[R] WG: Reference classes: error with missing arguments in method calls

[Janko Thyson]

Dear list,

a while ago I posted this at r-devel but got no answers. Hope it’s okay to
give it a shot again by cross-posting it here.

TIA for any comments,
Janko

Von: Janko Thyson [mailto:janko.thyson.rstuff at googlemail.com] 
Gesendet: Montag, 21. Februar 2011 00:58
An: r-devel at r-project. org (r-devel at r-project.org)
Betreff: Reference classes: error with missing arguments in method calls

Dear list,

I’m having problems in understanding an error that pops up in the context of
missing arguments with methods of reference class objects.

Because of the following statement taken from ‘?referenceClass’, my ref
class methods call explicit S4 methods:
“Reference methods should be kept simple; if they need to do some
specialized R computation, that computation should use a separate R function
that is called from the reference method“

So a ref class would look like this:
setRefClass(Class="Xmple", methods=list(foo=function(var.1, ...)
fooMthd(.self=.self, var.1=var.1, ...))) 

I’d like to keep the generics defs as simple as possible, thus their only
arg should be ‘.self’. The S4 methods are specified in a way that if ‘var.1’
is missing, it will be assigned some default value (I know I could
explicitly set the default value, yet I would like to rely on ‘missing()’
for that). Now, my problem is that this works fine if the generic contains
an argument ‘var.1’, but results in an error if it doesn’t. And I don’t
quite understand why since it seems to be related to whether the S4 method
is invoked from a call to a ref class method or not. Here’s an example which
demonstrates when it works as planed and when the error occurs. I tried to
keep as short as possible:

# 1) "Stand-alone" context
setGeneric(name="fooMthd", def=function(.self, ...)
standardGeneric("fooMthd"), signature=c(".self"))    

setMethod(f="fooMthd", signature=signature(.self="character"), 
    definition=function(.self, var.1, ...){
    cat("I'm having one additional argument compared to my generic:",
sep="\n")
    if(missing(var.1)) var.1 <- "some default value"
    cat(paste("* var.1: ", var.1, sep=""), sep="\n")    
})

fooMthd(.self="blabla", var.1="hello world!")
fooMthd(.self="blabla") # Works.

#+++++

# 2) Reference class context
setMethod(f="fooMthd", signature=signature(.self="Xmple"), 
    definition=function(.self, var.1, ...){
    cat("I'm having one additional argument compared to my generic:",
sep="\n")
    if(missing(var.1)) var.1 <- "some default value"
    cat(paste("* var.1: ", var.1, sep=""), sep="\n")    
})

setRefClass(Class="Xmple", methods=list(foo=function(var.1, ...)
fooMthd(.self=.self, var.1=var.1, ...)))

xmple <- getRefClass(Class="Xmple")$new()
xmple$foo(var.1="hallo") 
xmple$foo() # Does not work.

#+++++

# 3) "Fixed generic" context
setGeneric(name="fooMthd", def=function(.self, var.1, ...)
standardGeneric("fooMthd"), signature=c(".self"))     

setMethod(f="fooMthd", signature=signature(.self="Xmple"), 
    definition=function(.self, var.1, ...){
    cat("I'm having one additional argument compared to my generic:",
sep="\n")
    if(missing(var.1)) var.1 <- "some default value"
    cat(paste("* var.1: ", var.1, sep=""), sep="\n")    
})

xmple$foo(var.1=" blabla") 
xmple$foo() # Works.

I do understand that in the ref class ‘foo()’ has trouble passing an arg to
‘fooMthd()’ that hasn’t been specified. But why and how does simply
including ‘var.1’ in the generic def fix this?

Thanks for any comments,
Janko

R version 2.12.1 (2010-12-16)
Platform: i386-pc-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252   
[3] LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                   
[5] LC_TIME=German_Germany.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] codetools_0.2-6 tools_2.12.1