#' Check for a common mistake where length is applied in the wrong place
#'
#' Usage like `length(x == 0)` is a mistake. If you intended to check `x` is empty,
#' use `length(x) == 0`. Other mistakes are possible, but running `length()` on the
#' outcome of a logical comparison is never the best choice.
#'
#' @examples
#' # will produce lints
#' lint(
#' text = "length(x == 0)",
#' linters = length_test_linter()
#' )
#'
#' # okay
#' lint(
#' text = "length(x) > 0",
#' linters = length_test_linter()
#' )
#' @evalRd rd_tags("class_equals_linter")
#' @seealso [linters] for a complete list of linters available in lintr.
#' @export
length_test_linter <- function() {
xpath <- glue::glue("
parent::expr
/following-sibling::expr[{ xp_or(infix_metadata$xml_tag[infix_metadata$comparator]) }]
/parent::expr
")
Linter(linter_level = "expression", function(source_expression) {
xml_calls <- source_expression$xml_find_function_calls("length")
bad_expr <- xml_find_all(xml_calls, xpath)
expr_parts <- vapply(lapply(bad_expr, xml_find_all, "expr[2]/*"), xml_text, character(3L))
lint_message <- sprintf(
"Checking the length of a logical vector is likely a mistake. Did you mean `length(%s) %s %s`?",
expr_parts[1L, ], expr_parts[2L, ], expr_parts[3L, ]
)
xml_nodes_to_lints(
bad_expr,
source_expression = source_expression,
lint_message = lint_message,
type = "warning"
)
})
}
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