GH-35806: [R] Improve error message for null type inference with sparse CSV data

r/R/util.R

@@ -196,6 +196,18 @@ repeat_value_as_array <- function(object, n) {
 }
 
 handle_csv_read_error <- function(msg, call, schema) {
+  if (grepl("conversion error to null", msg)) {
+    msg <- c(
+      msg,
+      i = paste(
+        "If you have not specified the schema, this error may be due to the column type being",
+        "inferred as `null` because the first block of data contained only missing values.",
+        "See `?csv_read_options` for how to set a smaller value or specify a schema if you know the correct types."
+      )
+    )
+    abort(msg, call = call)
+  }
+
   if (grepl("conversion error", msg) && inherits(schema, "Schema")) {
     msg <- c(
       msg,

r/tests/testthat/test-dataset-csv.R

@@ -711,3 +711,21 @@ test_that("open_dataset() with `decimal_point` argument", {
     tibble(x = 1.2, y = "c")
   )
 })
+
+test_that("more informative error when column inferred as null due to sparse data (GH-35806)", {
+  tf <- tempfile()
+  on.exit(unlink(tf))
+
+  writeLines(c("x,y", paste0(1:100, ",")), tf)
+  write("101,foo", tf, append = TRUE)
+
+  expect_error(
+    open_dataset(
+      tf,
+      format = "csv",
+      read_options = csv_read_options(block_size = 100L)
+    ) |>
+      collect(),
+    "column type inferred as"
+  )
+})