On Wed, 9 May 2007, S Ellison wrote: > Brian, > >> If we make functions generic, we rely on package writers implementing >> the documented semantics (and that is not easy to check). That was >> deemed to be too easy to get wrong for var(). > > Hard to argue with a considered decision, but the alternative facing > increasing numbers of package developers seems to me to be pretty bad > too ... > > There are two ways a package developer can currently get a function > tailored to their own new class. One is to rely on a generic function to > launch their class-specific instance, and write only the class-specific > instance. That may indeed be hard to check, though I would be inclined > to think that is the package developer's problem, not the core team's. > But it has (as far as I know today ...?) no wider impact.
But it does: it gives the method privileged access, in this case to the stats namespace, even allowing a user to change the default method which namespaces to a very large extent protect against. If var is not generic, we can be sure that all uses within the stats namespace and any namespace that imports it are of stats::var. That is not something to give up lightly. > The other option, with no existing generic, is to mask the original > function by writing a new generic function that respects the original > syntax exactly, and then implement a fun.default that replicates the > original non-generic function's behaviour, hopefully by calling it > directly. As an example, library(circular) masks stats::var, though I'm > fairly sure its not the only case. This has obvious disadvantages, > including potentially system-wide (R-wide at least!) impact and > unfavourable interactions between packages masking each other's generics > and defaults). Just as can happen with methods. > I will use masking if I have to, at least for my own local use where its > only me that suffers if (when?) I get it wrong. But the idea makes me > very nervous, especially if I imagine folk who _don't_ get as nervous at > the idea. > > Hence the feeling that wider use of generics for fundamental and common > functions might make for a safer world. The opposite appears to be truer. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
