ARROW-15712 [R] Add a type method for Expression objects

This would allow for more consistent syntax when extracting the type of an expression.
A block like this:
```r
if (inherits(x, "Expression")) {
class <- x$type()$ToString()
} else {
class <- type(x)$ToString()
}
```

would be simplified to

```r
class <- type(x)$ToString()
```

Closes apache#12447 from dragosmg/type_method_for_expression

Authored-by: Dragoș Moldovan-Grünfeld <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

r/NAMESPACE

@@ -96,6 +96,7 @@ S3method(tail,RecordBatchReader)
 S3method(tail,Scanner)
 S3method(tail,arrow_dplyr_query)
 S3method(type,ArrowDatum)
+S3method(type,Expression)
 S3method(type,default)
 S3method(unique,ArrowDatum)
 S3method(vec_ptype_abbr,arrow_fixed_size_binary)

r/R/type.R

@@ -77,6 +77,9 @@ type.default <- function(x) Array__infer_type(x)
 #' @export
 type.ArrowDatum <- function(x) x$type
 
+#' @export
+type.Expression <- function(x) x$type
+
 #----- metadata
 
 #' @title class arrow::FixedWidthType

r/tests/testthat/test-type.R

@@ -229,3 +229,13 @@ test_that("Type strings are correctly canonicalized", {
     "Unrecognized"
   )
 })
+
+test_that("type() gets the right type for Expression", {
+  x <- Expression$scalar(32L)
+  y <- Expression$scalar(10)
+  add_xy <- Expression$create("add", x, y)
+
+  expect_equal(x$type, type(x))
+  expect_equal(y$type, type(y))
+  expect_equal(add_xy$type, type(add_xy))
+})