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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.

Comments

  1. It came up, but my impression from the meeting is that the only area under discussion is adding support for some kind of forwarding syntax.

    This is the way people were forwarding arguments for decades:

    foo([a, b, c]) {
    ...
    }

    bar([x, y, z]) {
    foo(x, y, z);
    }

    Except that it doesn't work in Dart... well it usually does work. But if foo uses the '?a' operator, then it can break.

    I can imagine two ways of adding syntax for forwarding: to the caller or to the callee.

    foo([a, b, c]) {
    ...
    }

    bar([x, y, z]) {
    foo(->x, ->y, ->z);
    }

    Well this sounds crazy to me, because in bar, I can't really know if foo uses the ?check or not. And I shouldn't. Adding forwarding syntax to the caller is likely to result in style rules like "always use the ->forwarding syntax when passing your function's arguments to other functions". Please don't.

    foo([->a, ->b, ->c]) {
    ...
    }

    bar([x, y, z]) {
    foo(x, y, z);
    }

    Meaning that if something was an argument in any caller of foo, then it will be treated in a special way when calling. This sounds intricate, but at least it's self-contained and the outer world doesn't need to care.

    Ultimately, Dart opened a can of worms with adding the ?check. It's like a restricted form of JavaScript's 'undefined' that can only happen implicitly when calling functions with optional parameters. But given the restriction, if a parameter wasn't passed, it actually has two observable values -- 'undefined' and the default value. Which is quite bad when forwarding, so we are debating adding a special syntax for that.

    Don't know about you, but this sounds pretty scary to me. Parameters in Dart are already a delicate subject, and my wild guess is that this ?check operator is one of the less known and less understood features of Dart. And honestly, I can't see a way out.

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