Allow stateful functions through static variables

Sometimes it would be cool if functions could have static variables, which would be about the same as an object with fields and implementing a java Functional interface.

So I could do:

value x = foo.filter((y) {
    static variable value done = false;
    if (!done) {
      doit();
      done=true;
    }
    return true;
  });

instead of

variable value done = false;
value x = foo.filter((y) {
    if (!done) {
      doit();
      done=true;
    }
    return true;
  });

This was discussed in gitter; (hopefully correct) summary here:

• @gavinking seemed interested
• @jvasileff was worried that “static” would have the appearance of being shared between all “instances” of the function (as opposed to how it was described by the above “instead of” example where every time the outmost scope is entered a new instance variable is created).
• @FroMage was worried about function transformations and calls in super calls

[zamfofex]

Doesn’t this have the same problems as classes did before constructors were a thing?

Like, the argument of the function (which is in scope) must not be referenced from the static value’s specifier, which is awkward.

[lucaswerkmeister]

A feature like this might be useful, but I don’t see what this has to do with the existing meaning of static, and I don’t think we should copy C’s habit of reusing keywords for wildly different behavior here. If I saw a static value in a function –

class C() {
    void f() {
        static variable Integer i = 0;
        print(++i);
    }
}

C c = C();

– then I would expect one of the following to be allowed:

Integer i = C.f.i;
Integer i = c.f.i;

Which one? No idea. (I suppose that’s the same as @jvasileff’s concern.)