[R] Method dispatch in S4

Simon Zehnder szehnder at uni-bonn.de
Fri Aug 9 16:40:02 CEST 2013


Hi Martin,

thank you very much for this profound answer! Your added design advice is very helpful, too! 

For the 'simple example': Sometimes I am still a little overwhelmed from a certain setting in the code and my ideas how I want to handle a process. But I learn from session to session. In future I will also span the lines more than 80 columns. I am used to the indent in my vim editor. 

I have one further issue: I do know, that you are one of the leading developers of the bioconductor package which uses (as far as I have read) extensively OOP in R. Is there a package you could suggest to me to learn from by reading and understanding the code? Where can I find the source code? 

Best

Simon


On Aug 8, 2013, at 10:00 PM, Martin Morgan <mtmorgan at fhcrc.org> wrote:

> On 08/04/2013 02:13 AM, Simon Zehnder wrote:
>> So, I found a solution: First in the "initialize" method of class C coerce
>> the C object into a B object. Then call the next method in the list with the
>> B class object. Now, in the "initialize" method of class B the object is a B
>> object and the respective "generateSpec" method is called. Then, in the
>> "initialize" method of C the returned object from "callNextMethod" has to be
>> written to the C class object in .Object. See the code below.
>> 
>> setMethod("initialize", "C", function(.Object, value) {.Object at c <- value;
>> object <- as(.Object, "B"); object <- callNextMethod(object, value);
>> as(.Object, "B") <- object; .Object <- generateSpec(.Object);
>> return(.Object)})
>> 
>> This setting works. I do not know though, if this setting is the "usual" way
>> such things are done in R OOP. Maybe the whole class design is
>> disadvantageous. If anyone detects a mistaken design, I am very thankful to
>> learn.
> 
> Hi Simon -- your 'simple' example is pretty complicated, and I didn't really follow it in detail! The code is not formatted for easy reading (e.g., lines spanning no more than 80 columns) and some of it (e.g., generateSpec) might not be necessary to describe the problem you're having.
> 
> A good strategy is to ensure that 'new' called with no arguments works (there are other solutions, but following this rule has helped me to keep my classes and methods simple). This is not the case for
> 
>  new("A")
>  new("C")
> 
> The reason for this strategy has to do with the way inheritance is implemented, in particular the coercion from derived to super class. Usually it is better to provide default values for arguments to initialize, and to specify arguments after a '...'. This means that your initialize methods will respects the contract set out in ?initialize, in particular the handling of unnamed arguments:
> 
>     ...: data to include in the new object.  Named arguments
>          correspond to slots in the class definition. Unnamed
>          arguments must be objects from classes that this class
>          extends.
> 
> I might have written initialize,A-method as
> 
>  setMethod("initialize", "A", function(.Object, ..., value=numeric()){
>      .Object <- callNextMethod(.Object, ..., a=value)
>      generateSpec(.Object)
>  })
> 
> Likely in a subsequent iteration I would have ended up with (using the convention that function names preceded by '.' are not exported)
> 
>  .A <- setClass("A", representation(a = "numeric", specA = "numeric"))
> 
>  .generateSpecA <- function(a) {
>      1 / a
>   }
> 
>  A <- function(a=numeric(), ...) {
>      specA <- .generateSpecA(a)
>      .A(..., a=a, specA=specA)
>  }
> 
>  setMethod(generateSpec, "A", function(object) {
>      .generateSpecA(object at a)
>  })
> 
> ensuring that A() returns a valid object and avoiding the definition of an initialize method entirely.
> 
> Martin
> 
>> 
>> Best
>> 
>> Simon
>> 
>> 
>> On Aug 3, 2013, at 9:43 PM, Simon Zehnder <simon.zehnder at googlemail.com>
>> wrote:
>> 
>>> setMethod("initialize", "C", function(.Object, value) {.Object at c <- value;
>>> .Object <- callNextMethod(.Object, value); .Object <-
>>> generateSpec(.Object); return(.Object)})
>> 
>> ______________________________________________ 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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