[design] when to validate objects

[wlandau]

Thanks for drafting the design document, @lawremi.

Early on, you wrote:

Initializing an instance of a class will:

1. Create the prototype, by either adding the class label to the parent instance or constructing an empty S4SXP,
2. Merge any given property values, and
3. Validate and return the result.

I like that we have a dedicated place for validation, but perhaps we should touch base about when we want it to happen. Just concerned about the potential overhead of validating on every initialization.

[mmaechler]

Good point. Has been quite relevant for S4, too.
I don't know the pre-history, but IIRC, it was important that you could incrementally build an object in a way that validation would not yet happen during the build up.