The subject heading is correct if referring to exportMethods() and
importMethodsFrom(). They refer to the methods tables, not the generic
functions, whatever the extensions manual says.
Looking into the code of namespaceImportMethods() will illustrate this.
It just deals with lists of method tables obtained from .getGenerics()
which in spite of its name also only looks for the method table metadata
objects.
As I vaguely recall, there was some concern at one time about having
extra copies of the generic version of the function.
The fundamental problem is that creating methods for unique(), say, does
not change the way calls to base::unique() work. Therefore, all
packages that want to use methods have to ensure that a generic version
of unique gets in between. Primitive functions are an exception
because the method dispatch is in the C code and has a rule for checking
when given an S4 object. There is no corresponding provision for
evaluating a call to a regular function.
If the importing package has a setGeneric() for the relevant function
then its own namespace has the generic version of the function. (That
is a workaround, but I inferred that was what you were trying to avoid.)
Fixes seem possible, but some care is needed. If exportMethods
automatically exported the generic function, it really is no different
from export() for that function.
namespaceImportMethods() could try to supply the generic function if it
is not already present. If it does not find the generic in the
namespace being imported, it would essentially have to call
setGeneric(), assuming the non-generic function exists in the specified
package (e.g., in base for unique()).
Comments?
John
On 12/16/11 6:16 AM, Martin Morgan wrote:
tar of Pkgs A, B, C attached. Martin
On 12/15/2011 03:34 PM, Martin Morgan wrote:
In
> R.version.string
[1] "R Under development (unstable) (2011-12-15 r57901)"
section 1.6.6 of 'Writing R Extensions' says
Note that exporting methods on a generic in the namespace will
also export the generic, and exporting a generic in the
namespace will also export its methods.
and
Note that importMethodsFrom will also import any generics defined in
the namespace on those methods
However, if PkgA promotes 'unique' to a generic and exports that
DESCRIPTION:
Imports: methods
R/f.R:
setGeneric("unique")
NAMESPACE:
export(unique)
and PkgB creates and exports a method on unique
DESCRIPTION
Imports: methods, PkgA
R/f.R:
setClass("B", representation(b="numeric"))
setMethod(unique, "B",
function(x, incomparables=FALSE, ...) unique(x@b))
NAMESPACE:
importFrom(PkgA, unique)
exportClasses(B)
exportMethods(unique)
and PkgC wants to import PkgB's classes and methods
DESCRIPTION
Imports: methods, PkgB
R/f.R
cunique <- function(x) unique(x)
NAMESPACE
importMethodsFrom(PkgB, unique)
export(cunique)
then
(a) the 'unique' generic is not available to the user of PkgB
> library(PkgB)
> unique(new("B", b=1:5))
Error in unique.default(new("B", b = 1:5)) :
unique() applies only to vectors
and (b) the generic has not been imported to PkgC's namespace
> cunique(new("B", b=1:5))
Error in unique.default(b) : unique() applies only to vectors
A workaround is for PkgB to also export(unique), and for PkgC to also
importFrom(PkgA, unique), but is this the intention?
This is arising from Bioconductor efforts to place commonly promoted
functions and S3 classes into a single package, to avoid conflicts when
the same function is promoted independently by several packages.
Martin
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