paleolimbot commented on a change in pull request #12324:
URL: https://github.com/apache/arrow/pull/12324#discussion_r806789891
##########
File path: r/R/array.R
##########
@@ -216,6 +216,76 @@ Array$create <- function(x, type = NULL) {
#' @include arrowExports.R
Array$import_from_c <- ImportArray
+
+#' Concatenate zero or more Arrays
+#'
+#' @param ... zero or more [Array] objects to concatenate
+#' @param type An optional `type` describing the desired
+#' type for the final Array.
+#'
+#' @return An [Array]
+#' @export
+#'
+#' @examples
+#' concat_arrays(Array$create(1:3), Array$create(4:5))
+#'
+concat_arrays <- function(..., type = NULL) {
+ dots <- lapply(list2(...), Array$create, type = type)
+
+ if (length(dots) == 0 && is.null(type)) {
+ return(Array$create(logical(), type = null()))
+ } else if (length(dots) == 0) {
+ return(Array$create(logical(), type = null())$cast(type))
+ }
+
+ if (!is.null(type)) {
+ dots <- lapply(dots, function(array) array$cast(type))
+ }
+
+ arrow__Concatenate(dots)
+}
+
+# The c() method uses non-standard dispatch in R
+# and has some peculiarities when multiple types are passed to ....
+# However, with a method defined for all subclasses of Array, it will
+# do what a user expects most of the time.
Review comment:
Good call! I thought this would fail:
``` r
library(arrow, warn.conflicts = FALSE)
arr1 <- Array$create(1:3)
arr2 <- Array$create(4:5)
c(arr1, arr2)
#> Array
#> <int32>
#> [
#> 1,
#> 2,
#> 3,
#> 4,
#> 5
#> ]
class(arr1) <- c("SomeSubclass", class(arr1))
class(arr2) <- c("SomeOtherSubclass", class(arr2))
c(arr1, arr2)
#> Array
#> <int32>
#> [
#> 1,
#> 2,
#> 3,
#> 4,
#> 5
#> ]
```
...but it didn't, and removing all the extra `c()` methods didn't cause any
of the tests to break, which means that for all practical purposes we don't
need them. A better example of the madness I was thinking of is this:
``` r
library(arrow, warn.conflicts = FALSE)
c(Array$create(1:3), wk::xy(1:2, 1:2))
#> Error: Invalid: arrays to be concatenated must be identically typed, but
int32 and list<item: double> were encountered.
c( wk::xy(1:2, 1:2), Array$create(1:3))
#> Error: Can't combine 'wk_rcrd' objects that do not have identical classes.
```
(where if more than one `c()` method is defined you'll get different results
depending on which one is first).
The "right way" to do this is a full vctrs implementation but I don't think
it's time for that quite yet.
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