[R] Forwarding missing arguments to the `[` method

[Martin Maechler]

>>>>> Ivan Krylov 
>>>>>     on Wed, 8 Dec 2021 16:52:00 +0300 writes:

I have always learnt a lot about programming when reading other
people's code .. notably if those people new what they are
doing.

In this case, studying R-core's

`[.data.frame`  and

`[<-.data.frame`

{or for S4 methods, the 'Matrix' or 'Rmpfr' packages}

should show you a lot.

What you should take from there:
Do work with *both*
    missing(drop)
and
    nargs()

(and more)
in order to distinguish m[i] from m[i,] etc


Best,
Martin


    > 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
> 
> ______________________________________________