ARROW-13886 [R] Expand documentation for decimal()

r/R/type.R

@@ -187,8 +187,11 @@ NestedType <- R6Class("NestedType", inherit = DataType)
 #' @param timezone For `timestamp()`, an optional time zone string.
 #' @param byte_width byte width for `FixedSizeBinary` type.
 #' @param list_size list size for `FixedSizeList` type.
-#' @param precision For `decimal()`, precision
-#' @param scale For `decimal()`, scale
+#' @param precision For `decimal()`, precision. The number of significant digits
+#'    the arrow `decimal` type can represent. Currently `decimal()` is mapped
+#'    to `DecimalType128`, having a maximum precision of 38 significant digits.
+#' @param scale For `decimal()`, scale. The number of digits after the decimal
+#'    point. It can be negative.
 #' @param type For `list_of()`, a data type to make a list-of-type
 #' @param ... For `struct()`, a named list of types to define the struct columns
 #'
@@ -360,6 +363,12 @@ decimal <- function(precision, scale) {
   } else {
     stop('"precision" must be an integer', call. = FALSE)
   }
+  if (precision > 38) {
+    stop('"precision" must be lower than or equal to 38', call. = FALSE)
+  }
+  if (precision < 1) {
+    stop('"precision" must be greater than 0', call. = FALSE)
+  }
   if (is.numeric(scale)) {
     scale <- as.integer(scale)
   } else {

r/tests/testthat/test-data-type.R

@@ -390,8 +390,8 @@ test_that("decimal type and validation", {
   expect_error(decimal(4))
   expect_error(decimal(4, "two"), '"scale" must be an integer')
   expect_error(decimal(NA, 2), '"precision" must be an integer')
-  expect_error(decimal(0, 2), "Invalid: Decimal precision out of range: 0")
-  expect_error(decimal(100, 2), "Invalid: Decimal precision out of range: 100")
+  expect_error(decimal(0, 2), "\"precision\" must be greater than 0")
+  expect_error(decimal(100, 2), "\"precision\" must be lower than or equal to 38")
   expect_error(decimal(4, NA), '"scale" must be an integer')
 
   expect_r6_class(decimal(4, 2), "Decimal128Type")