From 931d4ec971a6c002c7c6c537e91713ed608f44b6 Mon Sep 17 00:00:00 2001
From: Joe Thorley <joe@poissonconsulting.ca>
Date: Tue, 8 Sep 2026 12:15:41 -0700
Subject: [PATCH] Error informatively when xobs_only() arguments are named or
 unknown

A named argument inside xobs_only() failed with an internal stopifnot
from xcast() and a dplyr cross-join deprecation warning. Names are now
rejected up front, since a new column has no observed combinations to
preserve, and semi_crossing() errors when an argument does not refer
to a column of .data instead of falling through to an empty join.

Closes #111.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 R/xobs-only.R                      | 25 +++++++++++++++++++++--
 man/xobs_only.Rd                   |  5 ++++-
 tests/testthat/_snaps/xobs-only.md | 23 +++++++++++++++++++++
 tests/testthat/test-xobs-only.R    | 32 ++++++++++++++++++++++++++++++
 4 files changed, 82 insertions(+), 3 deletions(-)
 create mode 100644 tests/testthat/_snaps/xobs-only.md
 create mode 100644 tests/testthat/test-xobs-only.R

diff --git a/R/xobs-only.R b/R/xobs-only.R
index db7d620..09457a5 100644
--- a/R/xobs-only.R
+++ b/R/xobs-only.R
@@ -1,6 +1,9 @@
 #' Generate Observed Combinations Only
 #'
-#' @param ... One or more variables to generate combinations for.
+#' @param ... One or more unnamed variables in `.data` to generate observed
+#'   combinations for. Naming an argument is an error as a new column has no
+#'   observed combinations to preserve; use a named argument to [xnew_data()]
+#'   instead.
 #' @param .length_out A count to override the default length of sequences.
 #' @inheritParams xcast
 #' @return A tibble of the observed combinations of the variables.
@@ -19,6 +22,15 @@
 xobs_only <- function(..., .length_out = NULL, .data = xnew_data_env$data) {
   quos <- enquos(...)
 
+  named <- names2(quos)[nzchar(names2(quos))]
+  if (length(named)) {
+    err(
+      "`xobs_only()` arguments must not be named (",
+      cc(named, " and "),
+      ") as observed combinations must refer to columns of `.data`"
+    )
+  }
+
   translated <- map(quos, quo_translate_xobs_only, .length_out)
 
   out <- semi_crossing(!!!translated)
@@ -46,5 +58,14 @@ semi_crossing <- function(..., .data = xnew_data_env$data) {
   }
 
   out <- tidyr::crossing(...)
-  dplyr::semi_join(out, .data, by = intersect(names(out), names(.data)))
+
+  missing <- setdiff(names(out), names(.data))
+  if (length(missing)) {
+    err(
+      "`xobs_only()` arguments must refer to columns of `.data` (unrecognised: ",
+      cc(missing, " and "),
+      ")"
+    )
+  }
+  dplyr::semi_join(out, .data, by = names(out))
 }
diff --git a/man/xobs_only.Rd b/man/xobs_only.Rd
index 068996c..747548e 100644
--- a/man/xobs_only.Rd
+++ b/man/xobs_only.Rd
@@ -7,7 +7,10 @@
 xobs_only(..., .length_out = NULL, .data = xnew_data_env$data)
 }
 \arguments{
-\item{...}{One or more variables to generate combinations for.}
+\item{...}{One or more unnamed variables in \code{.data} to generate observed
+combinations for. Naming an argument is an error as a new column has no
+observed combinations to preserve; use a named argument to \code{\link[=xnew_data]{xnew_data()}}
+instead.}
 
 \item{.length_out}{A count to override the default length of sequences.}
 
diff --git a/tests/testthat/_snaps/xobs-only.md b/tests/testthat/_snaps/xobs-only.md
new file mode 100644
index 0000000..0b40893
--- /dev/null
+++ b/tests/testthat/_snaps/xobs-only.md
@@ -0,0 +1,23 @@
+# xobs_only errors informatively for named and unknown arguments (#111)
+
+    Code
+      xnew_data(data, xobs_only(z = b))
+    Condition
+      Error:
+      ! `xobs_only()` arguments must not be named ('z') as observed combinations must refer to columns of `.data`.
+    Code
+      xnew_data(data, xobs_only(a, z = new_seq(b, .length_out = 2)))
+    Condition
+      Error:
+      ! `xobs_only()` arguments must not be named ('z') as observed combinations must refer to columns of `.data`.
+    Code
+      xnew_data(data, xobs_only(b = b))
+    Condition
+      Error:
+      ! `xobs_only()` arguments must not be named ('b') as observed combinations must refer to columns of `.data`.
+    Code
+      xnew_data(data, xobs_only(new_seq(b, .length_out = 2)))
+    Condition
+      Error:
+      ! `xobs_only()` arguments must refer to columns of `.data` (unrecognised: 'new_seq(b, .length_out = 2)').
+
diff --git a/tests/testthat/test-xobs-only.R b/tests/testthat/test-xobs-only.R
new file mode 100644
index 0000000..f6c8b6a
--- /dev/null
+++ b/tests/testthat/test-xobs-only.R
@@ -0,0 +1,32 @@
+test_that("xobs_only preserves observed combinations", {
+  data <- tibble::tibble(
+    a = c(1.5, 2.5, 3.5),
+    b = factor(c("a", "a", "b"), levels = c("a", "b", "c"))
+  )
+
+  new_data <- xnew_data(data, xobs_only(a, b))
+  expect_identical(new_data$a, data$a)
+  expect_identical(new_data$b, data$b)
+
+  new_data <- xnew_data(data, xobs_only(a, xnew_seq(b, .length_out = 1)))
+  expect_identical(new_data$a, c(1.5, 2.5))
+  expect_identical(new_data$b, factor(c("a", "a"), levels = c("a", "b", "c")))
+})
+
+test_that("xobs_only errors informatively for named and unknown arguments (#111)", {
+  data <- tibble::tibble(
+    a = 1:5 + 0.5,
+    b = factor(letters[1:5])
+  )
+
+  expect_snapshot(error = TRUE, {
+    xnew_data(data, xobs_only(z = b))
+    xnew_data(data, xobs_only(a, z = new_seq(b, .length_out = 2)))
+    xnew_data(data, xobs_only(b = b))
+    xnew_data(data, xobs_only(new_seq(b, .length_out = 2)))
+  })
+
+  # naming the whole xobs_only() call still creates a new column
+  new_data <- xnew_data(data, z = xobs_only(b))
+  expect_named(new_data, c("a", "b", "z"))
+})