, "Sure, if there's no concrete inference context type, you can't use the shorthand", but I worry there are other complications related to this that we haven't realized yet. My experience is that basically anything touching name resolution gets complex.

What does it mean for enum-like classes?

In large part because enums are underpowered in Dart, it's pretty common to turn an enum into an enum-like class so that you can add other members. If this shorthand only works with actual enums, that breaks any existing code that was using the shorthand syntax to access an enum member. I think that would be really painful.

We could try to extend the shorthand to work with enum-like members, but that could get weird. Do we allow it at access any static member defined on the context type? Only static getters whose return type is the surrounding class's type? What if the return type is a subtype?

Or we could make enum types more full-featured so that this transformation isn't needed as often. That's great, but it means the shorthand is tied to a larger feature.

How does it interact with subtyping?

If we extend the shorthand to work with enum-like classes, or make enums more powerful, there's a very good chance you'll have enum or enum-like types that have interesting super- and subtypes. How does the shorthand play with those?

Currently, if I have a function:

foo(int n) {}

I can change the parameter type to accept a wider type:

foo(num n) {}

That's usually not a breaking change, and is a pretty minor, safe thing to do. But if that original parameter was an enum type and people were calling foo with the shorthand syntax, then widening the parameter type might break the context needed to resolve those shorthands. Ouch.

All of this does not mean that I think a shorthand is intractable or a bad idea. Just that it's more complex than it seems and we'll have to put some real thought into doing it right.

[Abion47]

@munificent

If changing the interface breaks the context to the point that name inference breaks, then that is probably a good thing in the same way that making a breaking change in a package should be statically caught by the compiler. It means that the developer needs to update their code to address the breaking change.

To your last example in particular

foo(int n) {}
// to
foo(num n) {}

if that original parameter was an enum type

Enums don't have a superclass type, so I don't really see how an inheritance issue could arise when dealing with enums. With enum-like classes, maybe, but if you have a function that takes an enum-like value of a specific type, changing the type to a wider superclass type seems like it would be an anti-pattern anyway, and regardless would also fall into what I said earlier about implementing breaking changes resulting in errors in the static analysis of your code being a good thing.

[mraleph]

Unfortunately, I don't think it's entirely simple to implement. At least two challenges are I know are:

FWIW you list design challenges, not implementation challenges. The feature as I have described it (treat .m as E.m if .m occurs in place where E is statically expected) is in fact extremely simple to implement. You just treat all occurrences of .m as a dynamic, run the whole inference and then at the very end return to .m shorthands - for each of those look at the context type E and check if E.m is assignable to E (this condition might be tightened to require E.m to be specifically static final|const E m). If it is - great, if it is not issue an error. Done. As described it's a feature on the level of complexity of double literals change that we did few years back (double x = 1 is equivalent to double x = 1.0).

I concede that there might be some design challenges here, but I don't think resolving them should be a blocker for releasing "MVP" version of this feature.

Obviously things like grammar ambiguities would need to be ironed out first: but I am not very ambitions here either, I would be totally fine shipping something that only works in parameter positions, lists and on the right hand side of comparisons - which just side steps known ambiguities.

Just that it's more complex than it seems and we'll have to put some real thought into doing it right.

Sometimes putting too much thought into things does not pay off because you are entering the area of diminishing returns (e.g. your design challenges are the great example of things which I think is not worth even thinking about in the context of this language feature) or worse you are entering analysis paralysis which prevents you from moving ahead and actually making the language more delightful to use with simple changes to it.

That's usually not a breaking change, and is a pretty minor, safe thing to do.

You break anybody doing this:

var x = foo;
x = (int n) { /* ... */ }

Does it mean we should maybe unship static tear-offs? Probably not. Same applies to the shorthand syntax being discussed here.

[lukepighetti]

I'm not a computer scientist but aren't the majority of these issues solved by making it only work with constructors / static fields that share return a type that matches the host class & enum values? That's my only expectation for it anyway, and none of those come through generic types to begin with. If the type is explicit, it seems like the dart tooling would be able to to know what type you're referring to.

I don't think the value of this sugar can be understated. In the context of Flutter it would offer a ton of positive developer experience.

enum FooEnum {
  foo,
  bar,
  baz
}

f(FooEnum t) {}

f(.foo) // tooling sees f(FooEnum .foo)
f(.bar) // tooling sees f(FooEnum .bar)
f(.baz) // tooling sees f(FooEnum .baz)

In the context of Flutter the missing piece that I find first is how to handle foo(Color c) and trying to do foo(.red) for Colors.red. That seems like it would be a nice feature but I'm not sure how you'd handle that quickly and cleanly. I don't think it's necessary to be honest, though.

