New function_brace_linter

DESCRIPTION

@@ -77,6 +77,7 @@ Collate:
     'expect_type_linter.R'
     'extract.R'
     'extraction_operator_linter.R'
+    'function_brace_linter.R'
     'function_left_parentheses.R'
     'get_source_expressions.R'
     'ids_with_token.R'

NAMESPACE

@@ -44,6 +44,7 @@ export(expect_s4_class_linter)
 export(expect_true_false_linter)
 export(expect_type_linter)
 export(extraction_operator_linter)
+export(function_brace_linter)
 export(function_left_parentheses_linter)
 export(get_source_expressions)
 export(ids_with_token)

R/function_brace_linter.R

@@ -0,0 +1,28 @@
+#' Require multi-line functions to use braces
+#'
+#' This linter catches function definitions spanning multiple lines of code
+#'   that aren't wrapped in braces
+#'
+#' @evalRd rd_tags("function_brace_linter")
+#' @seealso [linters] for a complete list of linters available in lintr.
+#' @export
+function_brace_linter <- function() {
+  Linter(function(source_file) {
+    if (length(source_file$xml_parsed_content) == 0L) {
+      return(list())
+    }
+
+    xml <- source_file$xml_parsed_content
+
+    bad_expr_xpath <- "//expr[FUNCTION and @line1 != @line2 and not(expr[OP-LEFT-BRACE])]"
+    bad_expr <- xml2::xml_find_all(xml, bad_expr_xpath)
+
+    return(lapply(
+      bad_expr,
+      xml_nodes_to_lint,
+      source_file = source_file,
+      lint_message = "Any function spanning multiple lines must use curly braces.",
+      type = "warning"
+    ))
+  })
+}

R/zzz.R

@@ -89,6 +89,7 @@ default_linters <- with_defaults(
   commented_code_linter(),
   cyclocomp_linter(),
   equals_na_linter(),
+  function_brace_linter(),
   function_left_parentheses_linter(),
   if_else_match_braces_linter(),
   infix_spaces_linter(),

inst/lintr/linters.csv

@@ -22,6 +22,7 @@ expect_s4_class_linter,package_development best_practices
 expect_true_false_linter,package_development best_practices readability
 expect_type_linter,package_development best_practices
 extraction_operator_linter,style best_practices
+function_brace_linter,default style readability
 function_left_parentheses_linter,style readability default
 if_else_match_braces_linter,default style readability
 implicit_integer_linter,style consistency best_practices

tests/testthat/default_linter_testcode.R

@@ -29,6 +29,10 @@ someComplicatedFunctionWithALongCamelCaseName <- function(x)
 # vector_logic
 if (1 & 2) FALSE else TRUE
 
+# function_brace
+my_metric <- function(x)
+  sum(x) + prod(x)
+
 # no_tab
 # pipe_continuation
 # seq_linter

tests/testthat/test-function_brace_linter.R

@@ -0,0 +1,22 @@
+test_that("function_brace_linter skips allowed usages", {
+  expect_lint("function(x) 4", NULL, function_brace_linter())
+
+  lines <- trim_some("
+    function(x) {
+      x + 4
+    }
+  ")
+  expect_lint(lines, NULL, function_brace_linter())
+})
+
+test_that("function_brace_linter blocks disallowed usage", {
+  lines <- trim_some("
+    function(x)
+      x+4
+  ")
+  expect_lint(
+    lines,
+    rex::rex("Any function spanning multiple lines must use curly braces."),
+    function_brace_linter()
+  )
+})