2 setGeneric's, same name, different method signatures
Martin, fantastic. thank you *very* much! that clears lots of things up for me. (for the record: i think that setGeneric overwriting a previous is more surprising -- thus violating the principle of least surprise -- than one function overwriting a previous, in that we think of (or, one way to think of) OOP is that, after finagling to have no clashes on *class* names, we should not worry about other than intra-class name clashes. as i said, for the record.) thanks again. cheers, Greg ----
Martin Morgan <mtmorgan at fhcrc.org> wrote:
Hi Greg --
this setGeneric is over-writing the first, as would
f = function() "first"
f = function() "second"
f() # "second"
If you'd like to dispatch on a single argument, then
setGeneric("one", function(x, ...) standardGeneric("one"))
setMethod("one", "A", function(x, ...) "A-method")
setMetohd("one", "B", function(x, y, ...) "B-method")
The '...' in the generic allow you to add arguments that are 'picked
off' by methods. The user could provide any value for y, not only an
object of class "B".
If you'd like to dispatch sometimes on two arguments then
setGeneric("two", function(x, y, ...) standardGeneric("two"))
setMethod("two", c("A", "ANY"), function(x, y, ...) "A,ANY-method")
setMethod("two", c("B", "B"), function(x, y, ...) "B,B-method")
then two(new("A")), two(new("A"), new("A")) and two(new("A"),
new("B")) end up in A,ANY,two-method while two(new("B"), new("B"))
ends up in "B,B,two-method". Other combinations are errors. One might
instead not define A,ANY but instead
setMethod("two", c("A", "missing"), function(x, y, ...) "A,missing-method")
and then two(new("A"), new("A")) would be an error.
Multiple dispatch is complicated, and perhaps best to avoid if possible.
It's possible to write a generic and methods that dispatch on '...',
with the requirement that all classes are the same; this in the spirit
of comparing two B's and returning the smaller; see ?dotsMethods
though again this is not a trivial use case.
Hope that helps enough.
Martin