[munificent]

FWIW you list design challenges, not implementation challenges.

Yes, good point. I mispoke there. :)

As described it's a feature on the level of complexity of double literals change that we did few years back

That feature has caused some problems around inference, too, though, for many of the same reasons. Any time you use the surrounding context to know what an expression means while also using the expression to infer the surrounding context, you risk circularity and ambiguity problems. If we ever try to add overloading, this will be painful.

I concede that there might be some design challenges here, but I don't think resolving them should be a blocker for releasing "MVP" version of this feature.

We have been intensely burned on Dart repeatedly by shipping minimum viable features:

• The cascade syntax is a readability nightmare when used in nested contexts. The language team at the time dismissed this as, "Well, users shouldn't nest it." But they do, all the time, and the code is hard to read because of it. No one correctly understands the precedence and god help you if you try to combine it with a conditional operator.

• We shipped minimal null-aware operators that were described as a "slam dunk" because of how simple and easy it was. If I recall right, the initial release completely forgot to specify what short-circuiting ??= does. The ?. specified no short-circuiting at all which made it painful and confusing to use in method chains. We are laboriously fixing that now with NNBD and we had to bundle that change into NNBD because it's breaking and needs an explicit migration.

• The generalized tear-off syntax was basically dead-on-arrival and ended up getting removed.

• Likewise, the "minimal" type promotion rules initially added to the language didn't cover many common patterns and we are again fixing that with NNBD (even though most of it is not actually related to NNBD) because doing otherwise is a breaking change.

• The crude syntax-driven exhaustiveness checking for switch statements was maybe sufficient when we were happy with any function possibly silently returning null if it ran past the end without a user realizing but had to be fixed for NNBD.

• The somewhat-arbitrary set of expressions that are allowed in const is a constant friction point and every couple of releases we end up adding a few more cherry-picked operations to be used there because there is no coherent principle controlling what is and is not allowed in a const expression.

• The completely arbitrary restriction preventing a method from having both optional positional and optional named parameters causes real pain to users trying to evolve APIs in non-breaking ways.

• The deliberate simplifications to the original optional type system—mainly covariant everything, no generic methods, and implicit downcasts—were the wrong choice (though made for arguably good reasons at the time) and had to be fixed with an agonizing migration in Dart 2.

I get what you're saying. I'm not arguing that the language team needs to go meditate on a mountain for ten years before we add a single production to the grammar. But I'm pretty certain we have historically been calibrated to underthink language designs to our detriment.

I'm not proposing that we ship a complex feature, I'm suggesting that we think deeply so that we can ship a good simple feature. There are good complex features (null safety) and bad simple ones (non-shorting ?.). Thinking less may by necessity give you a simple feature, but there's no guarantee it will give you a good one.

It's entirely OK if we think through something and decide "We're OK with the feature simply not supporting this case." That's fine. What I want to avoid is shipping it and then realizing "Oh shit, we didn't think about that interaction at all." which has historically happened more than I would like.

You break anybody doing this:

var x = foo;
x = (int n) { /* ... */ }

Does it mean we should maybe unship static tear-offs?

That's why I said "usually". :) I don't think we should unship that, no. But it does factor into the trade-offs of static tear-offs and it is something API maintainers have to think about. The only reason we have been able to change the signature of constructors in the core libraries, which we have done, is because constructors currently can't be torn off.

[tatumizer]

Typedefs looks like a viable option, but by itself, it won't help.
Consider

static extension on AlignmentGeometry {
  typedef Absolute = Alignment;
  typedef Directional = AlignmentDirectional;
}
AlignmentGeometry a = .Absolute.topRight; 

How is .Absolute.topRight better than Alignment.topRight ?

.topRight is strictly better.

What do you mean by "better"? It's shorter, but to be able to say it's better, you have to come up with a theory that explains how this constant makes its way into AlignmentGeometry namespace.
We can try

static extension on AlignmentGeometry {
  typedef Absolute = Alignment default;
  typedef Directional = AlignmentDirectional;
}
AlignmentGeometry a = $.topRight; 

where default enables a shorter syntax, but what syntax? That's what led me to $.topRight, but this is not the only option. We can experiment with different symbols: \ # % ^ & * + = : | < > /, with or without the dot. like

AlignmentGeometry a = #.topRight; 
AlignmentGeometry a = `.topRight; 
AlignmentGeometry a = `topRight; // no dot
//etc. 

[lrhn]

I'm not advocating that every static declaration on a subclass should necessarily be available on the superclass.
Only those that make sense.

you have to come up with a theory that explains how this constant makes its way into AlignmentGeometry namespace.

