[Rd] Problem calling $ inside a $ method

[Prof Brian Ripley]

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