From b8e79c09f61556c5f3efea9fc99904f7b409557f Mon Sep 17 00:00:00 2001
From: Matej Focko <me@mfocko.xyz>
Date: Sat, 29 Jul 2023 16:01:32 +0200
Subject: [PATCH] chore(cpp): add Y-combinator

Signed-off-by: Matej Focko <me@mfocko.xyz>
---
 .common/cpp/skeleton.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/.common/cpp/skeleton.cpp b/.common/cpp/skeleton.cpp
index da8fc11..cfea8de 100644
--- a/.common/cpp/skeleton.cpp
+++ b/.common/cpp/skeleton.cpp
@@ -93,6 +93,25 @@ std::vector<T> load_vector(std::size_t size) {
 }
 #pragma endregion /* input */
 
+#pragma region functional
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0200r0.html
+template <class Fun>
+class y_combinator_result {
+  Fun fun_;
+
+ public:
+  template <class T>
+  explicit y_combinator_result(T &&fun) : fun_(std::forward<T>(fun)) {}
+  template <class... Args>
+  decltype(auto) operator()(Args &&...args) {
+    return fun_(std::ref(*this), std::forward<Args>(args)...);
+  }
+};
+template <class Fun>
+decltype(auto) y_combinator(Fun &&fun) {
+  return y_combinator_result<std::decay_t<Fun>>(std::forward<Fun>(fun));
+}
+#pragma endregion /* functional */
 
 #define LOOP(var, n) for (auto var = 0; var < n; ++var)