combine() returns logical() (closes tidyverse#3365)

NEWS.md

@@ -69,6 +69,8 @@
 
 * Added an `.onDetach()` hook that allows for plyr to be loaded and attached without the warning message that says functions in dplyr will be masked, since dplyr is no longer attached (#3359, @jwnorman).
 
+* `combine()` returns `logical()` when all inputs are `NULL` (or when there are no inputs) (#3365, @zeehio).
+
 # dplyr 0.7.4
 
 * Fix recent Fedora and ASAN check errors (#3098).

src/bind.cpp

@@ -399,7 +399,7 @@ List cbind_all(List dots) {
 // [[Rcpp::export]]
 SEXP combine_all(List data) {
   int nv = data.size();
-  if (nv == 0) stop("combine_all needs at least one vector");
+  if (nv == 0) return LogicalVector();
 
   // get the size of the output
   int n = 0;
@@ -412,7 +412,7 @@ SEXP combine_all(List data) {
   for (; i < nv; i++) {
     if (!Rf_isNull(data[i])) break;
   }
-  if (i == nv) stop("no data to combine, all elements are NULL");
+  if (i == nv) return LogicalVector();
 
   // collect
   boost::scoped_ptr<Collecter> coll(collecter(data[i], n));

tests/testthat/test-combine.R

@@ -1,14 +1,10 @@
 context("combine")
 
-test_that("combine handles NULL (1596)", {
+test_that("combine handles NULL (#1596, #3365)", {
   expect_equal(combine(list(NULL, 1, 2)), c(1, 2))
   expect_equal(combine(list(1, NULL, 2)), c(1, 2))
   expect_equal(combine(list(1, 2, NULL)), c(1, 2))
-  expect_error(
-    combine(list(NULL, NULL)),
-    "no data to combine, all elements are NULL",
-    fixed = TRUE
-  )
+  expect_equal(combine(list(NULL)), logical())
 })
 
 test_that("combine complains about incompatibilites", {