Enhancement: static local variables

[DartBot]

This issue was originally filed by beatgam...@gmail.com

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In C/C++, we can do something like this:

int func() {
    static int val = 0;
    return val++;
}

printf("%d\n", func()); // outputs 0
printf("%d\n", func()); // outputs 1
printf("%d\n", func()); // outputs 2

In C++, for classes we can do the same:

class Thing {
  public:
    int func() {
      static int i = 0;
      return i++;
    }
};

int main() {
  Thing* a = new Thing();
  Thing* b = new Thing();

  printf("%d\n", a->func()); // outputs 0
  printf("%d\n", a->func()); // outputs 1
  printf("%d\n", a->func()); // outputs 2
  printf("%d\n", b->func()); // outputs 3
  printf("%d\n", b->func()); // outputs 4
  printf("%d\n", b->func()); // outputs 5
}

For translating this to JS, we can simulate this with:

function func() {
    if (!func.val) func.val = 0;
    return func.val++;
}

The only functional difference seems to be that func.val is visible outside func. This can also be done with prototypes in the same way.

I propose the syntax be just like the C/C++ syntax in Dart:

func() {
    static int val = 0; // or static var val = 0;
    return val++;
}

The best arguments against this that I could come up with are:

* doesn't exist in Java (weak)

• can get the same behavior with Isolates (issue Add support for generators #36), but without making val public when converting with dart2js

[kasperl]

Added Area-Language, Triaged labels.

[lrhn]

You can also get the same behavior with a global private value that you don't touch anywhere else. The "static local variable" is really just a way to reduce the scope of a global variable, In Dart's "library privacy" setting you pretty much have to trust the library you are in anyway.

So:
  int _funcVal = 0;
  int func() => _funcVal++;
and:
  class Thing {
    static int _funcVal = 0;
    int func() => _funcVal++;
  }

It's not as obvious that _funcVal is only used inside _func(), but it's just as expressive and uses the existing lazy initialization of global/static fields.

If static local fields were added, they would likely just desugar to the same code as above.

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Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by beatgam...@gmail.com

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True, but since this is a compile-time optimization, I couldn't accidentally re-use the same global in multiple places.

[DartBot]

This comment was originally written by esnellman2@gmail.com

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Encapsulation is a core OO principal. Yet the dart style guide:
https://www.dartlang.org/articles/style-guide/

Suggests one move every static field/method into the library. Yet the stated goal of the projected was structured web apps.

Does every function in Dart have a link to the parent environment in dartvm and/or dart2js?
example: var t = (int i) => i+2;

Does the compiler elevate a methods environment until it's body references something in that environment in dartvm and/or dart2js?

Likewise does it inline local variables(treat them like a static const) if they are never written to?

If no answers any of the cases above and these features are not planned or offered as a deployment compiler flag. Then Dart should offer syntactical sugar that lets the developer flag variables/functions as static which is translated at compile time. I want to avoid javascript closure leaks. When i am deep in some code somewhere I don't want to jump around from my current level to the class or library level to read my static helper method. By jumping around I lose structure.

[gbracha]

Set owner to @gbracha.
Added NotPlanned label.