Fix vroom_lines to respect n_max

Previously delim == '\n', which meant that the file had only delimiters
and no newlines, so n_max was never decremented internally. Switching
the delimiter to '\1', which should never appear in a file fixes this.

Fixes #142

NEWS.md

@@ -1,5 +1,7 @@
 # vroom (development)
 
+* `vroom_lines()` now properly respects the `n_max` parameter (#142)
+
 * The integer parser now returns NA values for invalid inputs (#135)
 
 * The column created by `id` is now stored as an run length encoded Altrep

R/vroom_lines.R

@@ -27,7 +27,11 @@ vroom_lines <- function(file, n_max = Inf, skip = 0, altrep_opts = "chr",
 
   col_select <- rlang::quo(NULL)
 
-  out <- vroom_(file, delim = "\n", col_names = "V1", col_types = "c",
+  # delim = "\1" sets the delimiter to be start of header, which should never
+  # appear in modern text. This essentially means the only record breaks will
+  # be newlines. Ideally this would be "\0", but R doesn't let you have nulls
+  # in character vectors.
+  out <- vroom_(file, delim = "\1", col_names = "V1", col_types = "c",
     id = NULL, skip = skip, col_select = col_select, na = character(), quote = "",
     trim_ws = FALSE, escape_double = FALSE, escape_backslash = FALSE, comment = "",
     locale = default_locale(),

tests/testthat/test-vroom_lines.R

@@ -46,3 +46,8 @@ test_that("vroom_lines works with files with no trailing newline", {
   writeBin(charToRaw("foo\nbar"), f2)
   expect_equal(vroom_lines(f2), c("foo", "bar"))
 })
+
+test_that("vroom_lines respects n_max", {
+  infile <- vroom_example("mtcars.csv")
+  expect_equal(vroom_lines(infile, n_max = 2), readLines(infile, n = 2))
+})