Disallow indexing by factors

NAMESPACE

@@ -11,4 +11,5 @@ export(strict_diag)
 export(strict_drop)
 export(strict_sample)
 export(strict_sapply)
+export(strict_square_bracket)
 import(rlang)

R/shim-square-bracket.R

@@ -0,0 +1,38 @@
+register_shims_square_bracket <- function(env) {
+  rlang::env_bind(env,
+    `[` = strict_square_bracket
+  )
+}
+
+#' Strict behaviour for square bracket indexing
+#'
+#' The effect of indexing with a factor is unclear, since factors can behave
+#' like character vectors or integer vectors, depending upon context. In
+#' strict mode, factor indexing throws an error.
+#'
+#' @param x The object to be indexed.
+#' @param ... Indexing values.
+#' @export
+#' @examples
+#' (x <- c(d = 1, c = 4, b = 9, a = 16))
+#' (index <- factor(letters[1:4]))
+#' # Character and integer indexing is OK.
+#' x[as.character(index)]
+#' x[as.integer(index)]
+#' # Factor indexing is disallowed.
+#' try(x[index])
+#'
+#' # For higher dimension objects, passing a factor
+#' # to any dimension throws an error.
+#' d <- data.frame(
+#'   a = 1:4, b = letters[1:4], c = runif(4),
+#'   stringsAsFactors = FALSE)
+#' try(d[factor(d$b), ])
+#' try(d[, factor(c("a", "b"))])
+strict_square_bracket <- function(x, ...) {
+  indexers <- rlang::dots_list(..., .ignore_empty = "all")
+  if (any(vapply(indexers, is.factor, logical(1L)))) {
+    stop("Indexing with a factor has unclear behavior. Explicitly coerce your index to a character or integer vector.")
+  } else base::`[`(x, ...)
+}
+

R/shims.R

@@ -14,6 +14,7 @@ register_shims <- function() {
   register_shims_apply(strict_shims)
   register_shims_risky(strict_shims)
   register_shims_scalar(strict_shims)
+  register_shims_square_bracket(strict_shims)
 }
 
 register_shim_T_F <- function(env) {