New empty_assignment_linter

DESCRIPTION

@@ -79,6 +79,7 @@ Collate:
     'declared_functions.R'
     'deprecated.R'
     'duplicate_argument_linter.R'
+    'empty_assignment_linter.R'
     'equals_na_linter.R'
     'exclude.R'
     'expect_comparison_linter.R'

NAMESPACE

@@ -42,6 +42,7 @@ export(default_settings)
 export(default_undesirable_functions)
 export(default_undesirable_operators)
 export(duplicate_argument_linter)
+export(empty_assignment_linter)
 export(equals_na_linter)
 export(expect_comparison_linter)
 export(expect_identical_linter)

NEWS.md

@@ -49,6 +49,8 @@
 
 * `for_loop_index_linter()` to prevent overwriting local variables in a `for` loop declared like `for (x in x) { ... }` (@MichaelChirico)
 
+* `empty_assignment_linter()` for identifying empty assignments like `x = {}` that are more clearly written as `x = NULL` (@MichaelChirico)
+
 ## Notes
 
 * `lint()` continues to support Rmarkdown documents. For users of custom .Rmd engines, e.g.

R/empty_assignment_linter.R

@@ -0,0 +1,62 @@
+#' Block assignment of `{}`
+#'
+#' Assignment of `{}` is the same as assignment of `NULL`; use the latter
+#'   for clarity. Closely related: [unneeded_concatenation_linter()].
+#'
+#' @examples
+#' # will produce lints
+#' lint(
+#'   text = "x <- {}",
+#'   linters = empty_assignment_linter()
+#' )
+#'
+#' cat("x = {\n}")
+#' lint(
+#'   text = "x = {\n}",
+#'   linters = empty_assignment_linter()
+#' )
+#'
+#' # okay
+#' lint(
+#'   text = "x <- { 3 + 4 }",
+#'   linters = empty_assignment_linter()
+#' )
+#'
+#' lint(
+#'   text = "x <- NULL",
+#'   linters = empty_assignment_linter()
+#' )
+#'
+#' @evalRd rd_tags("empty_assignment_linter")
+#' @seealso [linters] for a complete list of linters available in lintr.
+#' @export
+empty_assignment_linter <- function() {
+  # for some reason, the parent in the `=` case is <equal_assign>, not <expr>, hence parent::expr
+  xpath <- "
+  //OP-LEFT-BRACE[following-sibling::*[1][self::OP-RIGHT-BRACE]]
+    /parent::expr[
+      preceding-sibling::LEFT_ASSIGN
+      or preceding-sibling::EQ_ASSIGN
+      or following-sibling::RIGHT_ASSIGN
+    ]
+    /parent::*
+  "
+
+  Linter(function(source_expression) {
+    if (!is_lint_level(source_expression, "expression")) {
+      return(list())
+    }
+
+    xml <- source_expression$xml_parsed_content
+
+    bad_expr <- xml2::xml_find_all(xml, xpath)
+
+    xml_nodes_to_lints(
+      bad_expr,
+      source_expression = source_expression,
+      lint_message =
+        "Assign NULL explicitly or, whenever possible, allocate the empty object with the right type & size.",
+      type = "warning"
+    )
+  })
+}

inst/lintr/linters.csv

@@ -15,6 +15,7 @@ conjunct_test_linter,package_development best_practices readability
 consecutive_stopifnot_linter,style readability consistency
 cyclocomp_linter,style readability best_practices default configurable
 duplicate_argument_linter,correctness common_mistakes configurable
+empty_assignment_linter,readability best_practices
 equals_na_linter,robustness correctness common_mistakes default
 expect_comparison_linter,package_development best_practices
 expect_identical_linter,package_development

tests/testthat/test-empty_assignment_linter.R

@@ -0,0 +1,30 @@
+test_that("empty_assignment_linter skips valid usage", {
+  expect_lint("x <- { 3 + 4 }", NULL, empty_assignment_linter())
+  expect_lint("x <- if (x > 1) { 3 + 4 }", NULL, empty_assignment_linter())
+
+  # also triggers assignment_linter
+  expect_lint("x = { 3 + 4 }", NULL, empty_assignment_linter())
+})
+
+test_that("empty_assignment_linter blocks disallowed usages", {
+  linter <- empty_assignment_linter()
+  lint_msg <- rex::rex("Assign NULL explicitly or, whenever possible, allocate the empty object")
+
+  expect_lint("xrep <- {}", lint_msg, linter)
+
+  # assignment with equal works as well, and white space doesn't matter
+  expect_lint("x = {   }", lint_msg, linter)
+
+  # ditto right assignments
+  expect_lint("{} -> x", lint_msg, linter)
+  expect_lint("{} ->> x", lint_msg, linter)
+
+  # ditto data.table-style walrus assignments
+  expect_lint("x[, col := {}]", lint_msg, linter)
+
+  # newlines also don't matter
+  expect_lint("x <- {\n}", lint_msg, linter)
+
+  # LHS of assignment doesn't matter
+  expect_lint("env$obj <- {}", lint_msg, linter)
+})