`[.` method for `ReferenceClass` - r

Question

I would like to write a [. method for my ReferenceClass. So far, I have something like this:
DT <- data.table(Index=1:5)
MySeries <- setRefClass("MySeries", fields = list(data="data.table"))
setMethod("[","MySeries",function(x, i,j,drop) {
ii <- substitute(i)
x$data <- x$data[eval(ii)]
return(x)
})
S <- MySeries(data=DT)
... but it throws an error when I finally call S[Index>3]. How to fix the above to get this expected result?
Index
4: 4
5: 5

Answer 1

This is really about the use of eval(substitute()) as much as about S4 methods. Here is the generic that you are interested in
> getGeneric("[")
standardGeneric for "[" defined from package "base"
function (x, i, j, ..., drop = TRUE)
standardGeneric("[", .Primitive("["))
<bytecode: 0x42f4fe0>
<environment: 0x3214270>
Methods may be defined for arguments: x, i, j, drop
Use showMethods("[") for currently available ones.
Your method signature differs from the generic (no '...' and no default for 'drop') so the method has a nested '.local' function
> getMethod("[", "MySeries")
Method Definition:
function (x, i, j, ..., drop = TRUE)
{
.local <- function (x, i, j, drop)
{
ii <- substitute(i)
x$data <- x$data[eval(ii)]
return(x)
}
.local(x, i, j, ..., drop)
}
Signatures:
x
target "MySeries"
defined "MySeries"
and subsitute(i) is not what you think it is. Instead, write a method matching the generic signature
setMethod("[", "MySeries", function(x, i, j, ..., drop=TRUE) {
x$data <- x$data[eval(substitute(i))]
x
})
nested functions are a general problem with the eval(substitute()) paradigm, not just definition of S4 methods; see this question.