Logical compound assignment operators &&= and ||=

[eernstg]

Cf. issues dart-lang/sdk#26996, meta, and dart-lang/sdk#26997, analyzer, (there are others, but they should probably be replaced by a single front-end issue), the compound assignment operators &&= and ||= have been in the pipeline for a full specification and an implementation for a long time.

We ought to find a placement for this task in terms of milestones and priorities, or drop it.

[thosakwe]

The following prints out true:

void main() {
  var a = false;
  a |= true;
  print(a);
}

So maybe it's already implemented?

[eernstg]

No, that's a slightly different thing: The statement a |= true uses the operator | on bool, which is a regular operator. So expression evaluation for e1 | e2 is a regular instance method invocation. In particular, e2 will be evaluated even in the case where e1 evaluates to true, and the same property holds for a |= e2 because it's defined in terms of an invocation of |.

This matters in practice, because you can use done ||= doWork() to skip the execution of doWork() in the case where done is already true.

[thosakwe]

Ah, I see.

[marcglasberg]

Hey there, was this abandoned?

If I have this:

onPressed: doSomething,

I'd like to be able to do:

onPressed: doSomething && doSomethingElse || doSomethingElseEntirely,

By defining something like:

extension FunctionExtension on bool Function(String) {
  bool Function(String) operator &&=(bool Function(String) other) =>
      (String value) => this(value) || other(value);

  bool Function(String) ||=(bool Function(String) other) =>
      (String value) => this(value) && other(value);
}

[eernstg]

It is not abandoned, cf. #1077. However, you cannot declare an instance operator for any compound assignment (+=, *=, etc.), and also not for && or || (because evaluation of e1 && e2 is different from a method invocation). But you can do this:

typedef F = bool Function(String);

extension LiftBool on F {
  F operator &(F other) => (s) => this(s) && other(s);
  F operator |(F other) => (s) => this(s) || other(s);
}

bool succeed(String s) { print('Succeed: $s'); return true; }
bool fail(String s) { print('Fail: $s'); return false; }
bool notReached(String s) => throw 'Not reached';

void main() {
  (succeed & succeed | notReached)('Hello');
  (fail & notReached | succeed)('world!');
}