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Dec 10 Language Design Meeting Notes

Dart engineer Bob Nystrom has posted the notes from the December 10th language design meeting. He writes:

Mixins

We're down to arguing about syntax. Lars says, OK, here it is:

    class C extends S with M1, M2 { ... }

Mixins are linearized from right to left. This makes sense since the class body appears last but takes precedence, so we work backwards from that. Gilad has an updated proposal that he'll send out.

There is one corner case. If you want to mixin, you have to have an explicit superclass:

    class C extends Object with M1 { ... }

[Bob: This means that I believe we have the semantics and syntax for mixins nailed down now. Woo!]

throw as expression

They're looking to implement this in dart2js now. Even with the restriction on rethrows, it still feels tricky. Since throw takes an arbitrary expression on the right, including a conditional one, there are some corner cases related to how it binds.

Lars: Is rethrow an expression?
Gilad: It is in the spec, but the implementations don't support it.
Kasper: It is hard to parse. We have to lookahead to tell if we're at the beginning of an expression.
Lars: Let's make both kinds of throws an expression and rename rethrow to use a "rethrow" contextual keyword. It's a rarely used feature. Better?
Gilad: I'll talk to the implementors and work through something.

Metadata in corelib

Brian: There is a faction that says certain metadata constructs need to be in the language spec before they can be put into the core library. Are all tools required to have certain behavior because certain metadata is in the core library? For example, does dart2js have to emit some kind of notification if you use @deprecated?

Gilad: If we want metadata that does something magical then it should be in the spec. If it's just in tools, we shouldn't.

[Bob: I think an example of an annotation with magic behavior is one that says "don't tree-shake this".]

Kasper: If we use them in our libraries, we have to tell people what they mean. So it has to be specified somewhere, even if informally.

Gilad: Sure, but that applies to the library API itself too. You have to document it.

Lars: I would like to minimize the use of annotations in core library. We don't want using them to be part of normal style. Java uses them everywhere.

Configuration-specific code

Lars suggests we could have annotation to put on constants that lets you override its value from the outside from something like a command line argument.

We'll discuss this more after M2.

Cheers!

As always, we invite you to join our Dart mailing list, ask questions on Stack Overflow, or file feature requests on dartbug.com.

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