[Rd] Implementing a "plugin" paradigm with R methods

[Janko Thyson]

Hi Martin,

thanks a lot for the quick reply. I'll try your suggestions.

Regards,
Janko

On 24.08.2011 06:37, Martin Morgan wrote:
> On 08/23/2011 03:02 PM, Janko Thyson wrote:
>> Dear list,
>>
>> I was wondering how to best implement some sort of a "plugin" paradigm
>> using R methods and the dispatcher:
>> Say we have a function/method ('foo') that does something useful, but
>> that should be open for extension in ONE specific area by OTHERS using
>> my package. Of course they could go ahead and write a whole new 'foo'
>
> One possibility is to write class / method pairs. The classes extend 
> 'Plugin', and the methods are on generic 'plug', with the infrastructure
>
>   ## Approach 1: class / method pairs
>   setClass("Plugin")
>
>   setClass("DefaultPlugin", contains="Plugin")
>
>   DefaultPlugin <- function() new("DefaultPlugin")
>
>   setGeneric("plug",
>              function(plugin, src) standardGeneric("plug"),
>              signature="plugin",
>              valueClass="character")
>
>   setMethod(plug, "Plugin", function(plugin, src) {
>       src
>   })
>
>   foo <- function(src, plugin=DefaultPlugin()) {
>       plug(plugin, src)
>   }
>
> This is extended by writing class / method pairs
>
>   setClass("Punct", contains="Plugin")
>
>   Punct <- function() new("Punct")
>
>   setMethod(plug, "Punct", function(plugin, src) {
>       gsub("[[:punct:]]", "", src)
>   })
>
>
>   setClass("Digit", contains="Plugin")
>
>   Digit <- function() new("Digit")
>
>   setMethod(plug, "Digit", function(plugin, src) {
>       gsub("[[:digit:]]", "", src)
>   })
>
> The classes could have slots with state, accessible within the method. 
> An easier-on-the-user approach might have the Plugin class contain or 
> have slots of class "function". The user would only be obliged to 
> provide an appropriate function.
>
>   ## Approach 2:
>   setClass("Plugin", prototype=prototype(function(src) {
>       src
>   }), contains="function")
>
>   Plugin <- function() new("Plugin")
>
>   setGeneric("foo",
>              function(src, plugin) standardGeneric("foo"))
>
>   setMethod(foo, c("character", "missing"),
>             function(src, plugin) foo(src, Plugin()))
>
>   setMethod(foo, c("character", "Plugin"),
>             function(src, plugin) plugin(src))
>
>   ## 'Developer' classes
>   setClass("Punct", prototype=prototype(function(src) {
>       gsub("[[:punct:]]", "", src)
>   }), contains="Plugin")
>
>   Punct <- function() new("Punct")
>
>   setClass("Digit", prototype=prototype(function(src) {
>       gsub("[[:digit:]]", "", src)
>   }), contains="Plugin")
>
>   Digit <- function() new("Digit")
>
>   ## General-purpose 'user' class
>   setClass("User", contains="Plugin")
>
>   User <- function(fun) new("User", fun)
>
> This could have syntax checking in the validity method to catch some 
> mistakes early. In the S3 world, this is the approach taken by glm for 
> its 'family' argument, for instance str(gaussian().
>
> Martin
>
>> method including the features they'd like to see, but that's not really
>> necessary. Rather, they should be able to just write a new "plugin"
>> method for that part of 'foo' that I'd like to open for such plugins.
>>
>> The way I chose below works, but generates warnings as my method has
>> signature arguments that don't correspond to formal classes (which is
>> totally fine). Of course I could go ahead and make sure that such
>> "dummy" classes exist, but I was wondering if there's a better way.
>>
>> It'd be great if anyone could let me know how they handle "plugin"
>> scenarios based on some sort of method dispatch!
>>
>> Thanks,
>> Janko
>>
>> ##### CODE EXAMPLE #####
>>
>> setGeneric(name="foo", signature=c("src"), function(src, ...)
>> standardGeneric("foo"))
>> setGeneric(name="plugin", signature=c("src", "link", "plugin"),
>> function(src, link, plugin, ...) standardGeneric("plugin")
>> )
>> setMethod(f="plugin", signature=signature(src="character", link="foo",
>> plugin="punct"),
>> function(src, link, plugin, ...){
>> out <- gsub("[[:punct:]]", "", src)
>> return(out)
>> }
>> )
>> setMethod(f="plugin", signature=signature(src="character", link="foo",
>> plugin="digit"),
>> function(src, link, plugin, ...){
>> out <- gsub("[[:digit:]]", "", src)
>> return(out)
>> }
>> )
>> setMethod(f="foo", signature=signature(src="character"),
>> function(src, plugin=NULL, ...){
>> if(!is.null(plugin)){
>> if(!existsMethod(f="plugin",
>> signature=c(src=class(src), link="foo", plugin=plugin)
>> )){
>> stop("Invalid plugin")
>> }
>> .plugin <- selectMethod(
>> "plugin",
>> signature=c(src=class(src), link="foo", plugin=plugin),
>> useInherited=c(src=TRUE, plugin=FALSE)
>> )
>> out <- .plugin(src=src)
>> } else {
>> out <- paste("Hello world: ", src, sep="")
>> }
>> return(out)
>> }
>> )
>> foo(src="Teststring:-1234_56/")
>> foo(src="Teststring:-1234_56/", plugin="punct")
>> foo(src="Teststring:-1234_56/", plugin="digit")
>>
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>
>