fix: list_transpose() takes into account all elements for the templ…

…ate (#1136)

Closes #1128.

NEWS.md

@@ -1,5 +1,8 @@
 # purrr (development version)
 
+* `list_transpose()` inspects all elements to determine the correct
+  template if it's not provided by the user  (#1128, @krlmlr).
+
 # purrr 1.0.2
 
 * Fixed valgrind issue.

R/list-transpose.R

@@ -14,9 +14,9 @@
 #' @param x A list of vectors to transpose.
 #' @param template A "template" that describes the output list. Can either be
 #'   a character vector (where elements are extracted by name), or an integer
-#'   vector (where elements are extracted by position). Defaults to the names
-#'   of the first element of `x`, or if they're not present, the integer
-#'   indices.
+#'   vector (where elements are extracted by position). Defaults to the union
+#'   of the names of the elements of `x`, or if they're not present, the
+#'   union of the integer indices.
 #' @param simplify Should the result be [simplified][list_simplify]?
 #'   * `TRUE`: simplify or die trying.
 #'   * `NA`: simplify if possible.
@@ -74,8 +74,19 @@ list_transpose <- function(x,
 
   if (length(x) == 0) {
     template <- integer()
-  } else {
-    template <- template %||% vec_index(x[[1]])
+  } else if (is.null(template)) {
+    indexes <- map(x, vec_index)
+    call <- current_env()
+    withCallingHandlers(
+      template <- reduce(indexes, vec_set_union),
+      vctrs_error_ptype2 = function(e) {
+        cli::cli_abort(
+          "Can't combine named and unnamed vectors.",
+          arg = template,
+          call = call
+        )
+      }
+    )
   }
 
   if (!is.character(template) && !is.numeric(template)) {

tests/testthat/_snaps/list-transpose.md

@@ -64,3 +64,11 @@
       Error in `list_transpose()`:
       ! `template` must be a character or numeric vector, not a function.
 
+# fail mixing named and unnamed vectors
+
+    Code
+      test_list_transpose()
+    Condition
+      Error in `list_transpose()`:
+      ! Can't combine named and unnamed vectors.
+

tests/testthat/test-list-transpose.R

@@ -13,7 +13,7 @@ test_that("can use character template", {
   # Default:
   expect_equal(
     list_transpose(x, default = NA),
-    list(a = c(1, NA), b = c(2, 3))
+    list(a = c(1, NA), b = c(2, 3), c = c(NA, 4))
   )
 
   # Change order
@@ -130,3 +130,13 @@ test_that("validates inputs", {
     list_transpose(list(1), template = mean)
   })
 })
+
+test_that("fail mixing named and unnamed vectors", {
+  test_list_transpose <- function() {
+    x <- list(list(a = 1, b = 2), list(a = 3, b = 4))
+    list_transpose(list(x = list(a = 1, b = 2), y = list(3, 4)))
+  }
+  expect_snapshot(error = TRUE, {
+    test_list_transpose()
+  })
+})