[R] Unexpected result after auto-attaching of the Matrix package

[H B]

Dear  Ivan,

thanks a lot for the detailed analysis. This is excellent  and answers
my question. Unfortunately it is difficult to find any documentation on
this topic.

I now understand this is not a formal library  auto-loading feature.

What I still see as problematic is that in this way one can get two
different results when trying to access the  same object in the same 
way,  without any error or warning message.

Thanks again!

Best regards

Hilmar

On 12.02.25 14:09, Ivan Krylov wrote:
> В Tue, 11 Feb 2025 13:51:48 +0100
> H B via R-help <r-help using r-project.org> пишет:
>
>>   > class(d)
>> [1] "dgCMatrix"
>> attr(,"package")
>> [1] "Matrix"
>>   > dim(d)
>> Loading required package: Matrix
>> NULL
> Here's what happens here:
>
> 1. .Primitive("dim") considers dispatching a dim() method:
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/attrib.c#L1181
> 2. DispatchOrEval() first considers S4 dispatch:
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/eval.c#L4152
> 3. But then R_has_methods(dim) finds out that no S4 methods are
> currently registered for dim() - Matrix not being loaded yet - and S4
> dispatch is abandoned:
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/objects.c#L1578
> 4. Instead, R tries usemethod(...) for S3 dispatch:
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/eval.c#L4215
> 5. UseMethod() wants to find out the S3 class vector for 'd' and calls
> R_data_class2():
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/objects.c#L493
> 6. R_data_class2() sees it's an S4 object and calls
> methods:::.extendsForS3(class(d)), which finally loads Matrix in order
> to find out classes extended by "dgCMatrix":
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/attrib.c#L840
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/attrib.c#L736-L741
> 7. S3 dispatch fails for all those S4 classes or the "default" method,
> so dim(d) returns attr(d, "dim") (which is not set):
> https://github.com/r-devel/r-svn/blob/fa73948a1977da05788499807469e69d5a21bd98/src/main/attrib.c#L1184
>
> Then why does print(d) work without loading Matrix first?
> print.default() internally performs S4 dispatch after Matrix was loaded
> by R_data_class2() on step (6) above.
>
> So while Matrix is not being loaded by accident in your example, things
> are not set up to load the packages containing S4 classes
> automatically. It could become a feature request for DispatchOrEval()
> to ensure that the the package owning the S4 class is loaded before
> trying to dispatch an S4 method, although it could be challenging to
> implement while avoiding additional slowdowns.
>