[R] S4 vs Reference Classes

[Martin Morgan]

On 09/14/2011 06:01 AM, Joseph Park wrote:
> Thanks Martin.
>
> What i'm hoping to do is have a class object, with a member method
> that can change values of slots in the object, without having to
> assign values by external assignment to the object. Something like this:
>
> setClass ( "Element",
> representation ( x = "numeric", y = "numeric" ),
>
> prototype = list( x = 0, y = 1 )
> )
>
> setGeneric( name = "ComputeX",
> def = function( self ) standardGeneric("ComputeX") )
>
> setMethod( "ComputeX", signature = "Element",
> function ( self ) {
> if ( self @ y > 0 ) {
> self @ x = pi
> }
> }
> )
>
> so that a call to the method ComputeX assigns ('internally') a
> value to the slot x of the global object.

Hi Joseph --

I understand. In R generally and in S4 in particular self at x = pi 
triggers a 'copy-on-change', so self inside the function is now 
different from self outside the function.

You either need to change your expectations, or use reference classes 
(and change the expectations of your users).

For completeness, in your function above you would return self, and have

elt = ComputeX(elt)

you'd also likely implement some 'accessor' X (or better named) so

X(elt)

to get X. So there is no direct call to @ in your code.

It might help to understand a real use case; if it's just 'that's the 
way other programming languages do it' then there isn't much more to 
discuss. But maybe, like Doug Bates, you have a particular problem with 
the paradigm?

> One can do :
> a = new( 'Element' )
> a @ x = 2
>
> but i would prefer to have a class method do the work without
> having to explicitly call a @ x = 2. Having to do this means that
> i need code in my main processing app that does things on slots
> that normally i would do in a class method.
>
> As I understand it, Reference Classes provide this. So i'm
> naturally wondering if i should switch my app from S4 to RC.
>
> Fundamentally, I don't clearly understand S4 and what the difference
> is between creating a SetReplaceMethod vs a SetMethod, since it
> seems that in either case one has to 'externally' assign the slot
> value. My limitation, of course.

at some level they are differences in syntax only, e.g.,

slt(a) = 2

versus

setGeneric("updt", function(x, value, ...) standardGeneric("updt"))
setMethod(updt, c("A", "numeric"), function(x, value, ...) {
     initialize(x, a=value)
})

and then

a = updt(a, 3)

The 'updt' model easily extends to multiple arguments; both represent an 
abstraction between the API seen by the user, and the implementation of 
the class, so there's no reason to store '3' directly.

Martin

>
>
> On 9/14/2011 12:17 AM, Martin Morgan wrote:
>> On 09/13/2011 10:54 AM, Joseph Park wrote:
>>>
>>> Hi, I'm looking for some guidance on whether to use
>>> S4 or Reference Classes for an analysis application
>>> I'm developing.
>>> I'm a C++/Python developer, and like to 'think' in OOD.
>>> I started my app with S4, thinking that was the best
>>> set of OO features in R. However, it appears that one
>>> needs Reference Classes to allow object methods to assign
>>> values (other than the .Object in the initialize method)
>>> to slots of the object.
>>
>> With
>>
>> setClass("A", representation=representation(slt="numeric"))
>>
>> a slot can be updated with @<- and an object updated with a
>> replacement method
>>
>> setGeneric("slt<-", function(x, ..., value) standardGeneric("slt<-"))
>>
>> setReplaceMethod("slt", c("A", "numeric"), function(x, ..., value) {
>> x at slt <- value
>> x
>> })
>>
>> so
>>
>> > a = new("A", slt=1)
>> > slt(a) = 2
>> > a
>> An object of class "A"
>> Slot "slt":
>> [1] 2
>>
>> The default initialize method also works as a copy constructor with
>> validity check, e.g., allowing multiple slot updates
>>
>> setReplaceMethod("slt", c("A", "ANY"), function(x, ..., value) {
>> initialize(x, slt=as.numeric(value))
>> })
>>
>> > slt(a) = "1"
>>
>>
>>> This is typically what I prefer: creating an object, then
>>> operating on the object (reference) calling object methods
>>> to access/modify slots.
>>> So I'm wondering what (dis)advantages there are in
>>> developing with S4 vs Reference Classes.
>>
>> R's copy-on-change semantics leads me to expect that
>>
>> b = a
>> slt(a) = 2
>>
>> leaves b unchanged, which S4 does (necessarily copying and thus with a
>> time and memory performance cost). A reference class might be
>> appropriate when the entity referred to exists in a single copy, as
>> e.g., an on-disk data base, or an external pointer to a C++ class.
>>
>> Martin
>>
>>> Things of interest:
>>> Performance (i.e. memory management)
>>> Integration compatibility with R packages
>>> ??? other issues
>>> Thanks!
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> 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.
>>
>>


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