paleolimbot commented on code in PR #13786:
URL: https://github.com/apache/arrow/pull/13786#discussion_r959554332


##########
r/tests/testthat/test-dplyr-mutate.R:
##########
@@ -589,3 +588,106 @@ test_that("mutate() and transmute() with namespaced 
functions", {
     tbl
   )
 })
+
+test_that("Can use across() within mutate()", {
+  compare_dplyr_binding(
+    .input %>%
+      mutate(across(c(dbl, dbl2), round)) %>%
+      collect(),
+    tbl
+  )
+
+  compare_dplyr_binding(
+    .input %>%
+      mutate(across(c(dbl, dbl2), list(exp, sqrt))) %>%
+      collect(),
+    tbl
+  )
+
+  # across() arguments not in default order
+  compare_dplyr_binding(
+    .input %>%
+      mutate(across(.fns = round, c(dbl, dbl2))) %>%
+      collect(),
+    tbl
+  )
+
+  # ARROW-17364: .names argument not yet supported for across()
+  expect_error(
+    tbl %>%
+      arrow_table() %>%
+      mutate(across(c(dbl, dbl2), round, .names = "{.col}.{.fn}")) %>%
+      collect(),
+    regexp = "`.names` argument to `across()` not yet supported in Arrow",
+    fixed = TRUE
+  )
+
+  # ellipses (...) are a deprecated argument
+  expect_error(
+    tbl %>%
+      arrow_table() %>%
+      mutate(across(c(dbl, dbl2), round, digits = -1)) %>%
+      collect(),
+    regexp = "`...` argument to `across()` is deprecated in dplyr and not 
supported in Arrow",
+    fixed = TRUE
+  )
+
+  # alternative ways of specifying .fns - as a list
+  compare_dplyr_binding(
+    .input %>%
+      mutate(across(1:dbl2, list(round))) %>%
+      collect(),
+    tbl
+  )
+

Review Comment:
   Sorry for missing this comment...I think it's a good test to have in there 
because you're walking the syntax tree yourself. When I wrote this, I was 
worried somebody would get something like `object of type LANGSXP is not 
subsettable` or something.



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