[R] Method dispatch for function call operator?

Prof Brian Ripley ripley at stats.ox.ac.uk
Thu Jan 13 11:13:52 CET 2011

The details here are much more appropriate for R-devel, but please 
check the help pages for "(" and "[", and note

- "[" is generic and "(" is not.
- the primitive `(` is used to implement constructions such as
(x <- pi) and not x(...).

The special handling of operators such as "[" is part of the parser, 
and you are guessing incorrectly how function calls are parsed.

(Note to Duncan Murdoch whose reply came in whilst I was writig this: 
"(" is essentially a no-op, but it does turn visibility on, something 
often used with assignments.)

On Thu, 13 Jan 2011, Taras Zakharko wrote:

> Dear R gurus,
> I am trying to create a nicer API interface for some R modules I have
> written. Here, I heavily rely on S3 method dispatch mechanics and
> makeActiveBinding()  function
> I have discovered that I apparently can't dispatch on function call
> operator (). While .Primitive("(") exists, which leads me to believe that
> things like x(...) are internally translated to .Primitive("(")(x, ...), I
> can't seem to do something like:
> x <- integer()
> class(x) <- "testclass"
> "(.testclass" <- function(o, x, y) print(paste(x, y))
> x(1, 2)
> Similar code does work for other operators like "[".
> A workaround I have discovered is to make x a function from the beginning
> and then extend the functionality by via S3 methods. Unfortunately, it does
> not allow me to do something I'd really like to - use syntax like this:
> x(...) <- y
> For this, I'd need to dispatch on something like
> "(<-.testclass" <- function(x, arglist, value)
> Currently, I am using the index operator for this (i.e. x[...] <- y) - it
> works nicely, but I'd prefer the () syntax, if possible.
> Does anyone know a way to do this?
> -- 
> View this message in context: http://r.789695.n4.nabble.com/Method-dispatch-for-function-call-operator-tp3215381p3215381.html
> Sent from the R help mailing list archive at Nabble.com.
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
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

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