On 06/20/2012 07:25 PM, Hadley Wickham wrote:
Hi all,
Any ideas about this? As far as I can tell it should work - and I
don't understand why it's ok when run outside of a package.
from ?groupGeneric under 'Ops' (of which "+" is one)
used. If different methods are found, there is a warning
about 'incompatible methods': in that case or if no method is
found for either argument the internal method is used.
which doesn't really explain why it works at the command line but not in
a package. S4 methods can be defined on both arguments, so
setClass("A", contains="numeric")
setClass("B", contains="numeric")
setMethod("+", c("A", "A"), function(e1, e2) message("A + A"))
setMethod("+", c("A", "B"), function(e1, e2) message("A + B"))
setMethod("+", c("B", "A"), function(e1, e2) message("B + A"))
setMethod("+", c("B", "B"), function(e1, e2) message("B + B"))
with
exportClasses("A", "B")
exportMethods("+")
and then
> new("A") + new("B")
A + B
NULL
Martin
Hadley
On Wed, Jun 13, 2012 at 7:41 PM, Winston Chang<winstoncha...@gmail.com> wrote:
I'm trying to overload an operator, and I'm running into a strange problem.
It happens when I install and load the package, but not when I simply
source() the code.
I'm defining + for two classes. The R code looks like this:
#' @export
#' @method "+" a
`+.a`<- function (x1, x2) {
message("Running custom + function")
}
#' @export
#' @method "+" b
`+.b`<- `+.a`
In some cases I do a+b, and in other cases, I do b+b. I'm told that the +.a
and +.b functions must be identical to avoid the error about "Incompatible
methods". (In the actual code, the overloaded + function checks the classes
of x1 and x2, and then sends them off to other functions.)
This is the NAMESPACE file:
S3method("+",a)
S3method("+",b)
I've put the code up at https://github.com/wch/badadd.
If I just cut and paste the function definitions to my R session, it works
fine:
x + y
# Running + function
# NULL
However, if I install and load the package, it gives a warning about
incompatible methods, and then seems to fall back to the arithmetic +
operator:
library(badadd)
x + y
# [1] 3
# attr(,"class")
# [1] "a"
# Warning message:
# Incompatible methods ("+.a", "+.b") for "+"
Is this expected behavior? And if so, is there a workaround?
-Winston
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