Attribute lazy initialization

[@matejonnet] Looking for a shorter way to initialize an object attribute on a first use, and ability to use immutable attribute instead of variable

variable Foo? foo := null;
Foo getFoo() {
    if (exists f = foo) {
        return f;
    }
    foo := createFoo();
    if (exists f = foo) {
        return f;
    }
    throw;
}

I would like to write this as:

Foo? foo;
Foo getFoo() {
    if (!exists foo) {
        foo = createFoo();
    }
    return foo;
}

[Migrated from ceylon/ceylon-spec#438]

[bjansen]

🎉

[gavinking]

I propose the following syntax:

late Float pi = calculatePi();

Where I’m repurposing the late annotation to mean “initialized lazily” instead of just “initialized later”. There’s no ambiguity here, since currently a late field can’t have an initializer, and the compiler enforces that.

Reasons why I think this is reasonable:

• it seems to match the English meaning of “late”,
• the actual code generation is pretty similar: instead of throwing an exception, we initialize it to the given expression, and
• it seems to me that issue #6721 applies equally to lazy fields as to late fields (they can both be placed in the declaration section).