Skip to main content

Dart 1.21: Generic Method Syntax

Dart 1.21 is now available. It introduces support for generic method syntax along with a few popular convenience features. Get it now!

Generic method syntax




Until now, Dart's generic support was limited to classes, such as List<T>. Dart 1.21 introduces syntax allowing type arguments on methods and functions.

   T first<T>(List<T> ts) {
     return ts.length == 0 ? throw new ArgumentError('Empty list!') : ts[0];
   }

Note the return type, T. This enables call sites to preserve type information from arguments. Try to write the same function without a type argument, and you'll see that the return type must be Object – there is no other way we can make it work on all lists. For more examples, check out the Using Generic Methods article. For even more details, the informal specification is the place to go.

We've had generic methods and functions for a while in strong mode. 1.21 introduces support for generic method syntax even without strong mode. Libraries can use this feature and work in both contexts, providing a stepping stone to full generic methods in all Dart code.

However, this feature does not yet provide full runtime support for generic methods outside DDC. In particular, type arguments are not reified (i.e., they have no representation at runtime). Libraries must avoid constructs where the runtime value of type arguments matters, in order to work in both modes. For example, x is T will not work outside strong mode if T is a method type argument. The analyzer marks such cases as errors, so you'll have tool support to avoid them. (Note: If you are using IntelliJ, support is available in WebStorm 2016.3.2 RC / IntelliJ IDEA 2016.3.1 RC, and later.)

Convenience features


1.21 also introduces a couple of much requested convenience features. You can now use = to specify a default value for a named argument.

   enableFlags({bool hidden: false}) { … }

can now be replaced by

   enableFlags({bool hidden = false}) { … }

We’re also introducing access to initializing formals, e.g., x is now in scope in the initializer list such that we can use it to initialize y in this example:

   class C { 
     var x, y; 
     C(this.x): y = x; 
   } 

As with any new language feature, it is critical for package authors to update pubspec.yaml with a latest SDK version.

   name: my_cool_dart_package
   environment:
     sdk: '>=1.21.0 <2.0.0'

This ensures users on older SDKs aren't broken when they update their packages.

For more details on these and other changes in 1.21, see the full changelog. Otherwise, download the latest release, and let us know what you think!

Comments

Popular posts from this blog

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…

Dart 1.12 Released, with Null-Aware Operators and more

Dart 1.12.0 is now released! It contains the new null-aware operators language feature, and enhancements to pub, Observatory, dartdoc, and much more.

Null-aware operators

The new null-aware operators help you reduce the amount of code required to work with references that are potentially null. This feature is a collection of syntactic sugar for traversing (potentially null) object calls, conditionally setting a variable, and evaluating two (potentially null) expressions.

Click or tap the red Run button below to see them in action.

??

  if null operator. `expr1 ?? expr2` evaluates to `expr1` if not `null`, otherwise `expr2`.


??=

  null-aware assignment. `v ??= expr` causes `v` to be assigned `expr` only if `v` is `null`.

x?.p

  null-aware access. `x?.p` evaluates to `x.p` if `x` is not `null`, otherwise evaluates to `null`.

x?.m()

  null-aware method invocation. `x?.m()` invokes `m` only if `x` is not `null`.

Learn more about Dart's null-aware operators in our Language Tour.

.packages fi…

The new AdWords UI uses Dart — we asked why

Google just announced a re-designed AdWords experience. In case you’re not familiar with AdWords: businesses use it to advertise on google.com and partner websites. Advertising makes up majority of Google’s revenue, so when Google decides to completely redo the customer-facing front end to it, it’s a big deal. The Dart team is proud to say that this new front end is built with Dart and Angular 2. Whenever you asked us whether Google is ‘even using Dart for anything,’ this is what we had in mind but couldn’t say aloud. Until now. We asked Joshy Joseph, the primary technical lead on the project, some questions. Joshy is focusing on things like infrastructure, application latency and development velocity, so he’s the right person to ask about Dart.Q: What exactly did we launch on Monday?It’s a complete redesign of the AdWords customer experience that is rolling out slowly as a test to a small initial set of advertisers. The most noticeable thing is probably the Material Design look and f…