allow empty data list

NEWS.md

@@ -5,6 +5,10 @@
 * Fixed reading inverse mass matrix with values written in scientific format in
 the CSV. (#394)
 
+* Fixed error caused by an empty data list. Previously if a model didn't require
+data then `data` had to either be NULL or be a non-empty list, but now `list()`
+is allowed. (#403)
+
 ### New features
 
 * Added `$sample_mpi()` for MCMC sampling with MPI. (#350)

R/data.R

@@ -84,6 +84,9 @@ list_to_array <- function(x, name = NULL) {
 #'   CmdStan, or a named list of \R objects to pass to [write_stan_json()].
 #' @return Path to data file.
 process_data <- function(data) {
+  if (length(data) == 0) {
+    data <- NULL
+  }
   if (is.null(data)) {
     path <- data
   } else if (is.character(data)) {

tests/testthat/test-data.R

@@ -7,6 +7,10 @@ if (not_on_cran()) {
   fit_optimize <- testing_fit("bernoulli", method = "optimize", seed = 123)
 }
 
+test_that("empty data list converted to NULL", {
+  expect_null(process_data(list()))
+})
+
 test_that("NAs detected in data list", {
   expect_false(any_na_elements(list(y = 1)))
   expect_true(any_na_elements(list(y = 1, N = NA)))

tests/testthat/test-model-data.R

@@ -46,3 +46,21 @@ test_that("error if data contains NA elements", {
   expect_error(mod$sample(data = data_list2), "Data includes NA values")
   expect_error(mod$sample(data = data_list3), "Data includes NA values")
 })
+
+test_that("empty data list doesn't error if no data block", {
+  mod <- cmdstan_model(write_stan_file("
+  parameters {
+    real x;
+  }
+  model {
+    x ~ normal(0, 1);
+  }
+  "))
+
+  expect_sample_output(
+    fit <- mod$sample(data = list(), chains = 1)
+  )
+
+  # would error if fitting failed
+  expect_silent(fit$draws())
+})