[R] Reconsider behavior of as.<type>.ArrowDatum functions

[asfimport]

As discussed at #9942 (comment) the as.double(), as.integer(), and as.character() methods for ArrowDatum return R vectors of the specified R types, whereas in dplyr, these same functions perform casts to the analogous Arrow types 

Compare the definitions:

• ArrowDatum methods:

  arrow/r/R/arrow-datum.R

  Lines 139 to 145 in ace2bfc

  	as.double.ArrowDatum <- function(x, ...) as.double(as.vector(x), ...)
  	#' @export
  	as.integer.ArrowDatum <- function(x, ...) as.integer(as.vector(x), ...)
  	#' @export
  	as.character.ArrowDatum <- function(x, ...) as.character(as.vector(x), ...)

• dplyr functions:

  arrow/r/R/dplyr.R

  Lines 399 to 432 in f2db785

  	as.character = function(x) {
  	FUN("cast", x, options = cast_options(to_type = string()))
  	},
  	as.double = function(x) {
  	FUN("cast", x, options = cast_options(to_type = float64()))
  	},
  	as.integer = function(x) {
  	FUN(
  	"cast",
  	x,
  	options = cast_options(
  	to_type = int32(),
  	allow_float_truncate = TRUE,
  	allow_decimal_truncate = TRUE
  	)
  	)
  	},
  	as.integer64 = function(x) {
  	FUN(
  	"cast",
  	x,
  	options = cast_options(
  	to_type = int64(),
  	allow_float_truncate = TRUE,
  	allow_decimal_truncate = TRUE
  	)
  	)
  	},
  	as.logical = function(x) {
  	FUN("cast", x, options = cast_options(to_type = boolean()))
  	},
  	as.numeric = function(x) {
  	FUN("cast", x, options = cast_options(to_type = float64()))
  	},

  Consider whether the ArrowDatum methods should instead perform casts but keep the data in Arrow so that the user would have to also call as.vector() to return the data as an R vector.

Reporter: Ian Cook / @ianmcook
Assignee: Ian Cook / @ianmcook

Related issues:

• [R] Bindings for basic type convert functions in dplyr verbs (is related to)

_{Note: This issue was originally created as ARROW-12292. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
We might also want to define ArrowDatum methods for dplyr's pull() and collect() generics. These would do the same thing as as.vector() when dplyr is loaded.

[asfimport]

Ian Cook / @ianmcook:
Closing this for now because the current behavior seems fine. Can reopen later if we decide to reconsider this.