From: ruby-core@... Date: 2014-11-21T14:32:02+00:00 Subject: [ruby-core:66392] [ruby-trunk - Bug #10534] Enumerator methods other than "next" do not respect "peek" Issue #10534 has been updated by Marc-Andre Lafortune. Assignee deleted (Yukihiro Matsumoto) `next` and `peek` indeed live in their little world described as "external enumeration" in the documentation. The documentation of `next` and `next_values` states: * Note that +next_values+ does not affect other non-external enumeration * methods unless underlying iteration method itself has side-effect, e.g. * IO#each_line. The doc of `peek` and `peek_values` would benefit from a similar note. Maybe we should turn it around in this case, like: * Note that +peek+ will affect other non-external enumeration * methods if underlying iteration method itself has side-effect, e.g. * IO#each_line. ---------------------------------------- Bug #10534: Enumerator methods other than "next" do not respect "peek" https://bugs.ruby-lang.org/issues/10534#change-50032 * Author: Jonas Nicklas * Status: Open * Priority: Normal * Assignee: * Category: core * Target version: * ruby -v: ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-darwin14.0] * Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN ---------------------------------------- It seems like using "peek" and then calling other methods on the Enumerator consumes the value that was peeked at. While `next` correctly returns the peeked-at value the next time it is called, other Enumerator methods such as `to_a` and `each` do not. ~~~ >> enum = StringIO.new("foo").each => #:each> >> enum.peek => "foo" >> enum.to_a => [] ~~~ Here the final call to `enum.to_a` should have returned `["foo"]`, since we have not consumed anything from the enumerator yet. The peeked-at value is not included in the returned Array. Taking a glance at the code, it seems that these methods do not call `next` under the hood, but instead have some other mechanism of iterating over the Enumerator, this seems very counter-intuitive to me. -- https://bugs.ruby-lang.org/