Who said you can't? :) This actually works in Dart:
Dog? dog;
bool isDog = dog is Dog;
if (isDog) {
dog.bark();
}
i.e. boolean variables which serve as witnesses for type judgements are integrated into promotion machinery.
I do agree with you that
1. In general there is no limit to developers' expectations with respect to promotion, but there is a limit to what compiler can reasonably achieve. At some point rules become too complex for developers to understand and rely upon.
2. Creating local variables when you need to refine the type is conceptually simpler and more explicit, both for language designers, language developers and language users.
I don't hate excessive typing, especially where it helps to keep things simple and readable - so I would not be against a language that does not do any control flow based variable type promotion. Alas many developers don't share this view of the world and are vehemently opposed to typing anything they believe compiler can already infer.
Kotlin can't do this, which the language I have more experience with. It's good that Dart does a little better in that regard. And, I think we do agree, I just don't really like the viewpoint of doing this, because I feel like it's not really general enough.
Who said you can't? :) This actually works in Dart:
i.e. boolean variables which serve as witnesses for type judgements are integrated into promotion machinery.I do agree with you that
1. In general there is no limit to developers' expectations with respect to promotion, but there is a limit to what compiler can reasonably achieve. At some point rules become too complex for developers to understand and rely upon. 2. Creating local variables when you need to refine the type is conceptually simpler and more explicit, both for language designers, language developers and language users.
I don't hate excessive typing, especially where it helps to keep things simple and readable - so I would not be against a language that does not do any control flow based variable type promotion. Alas many developers don't share this view of the world and are vehemently opposed to typing anything they believe compiler can already infer.
It's all a matter of personal taste in the end.