Re: [Rd] Problem calling $ inside a $ method

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Tue, 08 May 2007 07:25:43 +0100 (BST)

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

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