First, by "doesn't work" you mean the printed output. The value
returned is the same.
Second, the problem is not a general one with S4 methods but with
primitive functions. To see this, define a real function with a similar
method:
> setGeneric("foo", function(e1, e2)
standardGeneric("foo"))
[1] "foo"
> setMethod("foo", "num", function(e1, e2) {
cat("Computing\n", deparse(substitute(e1)), "+",
deparse(substitute(e2)),
"\n")
e1 at x + e2@ x
})
[1] "foo"
> foo(a,b)
Computing
a + b
[1] 1.8
The problem with primitives, such as `+`, is that they aren't called in
the way functions are normally called. If my understanding is correct,
substitute() with one argument uses the "promise" objects
corresponding
to the formal arguments in order to extract the unevaluated expression.
There are no such things with primitives.
I think you need to use a different function for whatever you really wanted.
Franck Arnaud wrote:> Hi all,
> I don't understand why this does not what I expect :
>
> ## code start here ##############
> setClass("num",representation(x="numeric"))
>
> num<-function(x) new("num",x=x)
>
> add<-function(e1,e2) {
> cat("Computing
>
",deparse(substitute(e1)),"+",deparse(substitute(e2)),"\n")
> e1 at x+e2@x
> }
>
> setMethod("+","num",function(e1,e2) {
> cat("Computing
>
",deparse(substitute(e1)),"+",deparse(substitute(e2)),"\n")
> e1 at x+e2@x
> })
>
>
> a<-num(3.2)
> b<-num(-1.4)
>
> add(a,b)
> a+b
> ## code ends here ##############
>
> a+b does not work : I would like that add(a,b) and a+b give the exact same
> result
> I've seen a post on R-devel, but the answer seemed not to apply here.
> I've tried to use deparse(substitute(e1,sys.frame (-1))) and
> deparse(substitute(e1,sys.frame(-2))) (as it was advised by GG in january
> 2006). But it did not work.
>
> Therefore, i'm looking for 1) an explanation for this phenomenon (link
to a
> doc, anything) and/or 2) a way to do what i want, if it is possible.
>
> Thanks a lot
>
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>
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