mruby-enum-lazy: add Enumerator::Lazy#tap_each

add tap_each method that yields each element for side effects
(e.g. logging, debugging) and passes it through unmodified.
see https://bugs.ruby-lang.org/issues/21520

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-enum-lazy/mrblib/lazy.rb

@@ -349,6 +349,26 @@ class Enumerator
       }
     end
 
+    #
+    # call-seq:
+    #   lazy.tap_each {|obj| block } -> lazy_enumerator
+    #
+    # Yields each element to the block for side effects (e.g. logging,
+    # debugging) and passes it through unmodified.
+    #
+    #   (1..Float::INFINITY).lazy
+    #     .tap_each {|i| puts "saw: #{i}" }
+    #     .select(&:even?)
+    #     .first(3)
+    #   #=> [2, 4, 6]  (prints "saw: 1", "saw: 2", ... along the way)
+    #
+    def tap_each(&block)
+      Lazy.new(self){|yielder, val|
+        block.call(val)
+        yielder << val
+      }
+    end
+
     #
     # call-seq:
     #   lazy.force -> array

mrbgems/mruby-enum-lazy/test/lazy.rb

@@ -64,6 +64,19 @@ assert("Enumerator::Lazy#grep_v") do
   assert_equal [0, 1, 5, 6], lazy_grep_v.first(4)
 end
 
+assert("Enumerator::Lazy#tap_each") do
+  seen = []
+  result = [1, 2, 3, 4, 5].lazy.tap_each{|x| seen << x }.select{|x| x % 2 == 0 }.force
+  assert_equal [2, 4], result
+  assert_equal [1, 2, 3, 4, 5], seen
+end
+
+assert("Enumerator::Lazy#tap_each laziness") do
+  seen = []
+  [1, 2, 3, 4, 5].lazy.tap_each{|x| seen << x }.first(3)
+  assert_equal [1, 2, 3], seen
+end
+
 assert("Enumerator::Lazy#zip with cycle") do
   e1 = [1, 2, 3].cycle
   e2 = [:a, :b].cycle