I raised the questions below a couple of months ago by sending it to the
developers mailing list, but I don't know if it got through since no one
replied to it.
Looking at the [R] distribution, the UseMethod() is used in the following way:
alias <- function(object, ...) UseMethod("alias")
or
barplot <- function(height, ...) UseMethod("barplot")
What I would like to point out here is that there is a first argument
('object' and 'height'). This might give a conflict if some one
do like this
foo.Bar <- function(this, x=0, xidx=0) {
cat("In foo.Bar(): ", this, ", x=", x, ",
xidx=", xidx, ".\n", sep="");
}
foo.default <- function(...) {
str("Error: No default function found!");
}
foo <- function(x, ...) UseMethod("foo");
obj <- "Hello world!";
class(obj) <- "Bar";
foo(obj);
foo(obj, xidx=1);
foo(obj, x=1); # <- PROBLEM HERE!
foo(obj, x=1, xidx=1); # <- PROBLEM HERE!
foo(obj, xidx=1, x=1); # <- PROBLEM HERE!
foo.Bar(obj, x=1);
foo.Bar(obj, x=1, xidx=1);
you will get that all the calls with an 'x=1' will call the default
function. I just reported this as a bug, but as Peter Dalgaard kindly replied,
the problem is that I used the argument 'x' in the defintion of the
generic foo() function. Obvious, but I never saw since I kept all the foo()
functions in different files.
So to my question, is it correct to write a generic function without a first
argument, e.g.
foo <- function(...) UseMethod("foo");
It is obvious you would like to have '...'. It works for me, but can I
expect it to work in the future? Also, is there are reason for not doing like
this in the [R] core distribution? I think it would be safer and remove some
strange behaviour for programmers "down the stream".
A related question here is, is there a reason why not all methods are defined
generic with a '.default' definition too? For instance, now
'cat' is defined as
cat <- function(..., file="", sep=" ", fill=FALSE,
labels=NULL, append=FALSE) { ... }
If I create a class MyClass with a function cat (very probable), I would have to
create a generic cat() function. Now I would like to distribute my class in a
library/package and I do not have control of what libraries the user will load.
Then I would need to do
if (!exists("cat.default")) {
if (exists("cat"))
cat.default <- cat
else
cat <- function(x, ...) stop("No default method defined!");
}
cat <- function(x, ...) UseMethod("cat");
to make sure that 1) there is a generic cat() function and that possible
previously defined cat() functions are still available. I think this is
"almost" a safe strategy, but I am not sure.
Cheers
Henrik Bengtsson
Dept. of Mathematical Statistics @ Centre for Mathematical Sciences
Lund Institute of Technology/Lund University, Sweden (+2h UTC)
Office: P316, +46 46 222 9611 (phone), +46 46 222 4623 (fax)
hb at maths.lth.se, http://www.maths.lth.se/matstat/staff/hb/
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !) To: r-help-request at
stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
