is_chain_list: return FALSE for empty lists

NEWS.md

@@ -1,5 +1,6 @@
 # bayesplot (development version)
 
+* Fixed `is_chain_list()` to correctly reject empty lists instead of silently returning `TRUE`.
 * Added unit tests for `mcmc_areas_ridges_data()`, `mcmc_parcoord_data()`, and `mcmc_trace_data()`.
 * Added unit tests for `ppc_error_data()` and `ppc_loo_pit_data()` covering output structure, argument handling, and edge cases.
 * Added vignette sections demonstrating `*_data()` companion functions for building custom ggplot2 visualizations (#435)

R/helpers-mcmc.R

@@ -247,6 +247,9 @@ df_with_chain2array <- function(x) {
 #' @param x object to check
 #' @return TRUE or FALSE
 is_chain_list <- function(x) {
+  if (length(x) == 0) {
+    return(FALSE)
+  }
   check1 <- !is.data.frame(x) && is.list(x)
   dims <- try(sapply(x, function(chain) length(dim(chain))), silent=TRUE)
   if (inherits(dims, "try-error")) {

tests/testthat/test-helpers-mcmc.R

@@ -156,6 +156,7 @@ test_that("is_chain_list works", {
   expect_true(is_chain_list(chainlist))
   expect_true(is_chain_list(chainlist1))
   expect_true(is_chain_list(chainlist1chain))
+  expect_false(is_chain_list(list()))
 })
 
 test_that("validate_chain_list works", {