S3 dispatch for S4 subclasses only works if variable "extends" is accessible from global environment

Thanks for looking into it, your approach sounds good to me. See also
R_has_methods_attached()
(https://github.com/wch/r-source/blob/42ecf5f492a005f5398cbb4c9becd4aa5af9d05c/src/main/objects.c#L258-L265).

I'm fine with Rscript not loading "methods", as long as everything works
properly with "methods" loaded but not attached.


-Kirill



On 19.04.2016 04:10, Michael Lawrence wrote:

Right, the methods package is not attached by default when running R
with Rscript. We should probably remove that special case, as it
mostly just leads to confusion, but that won't happen immediately.

For now, the S4_extends() should probably throw an error when the
methods namespace is not loaded. And the check should be changed to
directly check whether R_MethodsNamespace has been set to something
other than the default (R_GlobalEnv). Agreed?

On Mon, Apr 18, 2016 at 4:35 PM, Kirill M?ller
<kirill.mueller at ivt.baug.ethz.ch> wrote:

Scenario: An S3 method is declared for an S4 base class but called for an
instance of a derived class.

Steps to reproduce:

Rscript -e "test <- function(x) UseMethod('test', x); test.Matrix <-
function(x) 'Hi'; MatrixDispatchTest::test(Matrix::Matrix())"

Error in UseMethod("test", x) :
   no applicable method for 'test' applied to an object of class
"lsyMatrix"
Calls: <Anonymous>
1: MatrixDispatchTest::test(Matrix::Matrix())

Rscript -e "extends <- 42; test <- function(x) UseMethod('test', x);
test.Matrix <- function(x) 'Hi';
MatrixDispatchTest::test(Matrix::Matrix())"

[1] "Hi"

To me, it looks like a sanity check in line 655 of src/main/attrib.c is
making wrong assumptions, but there might be other reasons.

(https://github.com/wch/r-source/blob/780021752eb83a71e2198019acf069ba8741103b/src/main/attrib.c#L655-L656)

Same behavior in R 3.2.4, R 3.2.5 and R-devel r70420.


Best regards

Kirill