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Notes from Feb 25th Language Meeting: Function Type Annotations, Optional Parameters and Exported Types


The invaluable Bob Nystrom fills us in on the language design discussions taking place amongst Dart engineers. Here are his notes from the February 25 language meeting:


Here's my notes from last week's meeting. I'll have this week's meeting notes out later this week:

function type annotations

Kasper noticed (or maybe someone noticed and brought it to his attention) that there is a place in Dart where seemingly innocuous type annotations do have a runtime effect. Given implicit closurization and is-checks with function types, you can:

1. Store a reference to a method in a variable.
2. Do an is check on that variable against some function type.

For that is check to perform correctly, it means we have to keep track of the methods type annotations at runtime.

The team discussed a bit about whether or not they want to make any changes around this, but didn't come to any conclusions.

optional parameters

We're entering a mode where changing the language gets increasingly hard. We should be careful.

Dan Grove said then if we're going to pull stuff out, we should do it soon. One thing people worry about is parameter syntax with optional parameters and forwarding.

Lars has some ideas for forwarding and will write up a proposal so we have something to discuss.

exported types

I asked if a library B exports Foo from library A and some library C imports both, are those Foos the same type?

I wasn't clear in my question, but after later verifying it myself, it seems Dart does do what I want here:

    // main.dart
    import 'foo.dart' as foo;
    import 'bar.dart' as bar;

    methodTakesFoo(foo.Baz baz) => print("ok");
    methodTakesBar(bar.Baz baz) => print("ok");

    main() {
      var fromFoo = new foo.Baz();
      var fromBar = new bar.Baz();
      
      print('foo.Baz is foo.Baz = ${fromFoo is foo.Baz}');
      print('foo.Baz is bar.Baz = ${fromFoo is bar.Baz}');
      print('bar.Baz is foo.Baz = ${fromBar is foo.Baz}');
      print('bar.Baz is bar.Baz = ${fromBar is bar.Baz}');
      methodTakesFoo(fromFoo);
      methodTakesFoo(fromBar);
      methodTakesBar(fromFoo);
      methodTakesBar(fromBar);
    }
    
    // foo.dart
    library foo;
    
    class Baz {}

    // bar.dart
    library bar;
    export 'foo.dart'; 
 On both the VM (in checked mode) and dart2js, this prints four "true"s and four "ok"s.

And as always, view the changelog for the full list of changes, and to get started with the Editor, see our tutorial.

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Const, Static, Final, Oh my!

Posted by Seth Ladd

(This is an "oldie but a goodie" misc@dartlang.org post originally written by Bob Nystrom. It is being posted here as the explanations still ring true.)

Bob writes:


"static", "final", and "const" mean entirely distinct things in Dart:

"static" means a member is available on the class itself instead of on instances of the class. That's all it means, and it isn't used for anything else. static modifies *members*.

"final" means single-assignment: a final variable or field *must* have an initializer. Once assigned a value, a final variable's value cannot be changed. final modifies *variables*.

"const" has a meaning that's a bit more complex and subtle in Dart. const modifies *values*. You can use it when creating collections, like const [1, 2, 3], and when constructing objects (instead of new) like const Point(2, 3). Here, const means that the object's entire deep state can be determ…