Ability to return and throw from ?? expression.

[BirjuVachhani]

I am familiar with Elvis operator ?: Kotlin. The dart equivalent of this would be ?? in most cases. While ?? works great, there is no way to return from it or throw in the same line of the expression without writing a bit of boilerplate code.

Example:

Dart as of now,

Version checkHasVersion(Map<String, dynamic> json) {
  final versionString = json['current'];
  if (versionString == null) throw Exception('Not found'); // extra line for null check.

  return Version.parse(versionString);
}

With ability to throw from ?? expression

Version checkHasVersion(Map<String, dynamic> json) {
  final versionString = json['current'] ?? throw Exception('Not found');

  return Version.parse(versionString);
}

Same for returning values:

Version checkHasVersion(Map<String, dynamic> json) {
  final String versionString = json['current'] ?? return Version(0, 0, 0);

  return Version.parse(versionString);
}

Kotlin version:

fun test(map: Map<String, Int>): Int {
    val version: Int = map["version"] ?: return -1
    return version + 2
}

[lrhn]

See #3052 for return.

You can use throw, it just needs parentheses:

  final versionString = json['current'] ?? (throw Exception('Not found'));

Closing as duplicate of #2025.

[BirjuVachhani]

I see. Thanks for the explanation and links to other related issues @lrhn.

However, I am wondering why these parenthesis are needed? It seems less intuitive.

[lrhn]

The reason the parentheses are needed is that someone argued, back whenthrow was made an expression, that it should bind weakly to its operand, which meant that it became a special <expression> instead of a prefix-operator.
That was me, I was wrong. Sorry.

That meant that there are places in the grammar it cannot occur. We have actually fixed that for conditional expressions. According to the spec grammar, you should be able to have a throw as the operand of a conditional expression, because <expressionWithoutCascade> can now be a throw. So no parentheses needed.

[BirjuVachhani]

Ah, I see. I didn't know that conditional expressions could have it w/o {}. I just tried it and it works as expected and I like it. Thanks!

Would it be possible to have the same behavior for ?? too? no parentheses? Would be great actually! I hate whenever I have to wrap something in parenthesis specially when it affects the readability of the code.