inheritance + check functions

[hadley]

Need equivalent of is()/inherits(), along with a helper that asserts that an object is of a class (extracting code out of the property checkers). Implementation already exists in class_inherits(); just needs a better name.

[hadley]

Other ideas

• instance (Dylan)
• isa (Julia)

[hadley]

Ensure that vector of NA can be coerced to any other vector type.

[lawremi]

Would is/inherits() not work already? I thought we were just using the S3 class vector approach.

[lawremi]

What do you mean by a vector of NA? Isn't the vector type a separate concept from missingness?

[hadley]

You could use inherits() currently, but you'd have to get the vector of class names somehow. I think you want some function where you can do:

foo <- new_class("foo")
x <- foo()

is_a(x, foo)
#> [1] TRUE

is_a(1, foo)
#> [1] FALSE

The NA case, I think, is needed for property checking of base types, since currently we have:

library(R7)
foo <- new_class("foo", properties = list(x = character))
foo(x = NA)
#> Error: <foo> object properties are invalid:
#> - @x must be <character>, not <logical>

^{Created on 2022-03-10 by the reprex package (v2.0.1)}

which I think is a bit unappealing. Maybe that needs to be handled elsewhere, but property type checking currently uses the internal class_inherits(), which is what I would imagine would power is_a().

[lawremi]

Ok, so is_a() would accept the class object instead of the class name, got it.

I'd prefer to handle the NA case as a convenience during construction, while ensuring that properties are always of the correct type, even if every value is NA.