nealrichardson commented on code in PR #12817:
URL: https://github.com/apache/arrow/pull/12817#discussion_r852392032
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r/R/array.R:
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@@ -217,6 +217,93 @@ Array$create <- function(x, type = NULL) {
Array$import_from_c <- ImportArray
+#' Convert an object to an Arrow Array
+#'
+#' Whereas `Array$create()` constructs an [Array] from the built-in data types
+#' for which the Arrow package implements fast converters, `as_arrow_array()`
+#' provides a means by which other packages can define conversions to Arrow
+#' objects.
+#'
+#' @param x An object to convert to an Arrow Array
+#' @param ... Passed to S3 methods
+#' @param type A [type][data-type] for the final Array. A value of `NULL`
+#' will default to the type guessed by [type()].
+#'
+#' @return An [Array].
+#' @export
+#'
+#' @examplesIf arrow_available()
+#' as_arrow_array(1:5)
+#'
+as_arrow_array <- function(x, ..., type = NULL) {
+ UseMethod("as_arrow_array")
+}
+
+#' @rdname as_arrow_array
+#' @export
+as_arrow_array.Array <- function(x, ..., type = NULL) {
+ if (is.null(type)) {
+ x
+ } else {
+ x$cast(type)
+ }
+}
+
+#' @rdname as_arrow_array
+#' @export
+as_arrow_array.ChunkedArray <- function(x, ..., type = NULL) {
+ concat_arrays(!!! x$chunks, type = type)
+}
+
+#' @rdname as_arrow_array
+#' @export
+as_arrow_array.vctrs_vctr <- function(x, ..., type = NULL) {
+ if (is.null(type)) {
+ vctrs_extension_array(x)
+ } else if (inherits(type, "VctrsExtensionType")) {
+ vctrs_extension_array(
+ x,
+ ptype = type$ptype(),
+ storage_type = type$storage_type()
+ )
+ } else {
+ NextMethod()
+ }
+}
+
+#' @export
+as_arrow_array.POSIXlt <- function(x, ..., type = NULL) {
+ as_arrow_array.vctrs_vctr(x, ..., type = type)
+}
+
+
+#' @export
+as_arrow_array.default <- function(x, ..., type = NULL, from_constructor =
FALSE) {
+ # If from_constructor is TRUE, this is a call from C++ for which S3 dispatch
+ # failed to find a method for the object. If this is the case, we error.
+ if (from_constructor && is.null(type)) {
+ abort(
+ sprintf(
+ "Can't create Array from object of type %s",
+ paste(class(x), collapse = " / ")
+ )
+ )
+ } else if (from_constructor) {
+ abort(
+ sprintf(
+ "Can't create Array<%s> from object of type %s",
+ format(type$code()),
+ paste(class(x), collapse = " / ")
+ )
+ )
+ }
+
+ # If from_constructor is FALSE, we use the built-in logic exposed by
+ # Array$create(). If there is no built-in conversion, C++ will call back
+ # here with from_constructor = TRUE to generate a nice error message.
Review Comment:
What happens if you don't error if `from_constructor = TRUE`?
I think I understand how you get here now, but I don't know that I'll
remember next time I look at this. IIUC it's because the extension type handler
is calling as_arrow_array() from C++, and if you have implemented
as_arrow_array.YourType, you won't hit this default method. But if you do, it
means that we don't know what to do with this extension type, so we error.
Right?
But aren't all extension types based on regular types, which we can convert?
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