Someone put it there. That's the answer every time.
If you don't want to put every constructor of Alignment into AlignmentGeometry, then don't. If it makes sense to have it there, then do.

[tatumizer]

I don't agree with your characterization of the proposal of copying/pasting/renaming stuff in bulk as a "theory". At least not a "good theory". I tried to list some arguments explaining why it was not a good theory every time it resurfaced, but the only important factor is whether Flutter people will like it or not, and I can bet a proverbial farm on them not liking it passionately enough. :-)

[TekExplorer]

Keep in mind, this feature making it in will have package maintainers move to improve their apis with this in mind.
Plus, static extensions will help as well, as accepted patterns will likely end up with static extensions on the context type eventually.

for AlignmentGeomentry I expect for all Alignment options to be basically grafted onto AlignmentGeomentry for the convenience. Future useful constructors will make their way into that static interface, if it makes sense to.

[tatumizer]

Keep in mind, this feature making it in will have package maintainers move to improve their apis with this in mind.

You have to convince the maintainers to do the following:

1. Mangle the names - because of name collisions, or because the original name does not make sense in another namespace. There's always at least one collision (default constructor). What to do about it? (In fact, the number of name collisions in general is unknown, and there's no theory saying how to rename them)

2. Fake the return type. Now not only some grafted constructors will have different names - they will also have different return types.

3. The proposal promises to support chaining. How can you explain to users why chaining is not supported for "grafted" methods? There's nothing in the dot syntax to suggest the method was grafted. This will look like a bug (because it is!)

4. Clutter the code with massive copy-pasted material

5. Realize that after all manipulations with names/types, your entire API changes - so you have to explain how to convert all your tutorials to the language of shortcuts. (This is not a mechanical procedure).

6. And for what? To enable the code like this?

Look inside

Container(
      decoration: const .box(
        border: .border(
          top: .new(color: .new(0xFFFFFFFF)),
          left: .new(color: .new(0xFFFFFFFF)),
          right: .new(),
          bottom: .new(),
        ),
      ),
      child: Container(
        padding: const EdgeInsets.symmetric(horizontal: 20.0, vertical: 2.0),
        decoration: const .box(
          border: .border(
            top: .new(color: .new(0xFFDFDFDF)),
            left: .new(color: .new(0xFFDFDFDF)),
            right: .new(color: .new(0xFF7F7F7F)),
            bottom: .new(color: .new(0xFF7F7F7F)),
          ),
          color: .new(0xFFBFBFBF),
        ),
        child: const Text('OK',
            textAlign: .center,
            style: .new(color: .new(0xFF000000))),
      ),
    );
  }

Please be honest: do you like this code?

[TekExplorer]

If mangling the name results in a bad API, then it's probably not a good default and you shouldn't use it that way anyway. Generally a "default constructor" conflict doesn't exist as the type names are different and can provide a reasonable name.

There is also nothing stopping you from namespacing things with records or middleman objects, riverpod style.

There is no problem with having the type of a getter or static method as a subtype, and I don't know what you're talking about regarding chaining and grafted members.

If you're copy pasting a lot, you're doing something wrong. It's just a single getter, static field, or factory/static method for most cases

A change to the API would be purely additive. And of course you need to take that into account for docs, it's an improvement on how defaults are accesses.

That code is mediocre because it abuses the feature. There is nothing stating we couldn't have an annotation + lint that says "don't dot-notate this member", or a lint that's specifically about .new

[tatumizer]

I don't know what you're talking about regarding chaining and grafted members.

Suppose you have grafted EdgeInset constructor all into EdgeInsetGeometry. Suppose also in EdgeInset, you have a method copyWith(...}.
Then

EdgeInsetGeometry g = EdgeInset.all(10).copyWith(right: 15); // works with full name
EdgeInsetGeometry g1 = .all(10).copyWith(right: 15); // error! 

The reason for an error is that the "grafted" constructor .all returns an object of type EdgeInsetGeometry, for which copyWith is not defined.

[Levi-Lesches]

I may have said it before, but with all the long conversations in this thread, I believe there is no one clear rule that is obvious and intuitive for everyone. Maybe this would be better suited as an IDE auto-complete suggestion that transforms eg .center --> Alignment.center by offering suggestions filtered by the context type and not an actual language feature that affects how code is read and written. It would ultimately end up with the developer experience most people want -- quick to write and simple to understand -- but I'm not sure some of the proposals here would accomplish both (again, as evidenced by many disagreements over what's natural or obvious)

[TekExplorer]

The reason for an error is that the "grafted" constructor .all returns an object of type EdgeInsetGeometry, for which copyWith is not defined.

