Check recursive indexing and NULLs

[hadley]

list("a" = list())[[c("a", "b", "c")]]

Maybe this is an R bug? Should at least bring up on R-devel, and consider a work around (tryCatch()?) in as_function.character()

[hadley]

I think we need a new recursive indexing operator that behaves consistently with respect to missing values. [[ is not consistent because:

list(a = list())[[c("a", "b", "c")]]
#> Error in list(a = list())[[c("a", "b", "c")]]: no such index at level 2
list(a = list())[["a"]][["b"]][["c"]]
#> NULL

It would also be nice to have be able to mix subsetting by name and position, so you could do (e.g.)

x <- list(a = list(list(), list(c = 3))
x[[list("a", 1, "c")]]

And we also need to be able to specify a default value to use when the indexed object does not - this makes it easier to use with map_int() etc. This will make this function slightly dangerous because it will always succeed (i.e. there are no situations in which it will fail), but I think that is necessary for the most common use case.

For efficiency, this will need to be implemented in C

cc @jennybc

[hadley]

What should happen if you do recursive indexing on an object that is not recursive?

x <- list(1:10)
index(x, 1, 2, 3)

Should that be an error?

[lionel-]

Should that be an error?

That would be consistent with base R

letters[[1]][[2]]
#> Error in letters[[1]][[2]] : subscript out of bounds

[hadley]

@lionel- I think the point here is to not be consistent with base R - we need a function that always returns a value so you can use it to regularise irregular data structures

[lionel-]

ok. Then IIUC it shouldn't be an error since recursive data structures always end with a non-recursive vector (or a NULL value).

We can return a NA or a NULL, but in either case we'll have the problem of distinguishing being actual NA/NULL values and implicitly filled ones. Maybe better to go with NULL then since NAs are used for actual data so we shouldn't have ambiguity for these.

[hadley]

@lionel- the default value will be user selectable

[smbache]

Often wish for something along the lines of Options: https://fsharpforfunandprofit.com/posts/the-option-type/

[lionel-]

R's NULL is pretty close, I guess it's only missing the ability to get attributes. If it were the case we could give a class or some other attribute to the filled in NULLs to distinguish them from other NULLs. ggplot2 uses a structure(list(NULL), class = "waiver")) for that purpose.

But this should probably be tackled in R core and not a package. Not sure it's possible to make NULL a proper SEXP type without breaking compatibility though (it's currently a singleton).

[smbache]

Yeah ... Similar but still not the same...

[lionel-]

No actually it's the same. I thought Fsharp none could be subtyped (hence my comment above) but I was mistaken. So all R data structures are actually option types.

[smbache]

Take a look at the differences mentioned in the article. Some of them would apply still...

[lionel-]

I did read the article. If you'd like to continue this discussion let's do it by mail so we don't ping Hadley all the time ;)

[smbache]

Not necessary :)

Point was more that NULL could be a valid value in the list and SOME (of whatever) would be returned (could then be NULL) and NONE would indicate that the element was non-existing. Just a thought..