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Notes from 5/14 Dart language meeting

Posted by Bob Nystrom


Once a week, Lars, Gilad, and Kasper meet to discuss the design of the language. For your delight and edification, here are my notes from this week's meeting:

var and Dynamic

We see people using Dynamic as a type argument even when they don't have to. Also see people trying to use var as a type argument. Gilad said there are very few cases where you actually do need to use Dynamic.

The source of the confusion is that var is used in a place (local variable declaration) where you can also use a type to declare a variable, so it makes people think var is itself a type.

He proposed always requiring var to declare variables to clarify that:

  var int foo = 123;

but doesn't think it will be liked.

Alternately, we could follow users' intuition and allow var to be used like a type:

  var map = new Map<int, var>();

Gilad is worried allowing var anywhere a type can appear may have some unpleasant corner cases we aren't thinking of. He likes the current behavior because he knows it is non-problematic.

No decision was reached.

I asked if when a user declares a variable with var are is the intent they are stating that they don't care what the variable's type is, or that they do care and want it to be dynamic? Gilad said from the language's point of view, it doesn't matter because there is no difference.

Re-export

Agreed to have an "export" parameter in #import directives. So to re-export, you do:

  #import('somelib.dart', export: true);

If you want to use a library internally but only re-export a subset of its names, you would do:

  #import('somelib.dart');
  #import('somelib.dart', show: ['foo', 'bar'], export: true);

One consideration is how this works with prefixes. If you re-export something you imported with a prefix, does the prefix compound?

The current answer to keep the semantics restricted is "no". Prefixes are for using a name internally, and do not affect how it gets exported.

Cast syntax

Our hangout flaked out and Gilad dropped off for a few minutes. In that interim, we briefly discussed an explicit cast syntax.

Kasper pointed out that if you want to support an optional second type system that doesn't allow implicit downcasts, you'll need some kind of cast syntax. Lars says "as" is probably a good idea.

Statics on interfaces

Lars does not like code appearing in interfaces. Kasper says default classes are confusing for people, and that in large interfaces, people don't see that the constructors are there.

We talked briefly about constructors in interfaces just working like factory constructors so that you wouldn't need "default" to do the delegation.

Lars had to leave at this point. I don't think there was a firm decision, but it seems like we do not intend to support static methods defined directly in interfaces but may support them by delegating to the default class.

Directives

Kasper mentioned that we may want to refresh the directive syntax at some point. Users don't seem to like it. Gilad favors dedicated syntax for imports.

Cheers!

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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…