I'm trying to make a vector class using the vctrs package, that stores an expression. Mainly because I want to use it in a different vctrs vector. An expression is not a vector type, so a naive implementation of a vector expression (named vexpr here) fails.
library(vctrs)
expr <- scales::math_format()(1:10)
new_vexpr <- function(x) {
new_vctr(x, class = 'vexpr')
}
new_vexpr(expr)
#> Error: `.data` must be a vector type.
So, I thought, maybe I can implement the expression itself as an attribute in parallel with the vector.
new_vexpr <- function(x) {
if (!is.expression(x)) {
stop()
}
new_vctr(seq_along(x),
expr = x,
class = "vexpr")
}
format.vexpr <- function(x, ...) {
ifelse(is.na(vec_data(x)), NA, format(as.list(attr(x, "expr"))))
}
# Works!
x <- new_vexpr(expr)
I quickly began running into trouble because I haven't implemented the boilerplate vec_ptype2() and vec_cast() methods yet.
# Looks like it might work
c(x, x)
#> <vexpr[20]>
#> [1] 10^1L 10^2L 10^3L 10^4L 10^5L 10^6L 10^7L 10^8L 10^9L 10^10L
#> [11] 10^1L 10^2L 10^3L 10^4L 10^5L 10^6L 10^7L 10^8L 10^9L 10^10L
# Expression not concatenated (as might be expected)
attr(c(x, x), "expr")
#> expression(10^1L, 10^2L, 10^3L, 10^4L, 10^5L, 10^6L, 10^7L, 10^8L,
#> 10^9L, 10^10L)
So I tried to implement the boilerplate methods.
vec_ptype2.vexpr.vexpr <- function(x, y, ...) {
new <- c(attr(x, "expr"), attr(y, "expr"))
new_vctr(integer(0), expr = new, class = "vexpr")
}
vec_cast.vexpr.vexpr <- function(x, to, ...) {
new_vctr(vec_data(x), expr = attr(to, "expr"),
class = "vexpr")
}
Which helped concatenating the vectors, but return erroneous subsetting results.
# Expression is concatenated!
attr(c(x, x), "expr")
#> expression(10^1L, 10^2L, 10^3L, 10^4L, 10^5L, 10^6L, 10^7L, 10^8L,
#> 10^9L, 10^10L, 10^1L, 10^2L, 10^3L, 10^4L, 10^5L, 10^6L,
#> 10^7L, 10^8L, 10^9L, 10^10L)
# Subsetting doesn't make sense, should be 10^2L
x[2]
#> <vexpr[1]>
#> [1] 10^1L
# Turns out, full expression still there
attr(x[2], "expr")
#> expression(10^1L, 10^2L, 10^3L, 10^4L, 10^5L, 10^6L, 10^7L, 10^8L,
#> 10^9L, 10^10L)
Fine, so I defined my own subsetting methods outside the vctrs system, which initially seemed to work.
# Define S3 subsetting method
`[.vexpr` <- function(x, i, ...) {
expr <- attr(x, "expr")
ii <- vec_as_location(i, length(expr), names = names(x),
missing = "propagate")
new_vctr(vec_data(x)[ii],
expr = expr[ii],
class = "vexpr")
}
# Subsetting works!
x[2]
#> <vexpr[1]>
#> [1] 10^2L
# Seemingly sensible concatenation
c(x[2], NA)
#> <vexpr[2]>
#> [1] 10^2L <NA>
But also started to generate nonsensical results.
# expr is duplicated? would have liked the 2nd expression to be `expression(NA)`
attr(c(x[2], NA), "expr")
#> expression(10^2L, 10^2L)
Created on 2021-01-18 by the reprex package (v0.3.0)
Obviously, I'm doing something wrong here, but I haven't succeeded in debugging this problem. I have also tried implementing the vec_restore() method for vexpr, but this confused me even more. Have you seen nice implementations of parallel attribute vctrs somewhere? Do you know what I might be doing wrong?
Related question here: How do I build an object with the R vctrs package that can combine with c() (concatenating vctrs with attributes)
Related discussion here: https://github.com/r-lib/vctrs/issues/559
EDIT: I'm not married to the parallel attribute idea. If vec_data(x) would be an index into attr(x, "expr") that would work too, but I haven't managed this either.
EDIT2: Wrapping the expression in a list of calls seems to fit everything tidily. However, I'd still be interested in the parallel attribute / index attribute stability. Example of list wrapping (seemingly all methods work as they should!):
new_vexpr <- function(x) {
if (!is.expression(x)) {
x <- as.expression(x)
if (!is.expression(x)) {
stop()
}
}
x <- as.list(x)
new_vctr(x,
class = "vexpr")
}
as.expression.vexpr <- function(x) {
do.call(expression, vec_data(x))
}
