On 05/01/2011 03:09 PM, John Chambers wrote:
Yes, as presented on that site it makes a little more sense:
"While experimenting with the new reference classes in R I noticed some
odd behaviour if you use the "[[ ]]" notation for methods
(X[["doSomething"]] instead of X$doSomething). This notation works for
fields, but I initially thought it wouldn't work for methods until I
found that if you execute "class(X$doSomething)" you can then use "[[
]]" afterwards. The simple example below illustrates the point."
For reference classes, "[[" is not meant to be used either for fields or
methods. That it "works" at all is an artifact of the implementation
using environments. And arguably the failure to throw an error in that
circumstance is a bug.
Please use the API as described in the ?ReferenceClasses documentation.
These are encapsulated methods, in the usual terminology, with the
operator "$" playing the role normally assigned to "." in other languages.
A separate but related issue: It is possible to define S4 methods for
reference classes, as discussed in a previous thread, arguably also an
artifact in that a reference class is implemented as an S4 class of the
same name. These are functional methods, associated with a generic
function, and so outside the encapsulation paradigm.
It would be interesting to get some experience and opinions on whether
this is a good idea or not. It breaks encapsulation, in that the
behavior of the class can no longer be inferred from the class
definition alone. On the other hand, it is convenient and relates to
"operator overloading" in some other languages.
I have written 'show' methods for reference classes (is there another
way to pretty-print them?) and S4 methods that dispatch to reference
methods (in particular, yield(x) on connection-like classes dispatching
to x$yield()). The latter partly to provide end-user familiarity
(limiting need for the beleaguered user to have to learn yet another
syntax for invoking methods, though maybe hiding hints about important
differences in object behavior -- I am dreading the introductory class
where one tries to explain S3, S4, and reference classes), and partly to
provide a distinction between a 'developer' API and a user API (again
with questionable merits).
Martin
John
On 4/30/11 7:54 PM, Hadley Wickham wrote:
If this message is garbled for anyone else, the original question on
stackoverflow is here:
http://stackoverflow.com/questions/5841339/using-notation-for-reference-class-methods
Hadley
On Sat, Apr 30, 2011 at 11:35 AM, Chad
Goymer<chad.goy...@hotmail.co.uk> wrote:
I've been trying to use methods for reference classes via the
notation "[[...]]" (X[["doSomething"]] rather than X$doSomething),
but it failed to work. However, I did find that if you use the usual
"$" notation first, "[[...]]" can be used afterwards. The following
simple example illustrates the point:
setRefClass("Number", + fields = list(+ value = "numeric"+ ),+
methods = list(+ addOne = function() {+ value<<- value + 1+ }+ )+ )>
X<- new("Number", value = 1)> X[["value"]][1] 1
X[["addOne"]]()Error: attempt to apply non-function>
class(X[["addOne"]]) # NULL[1] "NULL"
class(X$addOne)[1] "refMethodDef"attr(,"package")[1] "methods"
X[["addOne"]]()> X[["value"]][1] 2> class(X[["addOne"]])[1]
"refMethodDef"attr(,"package")[1] "methods"
Is this a bug?
Chad Goymer
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