[Rd] Inherited Methods in r-devel (for package maintainers mainly)

[John Chambers]

The revisions below have been re-committed (r47803), and appear to be 
compatible with the current Matrix package ('0.999375-19').  Thanks to 
Martin Maechler for help with Matrix.

John Chambers wrote:
> A recently committed revison of R-devel (47740) has introduced a new 
> mechanism for ordering superclasses consistently, with related changes 
> for selecting inherited methods.
>
> As part of the process, a function testInheritedMethods has been  
> introduced that examines method selection for the relevant subclasses 
> and reports ambiguities.
>
> Maintainers of packages that have methods involving multiple arguments 
> are encouraged to run testInheritedMethods for the relevant generic 
> functions (e.g., the binary operators).  The new method selection is 
> unambiguous for single-argument selection.
>
> It's preferable  to find such ambiguities during package development 
> or revision, rather than having users encounter ambiguous method 
> selection later on.  In that spirit, ambiguous method selection is no 
> longer a warning, just a message.
>
> The new mechanism for class ordering and method selection is described 
> in a draft paper at 
> http://stat.stanford.edu/~jmc4/classInheritance.pdf (later likely to 
> be part of a submission to the R Journal).
>
> John
>
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