Not necessarily. The static member can be a subtype, which we can act on, so long as the type we ended up with is still valid. How we get there doesn't matter.

After all, .all is not a constructor, it's a static getter/member.

[tatumizer]

.all is a constructor in EdgeInset. You have two choices while grafting it into EdgeInsetGeometry namespace:

• translate it to a factory constructor. Then the return type will be EdgeInsetGeometry, and you lose the ability to call copyWith
• translate it to a static method. Then you lose constness.

[jodinathan]

I may have said it before, but with all the long conversations in this thread, I believe there is no one clear rule that is obvious and intuitive for everyone. Maybe this would be better suited as an IDE auto-complete suggestion that transforms eg .center --> Alignment.center by offering suggestions filtered by the context type and not an actual language feature that affects how code is read and written. It would ultimately end up with the developer experience most people want -- quick to write and simple to understand -- but I'm not sure some of the proposals here would accomplish both (again, as evidenced by many disagreements over what's natural or obvious)

I disagree because this solves a lot of problems with source generated stuff.

[TekExplorer]

.all is a constructor in EdgeInset. You have two choices while grafting it into EdgeInsetGeometry namespace:

• translate it to a factory constructor. Then the return type will be EdgeInsetGeometry, and you lose the ability to call copyWith
• translate it to a static method. Then you lose constness.

Const is not exactly going to help with your copyWith.

Pick which is a better default. A factory for const, a static method for subtype context, or, if you don't need arguments, let it be a constant static variable.

This issue does not replace the normal way of doing things, it's only a useful shorthand for most usecases. If you need more, go back to the specific one.

[eernstg]

@tatumizer wrote:

.all is a constructor in EdgeInset. You have two choices while grafting it into EdgeInsetGeometry namespace:

• translate it to a factory constructor. Then the return type will be EdgeInsetGeometry, and you lose the ability to call copyWith
• translate it to a static method. Then you lose constness.

It wouldn't be difficult to preserve both: #4153.

[bernaferrari]

it's been a while people don't talk (fight) about this issue.

Flutter has this request which seems like could be used by whatever you do with EdgeInsetGeometry once this is shipped.
https://docs.google.com/document/d/1WMxm3_E3w58339Wvjt-j5WaqTALNSwSePQs3p0fxW64/edit?tab=t.0

[mateusfccp]

it's been a while people don't talk (fight) about this issue.

Flutter has this request which seems like could be used by whatever you do with EdgeInsetGeometry once this is shipped.
https://docs.google.com/document/d/1WMxm3_E3w58339Wvjt-j5WaqTALNSwSePQs3p0fxW64/edit?tab=t.0

Annotations can't interfere with the semantics of a program. The language can't decide the context type or accept a shorthand based on an annotation.

Considering this, this is not a viable alternative. We would have to have something in the language side to do this.

The current specification does not handle this case.

[mit-mit]

This feature is moving to implementation; tracking issue is dart-lang/sdk#57036

[rrousselGit]

Out of curiosity, would in T support generic parameters? For example:

abstract class Interface<T extends Interface<T>> {
  void method(num value in T) {}
}

class Concrete extends Interface<Concrete> {
  static final pi = 3.14;
}

class Another extends Interface<Another> {
  static final theAnswer = 42;
}

Then used as:

final concrete = Concrete();
concrete.method(.pi);

final another = Another();
another.method(.theAnswer);

[eernstg]

Out of curiosity, would in T support generic parameters? For example:

I assume this is concerned with the proposal about 'parameter default scopes', #3834? Of course, that particular proposal caused several community members to get really, really upset, so it wasn't part of the proposal which is now being implemented.

However, I still think something along the lines of #3834 would be a useful generalization of the proposal which is being implemented. This would simply amount to adding support for in T on formal parameters, and letting that determine the namespace where dot shorthands would be looked up (namely: in T).

When there is no in T clause, the parameter default scope proposal works exactly like the proposal which is being implemented, as it has done all the time (in particular, if dot shorthands are generalized to support in T on parameters then no existing dot shorthands will break or change behavior).

The parameter default scope proposal requires T to be a compile-time constant denotation of a declaration which is capable of declaring static members and/or constructors. For instance, it could denote a class, a mixin, or an extension (but if it is a mixin or an extension then it can't contain constructors (yet ;-), so in that case it's only about static members).

This means that there is no direct support for your example.

