Improper `this` usage in primary constructors

[gryznar]

In spec of primary constructors: working/2364 - primary constructors/feature-specification.md, this is used in constructor declaration. I see it as improper, as this in Dart refers to class instance which is the product of construction. IMO much more suitable will be to use new keyword

@eernstg

[eernstg]

We're considering to use new to specify that a given declaration is a (non-declaring) constructor declaration, in which case we need to use some other syntax to indicate that a declaration is a declaring constructor.

// Today.
class LoooooooooooooooooooongClassName {
  final int x;
  final int y;
  const LoooooooooooooooooooongClassName(int i, this.y) : x = i + 1;
  LoooooooooooooooooooongClassName.named({String s}): x = s.length, y = 1;
}

// With the small feature that allows us to use `new`.
class LoooooooooooooooooooongClassName {
  final int x;
  final int y;
  const new(int i, this.y) : x = i + 1;
  new.named({String s}): x = s.length, y = 1;
}

// Using a declaring constructor in the body.
class LoooooooooooooooooooongClassName {
  final int x;
  const this(int i, final int y) : x = i + 1;
  new.named({String s}): x = s.length, y = 1;
}

// Using a declaring constructor in the header.
class const LoooooooooooooooooooongClassName(int i, final int y) {
  final int x = i + 1;
  new.named({String s}): this(s.length, 1);
}

So it's a deliberate decision to reserve new for the ordinary (non-declaring) constructors. The use of this to indicate that a constructor in the body is a declaring constructor is perhaps not self-explanatory, but I think we'll get used to it quickly.

[eernstg]

I'll add the 'primary-constructors' label, which is definitely relevant. I'll remove the 'bug' label, though, because the use of this rather than new was a decision, not an oversight.

[gryznar]

I agree, this is not strictly a bug. new sounds just better and much proper

[munificent]

I agree that new is a more natural syntax for constructors. However, this looks pretty strange to me:

class C {
  const new();
}

Back when Dart had a mandatory new keyword for calling a constructor, that keyword was the antonym of const. So seeing both together looks confusing, like seeing a variable declared final var x.

However, we made that keyword optional years ago and I suspect most Dart users have never written new at a constructor call and don't think of it as the "opposite of const". If that's true, then using new for declaring constructors even when paired with const seems reasonable to me.

I wonder if this is something we can get some user data on.

[gryznar]

I see using new Foo() as artifact from the past, so it may be strange only for those who get used to using it. But is this a meaningful group? I don't think so

[cedvdb]

However, we made that keyword optional years ago and I suspect most Dart users have never written new

This doesn't look weird to me at all, it's natural and 25% shorter. As a language designer you may have a more intimate relation with the language history.

[lrhn]

FWIW, I don't think const new(); looks weird.
It takes almost no effort to start reading new as declaring a constructor, and then const new is immediately understandable.