Don’t use partial matching for attr_getter()

Changed the `attr_getter()` fuction to no longer uses partial matching.
For example, if an `x` object has a `labels` attribute but no `label`
attribute, `attr_getter("label")(x)` will no longer extract the `labels`
attribute (fixes tidyverse#460).

Also added some documentation on the `attr_getter()` function.

NEWS.md

@@ -1,5 +1,10 @@
 # purrr 0.2.4.9000
 
+* `attr_getter()` no longer uses partial matching. For example,
+  if an `x` object has a `labels` attribute but no `label` attribute,
+  `attr_getter("label")(x)` will no longer extract the `labels`
+  attribute (#460, @huftis).
+
 * `flatten_dfr()` and `flatten_dfc()` now aborts if dplyr is not installed. (#454)
 
 * `imap_dfr()` now works with `.id` argument is provided (#429)

R/pluck.R

@@ -16,6 +16,11 @@
 #' Furthermore, `pluck()` never partial-matches unlike `$` which will
 #' select the `disp` object if you write `mtcars$di`.
 #'
+#' `attr_getter()` generates an attribute accessor function;
+#' i.e., it generates a function for extracting an attribute with
+#' a given name. Unlike the base R `attr()` function with default
+#' options, it doesn't use partial matching.
+#'
 #' @details
 #'
 #' Since it handles arbitrary accessor functions, `pluck()` is a type
@@ -83,5 +88,5 @@ pluck <- function(.x, ..., .default = NULL) {
 #' @param attr An attribute name as string.
 attr_getter <- function(attr) {
   force(attr)
-  function(x) attr(x, attr)
+  function(x) attr(x, attr, exact = TRUE)
 }