R devel - Mar 2026 - Is "difftime" an appropriate addition to methods::.OldClassesList?

I found myself needing to add methods::setOldClass("difftime") in
order to also set a coercion method for my class to difftime:

methods::setAs("myClass", "difftime", ...)

And thus avoid the on-load message:

in method for ?coerce? with signature
?"myClass","difftime"?: no
definition for class ?difftime?

I see 10 or so other packages also doing the same [1].

I am wondering, though, since {base} owns this class, if it shouldn't
be registered already? I see inclusions for, e.g., closely-related S3
classes Date and POSIXt [2].

OTOH, I see .OldClassesList hasn't really been edited in 15+ years, so
maybe this approach has lost favor and is better not amended anymore
[3].

(or maybe this is really an r-package-devel question about whether
supporting as(<my class>, "difftime") is actually beneficial...)

Mike C

[1]
https://github.com/search?q=org%3Acran%20%2FsetOldClass.*difftime%2F&type=code
[2]
https://github.com/r-devel/r-svn/blob/b7850c8cae0956592019c5f8f59e885d3d39ae6f/src/library/methods/R/BasicClasses.R#L550-L552
[3]
https://github.com/r-devel/r-svn/blame/b7850c8cae0956592019c5f8f59e885d3d39ae6f/src/library/methods/R/BasicClasses.R#L541-L569

>>>>> Michael Chirico 
>>>>>     on Thu, 5 Mar 2026 14:26:16 -0800 writes:
    > I found myself needing to add
methods::setOldClass("difftime") in
    > order to also set a coercion method for my class to difftime:

    > methods::setAs("myClass", "difftime", ...)

    > And thus avoid the on-load message:

    > in method for ?coerce? with signature
?"myClass","difftime"?: no
    > definition for class ?difftime?

    > I see 10 or so other packages also doing the same [1].

    > I am wondering, though, since {base} owns this class, if it
shouldn't
    > be registered already? I see inclusions for, e.g., closely-related S3
    > classes Date and POSIXt [2].

In my vague recollection  "difftime" came a bit later than
"Date"
and "POSIXlt".  This may be the reason for its non-presence.

    > OTOH, I see .OldClassesList hasn't really been edited in 15+ years,
so
    > maybe this approach has lost favor and is better not amended anymore
    > [3].

well, favors related to S4 have always been somewhat segregated ..

    > (or maybe this is really an r-package-devel question about whether
    > supporting as(<my class>, "difftime") is actually
beneficial...)

The use of  as(., <...>)  is an R-help question really  not
restricted to package development at all.

I agree that "difftime" does naturally belong alongside
"POSIXt"
and I am happy to add it to the list;
(I did check to see if an unneeded   setOldClass(<already_set>)
 gives a warning, and it doesn't)

Martin


    > Mike C

    > [1]
https://github.com/search?q=org%3Acran%20%2FsetOldClass.*difftime%2F&type=code
    > [2]
https://github.com/r-devel/r-svn/blob/b7850c8cae0956592019c5f8f59e885d3d39ae6f/src/library/methods/R/BasicClasses.R#L550-L552
    > [3]
https://github.com/r-devel/r-svn/blame/b7850c8cae0956592019c5f8f59e885d3d39ae6f/src/library/methods/R/BasicClasses.R#L541-L569