From 5b7f8139fc44e2df3adfb05535fd5872d9e37218 Mon Sep 17 00:00:00 2001
From: jgabry <jgabry@gmail.com>
Date: Mon, 14 Dec 2020 11:39:57 -0700
Subject: [PATCH 1/4] allow empty data list

fixes #401
---
 R/data.R                         |  3 +++
 tests/testthat/test-model-data.R | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/R/data.R b/R/data.R
index 3e38be3a6..6fb7852d5 100644
--- a/R/data.R
+++ b/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)) {
diff --git a/tests/testthat/test-model-data.R b/tests/testthat/test-model-data.R
index b591efe12..6f81c28e1 100644
--- a/tests/testthat/test-model-data.R
+++ b/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())
+})

From 2378182967a6a5496a4f6049823ca7c43f13210a Mon Sep 17 00:00:00 2001
From: jgabry <jgabry@gmail.com>
Date: Mon, 14 Dec 2020 12:03:08 -0700
Subject: [PATCH 2/4] add test

---
 tests/testthat/test-model-data.R | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/testthat/test-model-data.R b/tests/testthat/test-model-data.R
index 6f81c28e1..adba5fdd5 100644
--- a/tests/testthat/test-model-data.R
+++ b/tests/testthat/test-model-data.R
@@ -47,6 +47,10 @@ test_that("error if data contains NA elements", {
   expect_error(mod$sample(data = data_list3), "Data includes NA values")
 })
 
+test_that("empty data list converted to NULL", {
+  expect_null(process_data(list()))
+})
+
 test_that("empty data list doesn't error if no data block", {
   mod <- cmdstan_model(write_stan_file("
   parameters {

From e0d6ae0c00a8422bd63f92a0e0f709476e94a7e1 Mon Sep 17 00:00:00 2001
From: jgabry <jgabry@gmail.com>
Date: Mon, 14 Dec 2020 12:04:48 -0700
Subject: [PATCH 3/4] move test

---
 tests/testthat/test-data.R       | 4 ++++
 tests/testthat/test-model-data.R | 4 ----
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/tests/testthat/test-data.R b/tests/testthat/test-data.R
index 8fa0d3a0b..248421615 100644
--- a/tests/testthat/test-data.R
+++ b/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)))
diff --git a/tests/testthat/test-model-data.R b/tests/testthat/test-model-data.R
index adba5fdd5..6f81c28e1 100644
--- a/tests/testthat/test-model-data.R
+++ b/tests/testthat/test-model-data.R
@@ -47,10 +47,6 @@ test_that("error if data contains NA elements", {
   expect_error(mod$sample(data = data_list3), "Data includes NA values")
 })
 
-test_that("empty data list converted to NULL", {
-  expect_null(process_data(list()))
-})
-
 test_that("empty data list doesn't error if no data block", {
   mod <- cmdstan_model(write_stan_file("
   parameters {

From cd5a74b29b605b63a138969f322fff7238a72793 Mon Sep 17 00:00:00 2001
From: jgabry <jgabry@gmail.com>
Date: Mon, 14 Dec 2020 12:33:37 -0700
Subject: [PATCH 4/4] Update NEWS.md

---
 NEWS.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index 50808d8f0..74d1142a0 100644
--- a/NEWS.md
+++ b/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)