Considerations for + and other special infix operators

[DavisVaughan]

I think one of the nice things about R7 would be to be able to write + methods which dispatch off both inputs. Like <R7Date> + <R7Duration> = <R7Date>.

I think to do this correctly in all cases we will need some base R changes.

The C code for x + y currently checks to see if there is a + method for either x or y. If there is a method for exactly one of them, or if they both have the same method, then that method is used. Otherwise if there are methods for both and they aren't the same then a fallback internal implementation is used with a warning about "incompatible methods" (See ?groupGeneric).

Practically this means that we can currently make <R7Date> + <R7Duration> work because both of them will share R7_object as a base class. We will need to write a + method for R7_object which then hands off to something that actually handles the double dispatch right (See vctrs:::+.vctrs_vctr and vctrs::vec_arith() for an example of this).

But we can't currently make <Date> + <R7Duration> work, because we'd end up with conflicting +.Date and +.R7_object methods and we'd end up getting the fallback behavior.

^ So figuring out some way to make <Date> + <R7Duration> work might be challenging, but would be a neat feature to promote about R7.

[hadley]

Maybe if either x or y is an R7 class, it automatically uses R7 dispatch?

[DavisVaughan]

I think that might be how it works for S4 too. This seems to try S4 dispatch if either is S4 https://github.com/wch/r-source/blob/430360aa394b02046dd7175c0fc55e347e947f80/src/main/eval.c#L3751-L3767

[hadley]

So implies it will already work if you create an R7 object that doesn't extend an existing base type (typeof(new_class("foo")())). (But obviously we'll also want it to work for R7 objects that do extend base types)

[hadley]

See also #222