S4 'properties' - creating 'slot' functions?

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Hi Tariq

Some discussion of this topic a while ago on the R-devel newsgroup;
bottom line is that there is no consensus and a certain amount of
resistance to make R conform to the implementation of other languages
(and not exploit R's unique strengths).

https://stat.ethz.ch/pipermail/r-devel/2006-September/042854.html
https://stat.ethz.ch/pipermail/r-devel/2006-September/042864.html

You could implement something fancier (e.g., a method for "$" or "[["
or "@" or a suite of generics "get_x"), but I kind of like (because
'slot' is a function with a well-defined purpose, so why not make it a
generic?)

setGeneric("slot")

setMethod("slot",
          signature=signature(object="A", name="character"),
          function(object, name)
          ## do anything, e.g., restrict access to specific slots
          ## alternatively, dispatch on name with 'switch'
          if (name %in% slotNames(class(object))) callNextMethod()
          else stop("slot '", name, "' undefined"))

setGeneric("slot<-",
           signature=c("object", "name", "value"))

setReplaceMethod("slot",
                 signature(object="A", name="character", value="ANY"),
                 function(object, name, check=TRUE, value)
                 if (name %in% slotNames(class(object))) callNextMethod()
                 else stop("slot '", name, "' cannot be assigned"))

and then
setClass("A",
+          representation=representation(x="numeric"))
[1] "A"
a <- new("A", x=1:5)
slot(a, "y")
Error in slot(a, "y") : slot 'y' undefined
slot(a, "x") <- 5:1
slot(a, "x")
[1] 5 4 3 2 1

A class could be defined to implement the dispatch, so other classes
just have to inherit from that.

Discussion may be veering toward the R-devel newsgroup.

Martin

"?Tariq Khan" <tariq.khan at gmail.com> writes:
Dear R users!

Several languages like C# or VB allow one to create properties; typically
these are under 'get', 'set' tags. My question is this really, what are
similar or comparable strategies in R, and what might be preferred? I have a
couple of situations where I want a certain computation to belong to a
class, but I do not really want to seperate it out to a stand-alone
function.

Here are a couple of options I see possible in R, and if I use the last
described option (see option C below), is there a way to access the object
with something like in C# or C++ where one would probably use the 'this'
keyword or the 'me' keyword in VB from within the property?:

A) MyObject at Property is static and must be handled carefully if it depends
on other slots of the class. If one of the slots changes, then because this
slot is static (containing only the value), it will not change to reflect
the update.

B) MyObject at Property is deprecated, and special accessor functions are used.
The bio group seems to favor this option. So one would use:
Property(MyObject). The drawback I see with this is that there is no strong
relationship between Property and MyObject; they are not explicitly related
via a class structure, and in some way this defeats the point of class
structures.

C) Let MyObject at Property be a function, so in order to fetch the slot object
we would use MyObject at Property(); the question here is can I use something
like a 'this' keyword to access other slots of thr object without having to
pass the whole class as a parameter to this slot function, ie the
round-about, clumsy way would have to look like MyObject at Property(MyObject).
Ideally, the pseudo-code I have in mind might look like this for an object
with two slots (A and B)

Object at A = function() return(.this at B)

Thoughts and considerations would be very interesting.

-Tariq

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