Wildcard variables

[munificent]

Pattern matching brought a new way to declare variables. Inside patterns, any variable whose name is _ is considered a "wildcard". It behaves like a variable syntactically, but doesn't actually create a variable with that name. That means you can use _ multiple times in a pattern without a name collision.

That leads to an irregularity:

var (_, _) = (1, 2); // OK.

var _ = 1;
var _ = 2; // Error. Already a variable in scope with this name.

Also, it's annoying that _ binds a name. When you have a lambda with more than one parameter that you don't use, you end up having to do:

takesCallback((_, __, ___) { ... });

I have a proposal to fix both by saying that local variables and parameters named _ are also non-binding. This is the tracking issue for that proposal.

[Mike278]

I always love seeing those usage-pattern breakdowns across huge corpuses of Dart code!

From the proposal:

From skimming some of the examples, it does look like some users sometimes name lambda parameters _ and then use the parameter in the body.

As one of those users I just wanted to add that my two use cases for doing this are to workaround #8 (unless there's a better issue to reference?)

xs.map((_) => _.isEven);
// vs
xs.map(.isEven); // strawman syntax

and #3001:

users.map((_) {
  final (user, selected) = _;
  ...
})
// vs
users.map((user, selected) {
  ...
})