Extension Methods

[justinfagnani]

Besides all the usual arguments for extension methods, they would help tremendously with filling in gaps in core libraries and reduce the pressure to include every feature, allowing us to keep the core libraries slim.

Extension methods in Dart should be type-based, and scoped to a library, so that only libraries that import the extensions can call them.

[justinfagnani]

One particularly useful case for extension methods is with unittest matchers. Right now the expect syntax is:

expect(value, matcher);

While most dynamic languages have unittest libraries that use a fluent syntax:

expect(value).matcher();

We could use noSuchMethod and some kind of registration to provide this, but that makes static analysis impossible. Extension methods would enable this while still being statically analyzable:

import 'package:my_matchers/my_matchers.dart';

main() {
  expect(value).myMatcher();
}

[justinfagnani]

Closing as deuplicate of issue #13

---

Added Duplicate label.
Marked as being merged into #13.

[jmesserly]

@justin, the bug you merged this with was closed because it specifically asks for C# extension methods. Those won't work in Dart: they dispatch on the static type.

However, similar ideas can work for dynamic languages:
http://wiki.ecmascript.org/doku.php?id=strawman:scoped_object_extensions

It might be better to open a different bug for scoped object extensions.

[justinfagnani]

I assumed that Gilad didn't close the issue just because C# uses the static type. He says:

"Some of these mechanisms depend on static typing (as in C# or Haskell's type classes) some do not (class boxes, selector namespaces)"

And of course, the ES proposal is linked in that bug too. I'll talk to Gilad and see if I should reopen.

[jmesserly]

See the comment that WontFixed it:

C# extension methods rely on mandatory static typing. That is not an option in Dart.
LINQ like mechanisms can be added to a language by other means. We might consider something in this space down the line.
Status: WontFix

[jmesserly]

I opened it myself, see https://code.google.com/p/dart/issues/detail?id=9991