ARROW-13853: [R] String to_title, to_lower, to_upper kernels

r/R/dplyr-functions.R

@@ -330,6 +330,37 @@ arrow_string_join_function <- function(null_handling, null_replacement = NULL) {
   }
 }
 
+# Currently, Arrow does not supports a locale option for string case conversion
+# functions, contrast to stringr's API, so the 'locale' argument is only valid
+# for stringr's default value ("en"). The following are string functions that
+# take a 'locale' option as its second argument:
+#   str_to_lower
+#   str_to_upper
+#   str_to_title
+#
+# Arrow locale will be supported with ARROW-14126
+stop_if_locale_provided <- function(locale) {
+  if (!identical(locale, "en")) {
+    stop("Providing a value for 'locale' other than the default ('en') is not supported by Arrow. ",
+    "To change locale, use 'Sys.setlocale()'", call. = FALSE)
+  }
+}
+
+nse_funcs$str_to_lower <- function(string, locale = "en") {
+  stop_if_locale_provided(locale)
+  Expression$create("utf8_lower", string)
+}
+
+nse_funcs$str_to_upper <- function(string, locale = "en") {
+  stop_if_locale_provided(locale)
+  Expression$create("utf8_upper", string)
+}
+
+nse_funcs$str_to_title <- function(string, locale = "en") {
+  stop_if_locale_provided(locale)
+  Expression$create("utf8_title", string)
+}
+
 nse_funcs$str_trim <- function(string, side = c("both", "left", "right")) {
   side <- match.arg(side)
   trim_fun <- switch(side,

r/R/expression.R

@@ -50,9 +50,9 @@
   "str_length" = "utf8_length",
   # str_pad is defined in dplyr-functions.R
   # str_sub is defined in dplyr-functions.R
-  "str_to_lower" = "utf8_lower",
-  "str_to_title" = "utf8_title",
-  "str_to_upper" = "utf8_upper",
+  # str_to_lower is defined in dplyr-functions.R
+  # str_to_title is defined in dplyr-functions.R
+  # str_to_upper is defined in dplyr-functions.R
   # str_trim is defined in dplyr-functions.R
   "stri_reverse" = "utf8_reverse",
   # substr is defined in dplyr-functions.R

r/tests/testthat/test-dplyr-string-functions.R

@@ -467,6 +467,27 @@ test_that("strsplit and str_split", {
   )
 })
 
+test_that("str_to_lower, str_to_upper, and str_to_title", {
+  df <- tibble(x = c("foo1", " \tB a R\n", "!apACHe aRroW!"))
+  expect_dplyr_equal(
+    input %>%
+      transmute(
+        x_lower = str_to_lower(x),
+        x_upper = str_to_upper(x),
+        x_title = str_to_title(x)
+      ) %>%
+      collect(),
+    df
+  )
+
+  # Error checking a single function because they all use the same code path.
+  expect_error(
+    nse_funcs$str_to_lower("Apache Arrow", locale = "sp"),
+    "Providing a value for 'locale' other than the default ('en') is not supported by Arrow",
+    fixed = TRUE
+  )
+})
+
 test_that("arrow_*_split_whitespace functions", {
   # use only ASCII whitespace characters
   df_ascii <- tibble(x = c("Foo\nand bar", "baz\tand qux and quux"))