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Notes from the June 10 Dart language design meeting

The incomporable Bob Nystrom fills us in on the language design discussions taking place amongst Dart engineers. Here are his notes from the June 10th language meeting:

Here's my notes from this week's (short) meeting:

Name collisions with dart: imports

[A customer recently ran into an issue where a new type appeared in dart:html whose name collided with a name they were importing from elsewhere. This broke their app.]

Gilad brought up this issue. Pete's suggestion is if there's a collision between a "dart:" import and another library, the other library wins. That way, we can add stuff to "dart:" libraries without breaking people. I explained some more details here.

Lars asked if there are any negative consequences from that suggestion and when it needs to be fixed. Kasper suggested we start making it a warning [instead of an error?] now.

Ivan asked how JS handles this. Kasper said DOM names are usually higher up on the prototype chain so you get similar shadowing behavior there to what we're suggesting for Dart now.

Type guards

Folks in Aarhus had a discussion about type guards. When you do "if (x is Foo)" in the then branch can you statically tell x is of type Foo? The conclusion they came to is that if locals aren't final, the rules are so complicated it's not worth doing. With mutable locals, you get lots of problems with closures that assign to that local.

For finals, though, they are fine with inferring the type here as long as it doesn't make the rules too weird.

I asked about parameters which are non-final by default. Kasper says you can mark those final.

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…