However, if you're only going to use the in clause with subtypes anyway then you can just change the signature to have that clause on the subtypes:

abstract class Interface<T extends Interface<T>> {
  void method(num value) {}
}

class Concrete extends Interface<Concrete> {
  void method(num value in Concrete);
  static final pi = 3.14;
}

class Another extends Interface<Another> {
  void method(num value in Another);
  static final theAnswer = 42;
}

void main() {
  final concrete = Concrete();
  concrete.method(.pi);

  final another = Another();
  another.method(.theAnswer);
}

Note that the additional declarations of method are abstract (we're only changing the signature, not the implementation). If you need to change the implementation then those declarations can just be concrete.

We could consider making in T in Interface.method mean exactly this (such that the abstract declarations of method in Concrete and in Another would be implicitly induced by the compiler and analyzer).

abstract class Interface<T extends Interface<T>> {
  void method(num value in T) {}
}

class Concrete extends Interface<Concrete> {
  // Implicitly induced: `void method(num value in Concrete);`
  static final pi = 3.14;
}

class Another extends Interface<Another> {
  // Implicitly induced: `void method(num value in Another);`
  static final theAnswer = 42;
}

void main() {
  final concrete = Concrete();
  concrete.method(.pi);

  final another = Another();
  another.method(.theAnswer);

  // Forget all about `Concrete` by using a super-bounded type.
  Interface<Object?> interface = concrete;
  interface.method(-1.75); // `.pi` and `.theAnswer` are both errors.
}

This could be a quite interesting generalization of the parameter default scopes feature!

However, it (probably) implies that in T would be useless in the declaration of Interface because we have no idea where to look for the static members and/or constructors that we'd like to provide access to using the dot shorthand notation. We can only support dot shorthands when T has been "frozen" in some subtype using extends Interface<Something> where Something denotes a declaration that has static members and/or constructors. With invocations of method on that subtype we would then look up dot shorthands in Something. In the example using interface.method(...) above, we do not know where to look. So every dot shorthand (which isn't based on the context type num) is a compile-time error.

However, we could even allow in T in the formal parameter of Interface.method to be treated as in Concrete in the case where the receiver type is Interface<Concrete> (and similarly for the type Interface<S> for any S that denotes a declaration that has static members and/or constructors). This would allow the following:

void main() {
  ...
  Interface<Concrete> interface = concrete;
  interface.method(.pi);
}

You could then say that in T is considered to be an "abstract" namespace for dot shorthands when the receiver has type Interface<Object?> or a similar type that doesn't determine a namespace for the lookups. Every situation which implies that T has a value which is a declaration D that has static members and/or constructors will then make it "concrete", such that we can start using dot shorthands based on D.

We're cheating, though, because the actual type of such an object could very well be a type that has Interface<SubtypeOfConcrete> as a superinterface, in which case it would seem more consistent to say that (1) we don't know where to find the static members/constructors, so it's an error, or (2) we should look up the dot shorthands (like .pi) in the actual run-time value of T.

The latter would require a feature like 'more capable type objects', #4200, because that's a feature that allows us to use static members and constructors based on the actual value of a type parameter.

This approach only makes sense if we have multiple static members or constructors with the same name, because the whole point is that we can have one call site which is capable of calling one or the other, and it isn't known until run time which one it is. For example:

abstract class HasAnswer {
  num get theAnswer;
}

abstract class Interface<T extends Interface<T>> {
  void method(num value in T) => print(value);
}

class Concrete extends Interface<Concrete> static implements HasAnswer {
  static final theAnswer = 3.14;
}

class Another extends Interface<Another> static Implements HasAnswer {
  static final theAnswer = 42;
}

void foo<T extends Interface<T> static extends HasAnswer>(T t) {
  t.method(.theAnswer); // Means `T.theAnswer`, which uses #4200.
}

void main() {
  foo(Concrete()); // Prints '3.14'.
  foo(Another()); // Prints '42'.
}

This is yet another generalization of the parameter default scope mechanism, based on the 'more capable type objects' in #4200.

In the basic dot shorthand feature, we can pass .name to a parameter of type T (which must then denote a specific declaration, e.g., a class declaration, which means that T is not a type variable), and it means T.name. Similarly, with parameter default scopes where we can have an in T clause on the parameter (same constraints on T), in which case .name means T.name (but T is now decoupled from the parameter type).

In the approach where we use 'more capable type objects' we could again obtain the T from the context type or from an in T clause, but in this case (for this mechanism to kick in) T would be known to have a static implements relation to a given type I, and we can then use members of I using dot shorthands.

All in all, I think it's fair to say that we can find some ways to generalize the dot shorthands such that they can be used with types that do not directly denote a declaration. However, it does get somewhat hairy. ;-)