[R] A technical question on methods for "+"

[Deepayan Sarkar]

On Fri, Dec 3, 2021 at 3:14 AM Bert Gunter <bgunter.4567 using gmail.com> wrote:
>
> Those are both helpful comments. Would it not be useful to say
> something along these lines in ?class. ?
> The key point I missed is that there often(but not always) must be an
> *explicit* class **attribute** for method dispatch; and class() does
> not indicate whether its value is explicit or not (just recapitulating
> what you both said). Ops methods are such a case (for anything but
> classes inheriting from numeric?), and perhaps a mention there might
> be useful, too.

?Ops already says

     Note that a method will be used for one of these groups or one of
     its members _only_ if it corresponds to a '"class"' attribute, as
     the internal code dispatches on 'oldClass' and not on 'class'.
     This is for efficiency: having to dispatch on, say, 'Ops.integer'
     would be too slow.

Can you suggest some text to make this clearer?

FWIW, the following seems to work as you would want (at the cost of
global inefficiency):

`+` <- function(e1, e2) UseMethod("+")
`+.default` <- function(e1, e2 = 0) .Primitive("+")(e1, e2)

and then

"+.character" <- function(e1, e2) paste0(e1, e2)

> "jan" + "feb"
[1] "janfeb"
> Reduce("+", letters)
[1] "abcdefghijklmnopqrstuvwxyz"

-Deepayan

> Again, thanks for the help.
>
> Bert
>
> On Thu, Dec 2, 2021 at 12:40 PM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
> >
> > The reason for this behaviour is that finding methods is a lot slower
> > than just evaluating the built-in function.  So R takes the time to
> > determine if there's an attribute named "class" attached, but doesn't go
> > searching further if there isn't one.
> >
> > Duncan Murdoch
> >
> > On 02/12/2021 3:10 p.m., Andrew Simmons wrote:
> > > This is because + dispatches on the class attribute, which a string like
> > > "test" has set to NULL, so it doesn't dispatch. You can add the class
> > > yourself like structure("test", class = "character") and that should work.
> > >
> > > I'm not sure where it's explained, but most primitive functions dispatch on
> > > the class attribute, which is different from UseMethod which calls class()
> > > if the class attribute is NULL.
> > >
> > > I think if you want to define something like what you have written, you
> > > could write a function `%+%` use that instead
> > >
> > > On Thu, Dec 2, 2021, 14:32 Bert Gunter <bgunter.4567 using gmail.com> wrote:
> > >
> > >> ... and probably a dumb one and almost certainly not of interest to
> > >> most R users. But anyway...
> > >>
> > >> ?"+" says:
> > >> "The unary and binary arithmetic operators are generic functions:
> > >> methods can be written for them individually or via the Ops group
> > >> generic function. "
> > >>
> > >> So:
> > >> "+.character" <- function(e1, e2) paste0(e1, e2)
> > >> ## but this doesn't 'work':
> > >>> "a" + "b"
> > >> Error in "a" + "b" : non-numeric argument to binary operator
> > >>
> > >> ## but explicitly invoking the method does 'work' :
> > >>> "+.character"('a','b')
> > >> [1] "ab"
> > >>
> > >> ##Note also:
> > >>> methods("+")
> > >> [1] +.character +.Date      +.IDate*    +.POSIXt    +.trellis*
> > >>
> > >> So what am I failing to understand?
> > >> Thanks.
> > >>
> > >> Bert Gunter
> > >>
> > >> ______________________________________________
> > >> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> > >> PLEASE do read the posting guide
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> > >> and provide commented, minimal, self-contained, reproducible code.
> > >>
> > >
> > >       [[alternative HTML version deleted]]
> > >
> > > ______________________________________________
> > > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.
> > >
> >
>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.