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  • subclassof

subclassof

# P T Withington (15 years ago)

In the various classes-as-sugar proposals, is there a way to test if one class is a subclass of another? Is it as simple as:

classa.prototype instanceof classb

?

Should there be a subclassof operator?

# Mark S. Miller (15 years ago)

On Mon, Jan 18, 2010 at 7:32 AM, P T Withington <ptw at pobox.com> wrote:

In the various classes-as-sugar proposals, is there a way to test if one class is a subclass of another? Is it as simple as:

classa.prototype instanceof classb

?

In some of them, yes. For the desugaring presented at <

esdiscuss/2008-August/006941>,

WobblyPoint.prototype instanceof Point

would work for the right reason. In some later proposals, no, because the concept of subclassing is being replaced by the concept of trait combination. Since this gives non-tree-like subtype relationships, it can't accommodate instanceof.

# P T Withington (15 years ago)

On 2010-01-18, at 15:41, Mark S. Miller wrote:

On Mon, Jan 18, 2010 at 7:32 AM, P T Withington <ptw at pobox.com> wrote:

In the various classes-as-sugar proposals, is there a way to test if one class is a subclass of another? Is it as simple as:

classa.prototype instanceof classb

?

In some of them, yes. For the desugaring presented at < esdiscuss/2008-August/006941>,

WobblyPoint.prototype instanceof Point

would work for the right reason.

Yeah, I saw (and liked) that.

In OpenLaszlo we allow mixins (which are flattened into a single-inheritance prototype chain + multiple-inheritance interfaces), and we adopted the AS3 is operator, so we say:

subclass.prototype is superclass

and superclass can be either a class or a mixin. [Sadly this doesn't work in AS3!]

In some later proposals, no, because the concept of subclassing is being replaced by the concept of trait combination. Since this gives non-tree-like subtype relationships, it can't accommodate instanceof.

Should there be a subclassof operator?

Do you have a pointer to the later proposals?

I guess my question stands. Even if I implement classes using traits, I would want to be able to ask if a class had a particular trait. I guess that would have to be part of the class protocol then, not a language primitive.