Problem calling $ inside a $ method
In x$name, 'name' is supposed to be a symbol, unevaluated, or a literal character string (but not a character vector).
f <- list(bob=1, alice=2) `$`(f, "bob")
[1] 1
nm <- "bob" `$`(f, nm)
NULL
illustrates your misunderstanding.
Note that the S4 generic for $ in fact is
"$" = function(x, name)
{
name <- as.character(substitute(name))
standardGeneric("$")
}
so within S4 dispatch you will have a character vector for 'name'.
You need substitute() to get back to a literal character string.
I think the latter should be documented somewhere, but like many details
of S4, it is not AFAIK.
On Mon, 7 May 2007, Seth Falcon wrote:
Hello,
I wonder if this will make it through the spam filters given the
subject line.
I'm seeing the following when trying to call a dollar method inside of
a dollar method.
setClass("Foo", representation(d="list"))
[1] "Foo"
f <- new("Foo", d=list(bob=1, alice=2))
## We can call dollar at this level and it works as expected
`$`(f, "bo")
[1] 1
`$`(f, "al")
[1] 2
## So set a method on Foo that does this
setMethod("$", "Foo", function(x, name) `$`(x at d, name))
[1] "$"
## But it doesn't work. Why?
f$bo
NULL
f$al
NULL
## Here is a hackish workaround.
setMethod("$", "Foo", function(x, name)
eval(substitute(x at d$FOO, list(FOO=name))))
[1] "$"
f$bo
[1] 1
f$al
[1] 2
Other suggestions for workarounds? Is this a bug?
+ seth
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595