Dispatching on "missing"

[lawremi]

S4 lets us dispatch on "missing", i.e., an argument in the signature not being provided. This is a somewhat useful feature for multiple dispatch. Should/could we support that in R7?

[hadley]

Is there an advantage over providing NULL as a default value (which makes it clear that the argument is indeed optional) and then dispatching on that?

[lawremi]

One example is [(), where x[i,] is not the same as x[i,NULL]. Another is IRanges::findOverlaps(pattern, subject), where omitting subject computes overlaps of pattern with itself. Overlaps with NULL might intuitively have a different semantic (no overlaps). Making subject default to pattern does not work either, because the missing method has arguments past ... that only make sense in a self comparison. There are of course work arounds, but dispatching on missing has proven convenient.

[mmaechler]

I strongly agree with @lawremi : R like optional arguments make sense a lot. Indexing (and subsetting, i.e., the [<- generic) semantics in S and R need this distinction (and the method implementation in some of my cases in Matrix then may additionally use nargs() to distinguish cases.. because there you also need to distinguish x[i,] from x[i].

[hadley]

I think this is a confusing API (since you can't tell from the function formals which arguments are required and which are optional), but it seems like we'll need it for backward compatibility.

[lawremi]

Having this information gives us the opportunity to automatically clarify argument handling in the generic documentation, so that we do not have to depend on the formals and the presence of a default value to communicate optionality.

[hadley]

Any syntax preferences for this? We could use "missing" or we could make it a special symbol (e.g. missing_arg).

(It looks like method dispatch for missing arguments is already implemented, but method registration is not.)

[lawremi]

For consistency, what about making something like missing_arg be a class object, representing a dummy virtual, final class? That might even simplify the implementation?

[hadley]

Yeah, sorry that's what I meant by "special symbol". Does missing_arg seem like the right name? Another option would maybe be r7_missing? We could also make missing work, although it might be a bit confusing since we'd just be using the existing function as a sentinel (like we do for character, integer etc, but there the connection is much closer).

[lawremi]

missing_arg seems good enough for now at least.

[hadley]

Also need to implement/test dispatch on "ANY", which I think might have gotten muddled up with dispatch on NULL.

[hadley]

What do you think of "absent" instead of "missing"? It avoids some of the confusion associated with NA.

And I think maybe "_class" works better than arg, since we'll want to write new_property("foo", class = any_class).

[hadley]

I've gone with missing_class and any_class for now; if needed we can revisit when we look holistically at all the built in class names.