Language idea: Generators (yield fn, yield ret, yield break)

[alexrp]

The keyword is already reserved.

Something like:

yield fn range(x, y) {
    if y >= x {
        yield break;
    };
    let mut i = x;
    while i < y {
        yield ret i;
        i = i + 1;
    };
}

• yield fns may use yield ret and yield break; normal fns may not.
• yield fns must have at least one yield ret or yield break expression.
• yield fns may not use raise expressions, normal ret expressions, and error-propagating calls.
• yield fns do not have an implicit return value like normal fns.
• yield fn is mutually exclusive with ext fn and err fn.
• yield fn lambdas are supported.

The transformation into a state machine will happen when the module is loaded by the runtime. If a yield fn passes semantic analysis, it must be transformable.

This is tentatively approved for 2.0.