Got some progress on this, but still not sure how to continue. First, a
much more simple script reproducing the behaviour that confuses me:
foo <- function(x, ...) structure(x, class = 'foo')
`[.foo` <- function(x, i, ..., drop = TRUE) {
print(sys.call())
print(match.call())
foo(NextMethod()) # x[] should return an object of class foo
}
`[<-.foo` <- function(x, i, ..., value) {
print(sys.call())
print(match.call())
x[i, ..., drop = FALSE]
# imagine that we need to perform some checks on the
# subset of x that we're about to replace
NextMethod()
}
x <- foo(42)
x[] # this works
x[] <- 1 # this doesn't
Now, the crucial difference between the x[] and x[] <- 1 calls is that
in the former case, the `i` argument isn't even specified:
x[]
# `[.foo`(x, )
# `[.foo`(x = x)
# [1] 42
# attr(,"class")
# [1] "foo"
While in the latter case, `i` is specified in the x[] call (but
missing):
x[] <- 1
# `[<-.foo`(`*tmp*`, , value = 1)
# `[<-.foo`(x = `*tmp*`, value = 1)
# x[i, ..., drop = FALSE]
# `[.foo`(x = x, i = i, drop = FALSE)
Error in NextMethod() : argument "i" is missing, with no default
What's the best way to forward the arguments from [<-.class methods to
the [.class methods in order to handle cases like x[] properly?
--
Best regards,
Ivan
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