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#' Extraction operator linter
#'
#' Check that the `[[` operator is used when extracting a single element from an object,
#' not `[` (subsetting) nor `$` (interactive use).
#'
#' @details
#'
#' There are three subsetting operators in R (`[[`, `[`, and `$`) and they interact differently
#' with different data structures (atomic vector, list, data frame, etc.).
#'
#' Here are a few reasons to prefer the `[[` operator over `[` or `$` when you want to extract
#' an element from a data frame or a list:
#'
#' - Subsetting a list with `[` always returns a smaller list, while `[[` returns
#' the list element.
#'
#' - Subsetting a named atomic vector with `[` returns a named vector, while `[[` returns
#' the vector element.
#'
#' - Subsetting a data frame (but not tibble) with `[` is type unstable; it can return
#' a vector or a data frame. `[[`, on the other hand, always returns a vector.
#'
#' - For a data frame (but not tibble), `$` does partial matching (e.g. `df$a` will subset
#' `df$abc`), which can be a source of bugs. `[[` doesn't do partial matching.
#'
#' For data frames (and tibbles), irrespective of the size, the `[[` operator is slower than `$`.
#' For lists, however, the reverse is true.
#'
#' @examples
#' # will produce lints
#' lint(
#' text = 'iris["Species"]',
#' linters = extraction_operator_linter()
#' )
#'
#' lint(
#' text = "iris$Species",
#' linters = extraction_operator_linter()
#' )
#'
#' # okay
#' lint(
#' text = 'iris[["Species"]]',
#' linters = extraction_operator_linter()
#' )
#'
#' @references
#' - Subsetting [chapter](https://adv-r.hadley.nz/subsetting.html) from _Advanced R_ (Wickham, 2019).
#'
#' @evalRd rd_tags("extraction_operator_linter")
#' @seealso [linters] for a complete list of linters available in lintr.
#' @export
extraction_operator_linter <- function() {
constant_nodes_in_brackets <- paste0("self::", c("expr", "OP-PLUS", "NUM_CONST", "STR_CONST"))
xpath <- glue("
//OP-DOLLAR[not(preceding-sibling::expr[1]/SYMBOL[text() = 'self' or text() = '.self'])]
|
//OP-LEFT-BRACKET[
not(following-sibling::expr[1]/descendant::*[not({xp_or(constant_nodes_in_brackets)})]) and
not(following-sibling::OP-COMMA)
]
")
Linter(function(source_expression) {
if (!is_lint_level(source_expression, "expression")) {
return(list())
}
xml <- source_expression$xml_parsed_content
bad_exprs <- xml_find_all(xml, xpath)
msgs <- sprintf("Use `[[` instead of `%s` to extract an element.", xml_text(bad_exprs))
xml_nodes_to_lints(
bad_exprs,
source_expression = source_expression,
lint_message = msgs,
type = "warning"
)
})
}
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