From 82b56a2825dc9d4facb6d1d0d8cb659feccd6209 Mon Sep 17 00:00:00 2001
From: Red <nuzer501@gmail.com>
Date: Tue, 17 Mar 2026 17:32:45 +0900
Subject: [ruby-core:122847] Implements `Enumerator::Lazy#tee`

---
 enumerator.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

(limited to 'enumerator.c')

diff --git a/enumerator.c b/enumerator.c
index b60036a3de..42c11a08f8 100644
--- a/enumerator.c
+++ b/enumerator.c
@@ -2771,6 +2771,52 @@ lazy_with_index(int argc, VALUE *argv, VALUE obj)
     return lazy_add_method(obj, 0, 0, memo, rb_ary_new_from_values(1, &memo), &lazy_with_index_funcs);
 }
 
+static struct MEMO *
+lazy_tee_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
+{
+    struct proc_entry *entry = proc_entry_ptr(proc_entry);
+
+    rb_proc_call_with_block(entry->proc, 1, &result->memo_value, Qnil);
+
+    return result;
+}
+
+static const lazyenum_funcs lazy_tee_funcs = {
+    lazy_tee_proc, 0,
+};
+
+/*
+ *  call-seq:
+ *     lazy.tee { |item| ... } -> lazy_enumerator
+ *
+ *  Passes each element through to the block for side effects only,
+ *  without modifying the element or affecting the enumeration.
+ *  Returns a new lazy enumerator.
+ *
+ *  This is useful for debugging or logging inside lazy chains,
+ *  without breaking laziness or misusing +map+.
+ *
+ *     (1..).lazy
+ *           .tee { |x| puts "got #{x}" }
+ *           .select(&:even?)
+ *           .first(3)
+ *     # prints: got 1, got 2, ..., got 6
+ *     # returns: [2, 4, 6]
+ *
+ *  Similar in intent to Java's Stream#peek.
+ */
+
+static VALUE
+lazy_tee(VALUE obj)
+{
+    if (!rb_block_given_p())
+    {
+        rb_raise(rb_eArgError, "tried to call lazy tee without a block");
+    }
+
+    return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_tee_funcs);
+}
+
 #if 0 /* for RDoc */
 
 /*
@@ -4646,6 +4692,7 @@ InitVM_Enumerator(void)
     rb_define_method(rb_cLazy, "uniq", lazy_uniq, 0);
     rb_define_method(rb_cLazy, "compact", lazy_compact, 0);
     rb_define_method(rb_cLazy, "with_index", lazy_with_index, -1);
+    rb_define_method(rb_cLazy, "tee", lazy_tee, 0);
 
     lazy_use_super_method = rb_hash_new_with_size(18);
     rb_hash_aset(lazy_use_super_method, sym("map"), sym("_enumerable_map"));
-- 
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