R help - Dec 2021 - Forwarding missing arguments to the `[` method

I think you need a reprex... I don't think your claim is correct as stated.

On December 2, 2021 1:00:01 PM PST, Ivan Krylov <krylov.r00t at gmail.com>
wrote:
>Sorry for sending an unfinished message!
>
>On Thu, 2 Dec 2021 23:57:11 +0300
>Ivan Krylov <krylov.r00t at gmail.com> wrote:
>
>> Why can I forward missing i,j to built-in `[`
>
>Why can I forward missing i,j to the built-in `[` method but not
>to user-defined methods? How can I fix this? Default i and j to TRUE?
>Are there less involved methods of handling my problem of subsetting
>attr(x, 'foo') with the dimnames of x? Preferrably without setting
>dimnames on attr(x, 'foo') itself.
>
-- 
Sent from my phone. Please excuse my brevity.

On Thu, 02 Dec 2021 13:41:52 -0800
Jeff Newmiller <jdnewmil at dcn.davis.ca.us> wrote:
> I think you need a reprex... I don't think your claim is correct as
> stated.
Sorry, let me try again. The following "works" for me in the sense of
throwing the same error on R 3.3 and R-devel:

foo <- function(x, ...) UseMethod('foo')

# foo is a matrix tagged with a vector of length() == nrow(foo)
foo.matrix <- function(x, y, ...) {
	stopifnot(
		!is.recursive(y),
		length(y) == nrow(x),
		length(list(...)) == 0
	)
	structure(x, class = 'foo', foo = y)
}

# the vector must be subsetted at the same time as x
`[.foo` <- function(x, i, j, ..., drop = TRUE) {
	ret <- NextMethod() # could drop dimensions
	if (is.matrix(ret)) foo(
		ret,
		unname(setNames(attr(x, 'foo'), dimnames(x)[[1]])[i])
	)
	else ret
}

`[<-.foo` <- function(x, i, j, ..., value) {
	xsub <- x[i, j, drop = FALSE]
	# TODO: some checks on xsub
	NextMethod()
}

x <- foo(volcano, 1:nrow(volcano))
invisible(x[]) # doesn't stop despite missing i
x[] <- 1       # fails in `[.foo` with "i missing"
# Error in NextMethod() : argument "i" is missing, with no default
traceback()
# 9: NextMethod() at foo.R#15
# 8: `[.foo`(x, i, j, drop = FALSE) at foo.R#24
# 7: x[i, j, drop = FALSE] at foo.R#24
# 6: `[<-.foo`(`*tmp*`, , value = 1) at foo.R#30
# 5: `[<-`(`*tmp*`, , value = 1) at foo.R#30
# 4: eval(ei, envir)
# 3: eval(ei, envir)
# 2: withVisible(eval(ei, envir))
# 1: source("foo.R", echo = TRUE)

Or did you mean some other claim?

-- 
Best regards,
Ivan

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