https://git.ruby-lang.org/ruby.git/commit/?id=f6da4a5447

From f6da4a544760e00d932fea9a586bd869e82ad339 Mon Sep 17 00:00:00 2001
From: Akinori MUSHA <knu@i...>
Date: Wed, 4 Sep 2019 16:07:40 +0900
Subject: Describe #eager in the Enumerator::Lazy section


diff --git a/enumerator.c b/enumerator.c
index 7a0a17b..a8d14bb 100644
--- a/enumerator.c
+++ b/enumerator.c
@@ -1676,15 +1676,34 @@ lazy_generator_init(VALUE enumerator, VALUE procs) https://github.com/ruby/ruby/blob/trunk/enumerator.c#L1676
  *
  *    # This will fetch all URLs before selecting
  *    # necessary data
- *    URLS.map { |u| JSON.parse(open(u).read) }.
- *      select { |data| data.key?('stats') }.
- *      first(5)
+ *    URLS.map { |u| JSON.parse(open(u).read) }
+ *      .select { |data| data.key?('stats') }
+ *      .first(5)
  *
  *    # This will fetch URLs one-by-one, only till
  *    # there is enough data to satisfy the condition
- *    URLS.lazy.map { |u| JSON.parse(open(u).read) }.
- *      select { |data| data.key?('stats') }.
- *      first(5)
+ *    URLS.lazy.map { |u| JSON.parse(open(u).read) }
+ *      .select { |data| data.key?('stats') }
+ *      .first(5)
+ *
+ * Ending a chain with ".eager" generates a non-lazy enumerator, which
+ * is suitable for returning or passing to another method that expects
+ * a normal enumerator.
+ *
+ *    def active_items
+ *      groups
+ *        .lazy
+ *        .flat_map(&:items)
+ *        .reject(&:disabled)
+ *        .eager
+ *    end
+ *
+ *    # This works lazily; if a checked item is found, it stops
+ *    # iteration and does not look into remaining groups.
+ *    first_checked = active_items.find(&:checked)
+ *
+ *    # This returns an array of items like a normal enumerator does.
+ *    all_checked = active_items.select(&:checked)
  *
  */
 
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