#' Require usage of `expect_null` for checking `NULL`
#'
#' Require usage of `expect_null(x)` over `expect_equal(x, NULL)` and similar
#' usages.
#'
#' [testthat::expect_null()] exists specifically for testing for `NULL` objects.
#' [testthat::expect_equal()], [testthat::expect_identical()], and
#' [testthat::expect_true()] can also be used for such tests,
#' but it is better to use the tailored function instead.
#'
#' @examples
#' # will produce lints
#' lint(
#' text = "expect_equal(x, NULL)",
#' linters = expect_null_linter()
#' )
#'
#' lint(
#' text = "expect_identical(x, NULL)",
#' linters = expect_null_linter()
#' )
#'
#' lint(
#' text = "expect_true(is.null(x))",
#' linters = expect_null_linter()
#' )
#'
#'
#' # okay
#' lint(
#' text = "expect_null(x)",
#' linters = expect_null_linter()
#' )
#'
#' @evalRd rd_tags("expect_null_linter")
#' @seealso [linters] for a complete list of linters available in lintr.
#' @export
expect_null_linter <- function() {
# two cases two match:
# (1) expect_{equal,identical}(x, NULL) (or NULL, x)
# (2) expect_true(is.null(x))
expect_equal_identical_xpath <- "
parent::expr
/following-sibling::expr[position() <= 2 and NULL_CONST]
/parent::expr
"
expect_true_xpath <- "
parent::expr
/following-sibling::expr[1][expr[1]/SYMBOL_FUNCTION_CALL[text() = 'is.null']]
/parent::expr
"
Linter(linter_level = "expression", function(source_expression) {
expect_equal_identical_calls <- source_expression$xml_find_function_calls(c("expect_equal", "expect_identical"))
expect_true_calls <- source_expression$xml_find_function_calls("expect_true")
bad_expr <- combine_nodesets(
xml_find_all(expect_equal_identical_calls, expect_equal_identical_xpath),
xml_find_all(expect_true_calls, expect_true_xpath)
)
matched_function <- xp_call_name(bad_expr)
msg <- ifelse(
matched_function %in% c("expect_equal", "expect_identical"),
sprintf("expect_null(x) is better than %s(x, NULL)", matched_function),
"expect_null(x) is better than expect_true(is.null(x))"
)
xml_nodes_to_lints(
bad_expr,
source_expression,
lint_message = msg,
type = "warning"
)
})
}
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