operator :: and symbols not in the namespace of a package with a namespace (PR#9279)

On Fri, 6 Oct 2006, Duncan Murdoch wrote:

On 10/6/2006 9:05 AM, thomas.friedrichsmeier at rub.de wrote:

Full_Name: Thomas Friedrichsmeier
Version: 2.4.0
OS: GNU/Linux
Submission from: (NULL) (84.61.116.51)


Since R 2.4.0, operator "::" also returns objects in the package environment, if
the package does not have a namespace. This is a very welcome addition.

Additional wish:
If a package has a namespace, but does not place all symbols in that namespace,
lookup will still fail. For example in package boot (version 1.2-26):

library (boot)
exists ("motor", envir=as.environment ("package:boot"))   # TRUE
getAnywhere ("motor")                                     # found in
package:boot
boot::motor                                               # error not in
namespace

This is as documented, but I think it would be convenient, if boot::motor would
also return the object in this case.

At first I was ready to disagree with you, telling you to use the ":::"
operator for something that is not exported:  but in fact motor is a
dataset, so it is exported from the package by being loaded into the
package environment when the package is loaded.  I agree that the "::"
operator should find it.

It might be a good idea to rethink the decision to have data be
handled specially instead of having data sets be treated as ordinary
exported variables.  There may be a good reason for doing what we do
now but I don't know what it is.  If there really is a good reason to
continue this then maybe we should think about this addition.  It
doesn't feel like the right thing to do but may be harmless.

I think the rule should be:  the "::" operator looks at the symbols that
are publicly available from the package, i.e. boot::motor should
effectively be an abbreviation for

get("motor", pos="package:boot", inherits=FALSE)

after making sure boot is loaded.  I think getExportedValue() is a much
more rarely needed function, so users should need to use it explicitly.

No. :: exists precisely to allow code to access exported variables
without attaching the namespace.  It is the only way I use this feature
and the reason the feature exists.

Best,

luke

Duncan Murdoch

This might be achieved by adding an additional tryCatch in "::": If the package
has a namespace, try getExportedValue (), if that fails, try to get from package
environment instead. E.g.:

"::" <- function (pkg, name)
{
    pkg <- as.character(substitute(pkg))
    name <- as.character(substitute(name))
    ns <- tryCatch(asNamespace(pkg), hasNoNamespaceError = function(e) NULL)
    if (is.null(ns)) {
        pos <- match(paste("package", pkg, sep = ":"), search(),
            0)
        if (pos == 0)
            stop(gettextf(paste("package '%s' has no name space and",
                "is not on the search path"), pkg), domain = NA)
        get(name, pos = pos, inherits = FALSE)
    }
    else tryCatch(getExportedValue(pkg, name), error=function (e) {
                       pos <- match(paste("package", pkg, sep=":"), search(),
0)
                       get(name, pos, inherits=FALSE)
                  })
}