Don't handle 0-length vectors like NULL

Fixes #480

NEWS.md

@@ -11,6 +11,9 @@
 
 ## Features and fixes
 
+* `pluck()` no longer replaces 0-length vectors with `default`; it now
+  only applies absent and `NULL` components.
+
 * `map2()` and `pmap()` now recycle names of their first input if
   needed (#783).
 

R/pluck.R

@@ -3,7 +3,7 @@
 #' `pluck()` and `chuck()` implement a generalised form of `[[` that
 #' allow you to index deeply and flexibly into data structures.
 #' `pluck()` consistently returns `NULL` when an element does not
-#' exist, `chuck()` always throws (or chucks) an error in that case.
+#' exist while `chuck()` always throws (or chucks) an error.
 #'
 #' @param .x,x A vector or environment
 #' @param ... A list of accessors for indexing into the object. Can be
@@ -15,7 +15,7 @@
 #'   [Dynamic dots][rlang::dyn-dots] are supported. In particular, if
 #'   your accessors are stored in a list, you can splice that in with
 #'   `!!!`.
-#' @param .default Value to use if target is empty or absent.
+#' @param .default Value to use if target is `NULL` or absent.
 #'
 #' @details
 #' * You can pluck or chuck with standard accessors like integer

src/pluck.c

@@ -256,7 +256,7 @@ SEXP pluck_impl(SEXP x, SEXP index, SEXP missing, SEXP strict_arg) {
 
  end:
   UNPROTECT(1);
-  return (Rf_length(x) == 0) ? missing : x;
+  return Rf_isNull(x) ? missing : x;
 }
 
 

tests/testthat/test-as-mapper.R

@@ -28,13 +28,14 @@ test_that(".default replaces absent values", {
   expect_equal(map_dbl(x, "b", .default = NA), c(2, NA, NA))
 })
 
-test_that(".default replaces elements with length 0", {
+test_that(".default only replaces NULL elements", {
   x <- list(
     list(a = 1),
+    list(a = numeric()),
     list(a = NULL),
-    list(a = numeric())
+    list()
   )
-  expect_equal(map_dbl(x, "a", .default = NA), c(1, NA, NA))
+  expect_equal(map(x, "a", .default = NA), list(1, numeric(), NA, NA))
 })
 
 test_that("Additional arguments are ignored", {
@@ -75,3 +76,4 @@ test_that("complex types aren't supported for indexing", {
 test_that("raw vectors are supported for indexing", {
   expect_equal( as_mapper(1)(raw(2)), raw(1) )
 })
+

tests/testthat/test-pluck.R

@@ -59,15 +59,17 @@ test_that("special indexes never match", {
 })
 
 test_that("special values return NULL", {
-  # unnamed input
+  # absent name
   expect_null(pluck(list(1, 2), "a"))
+  expect_null(pluck(list(a = 1, b = 2), "c"))
 
-  # zero length input
-  expect_null(pluck(integer(), 1))
-
-  # past end
+  # outside of range
+  expect_null(pluck(1:4, 0))
   expect_null(pluck(1:4, 10))
-  expect_null(pluck(1:4, 10L))
+})
+
+test_that("can pluck 0-lengths object", {
+  expect_equal(pluck(list(integer()), 1), integer())
 })
 
 test_that("handles weird names", {