From the documentation for setMethod():
Method definitions can have default expressions for arguments, but a
current limitation is that the generic function must have some default
expression for the same argument in order for the method's defaults to
be used. <....>
(It would be nice to fix this, but would require some significant change
to how methods are dispatched, with some possible inefficiencies. The
issue is that R processes formal arguments differently if there is no
default expression. Volunteers among the R hackers welcome, though.)
Meanwhile, if you own the generic function just give it a default, say:
setGeneric("TSdoc",
def= function(x, con = stop("Sorry, no generic default"), ...)
standardGeneric("TSdoc"))
John
On 5/14/10 9:50 AM, Paul Gilbert wrote:
I am trying to understand the missing vs default value for an argument
to an S4 method. (I'm not sure if this is a bug or my confusion.) In S3
I can specify a default value for an argument and then both check if the
argument was missing in the call, and use it because it takes on the
default value. I do not seem to be able to do that in S4 as shown by
this simplified example:
setGeneric("TSdoc",
def= function(x, con, ...) standardGeneric("TSdoc"))
setMethod("TSdoc", signature(x="character", con="missing"),
definition= function(x, con=getOption("TSconnection"), ...){
if(is.null(con))
stop("con should be specified or set with
options(TSconnection=con).")
else return('ok')} )
Then when I try
TSdoc("B103")
In my original problem I got
Error in is.null(con) : 'con' is missing
And in the simplified example I get
Error in TSdoc("B103") :
element 1 is empty;
the part of the args list of 'is.null' being evaluated was:
(con)
I'm not sure why there is a difference, but the implication is the same
in both, the default
getOption("TSconnection")
NULL
Is not getting through to the is.null(). I can fix (or work around)
this with
setMethod("TSdoc", signature(x="character", con="missing"),
definition= function(x, con, ...){
if(is.null(con<- getOption("TSconnection")))
stop("con should be specified or set with
options(TSconnection=con).")
else return('ok') } )
TSdoc("B103")
Error in TSdoc("B103") :
con should be specified or set with options(TSconnection=con).
So my question is, is this a bug or do I have to adjust my programming
style for the way missing/default arguments are handled in S4?
Paul Gilbert
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