From 2be45e896a74e7ca032b6e3bafc142a94c9193c3 Mon Sep 17 00:00:00 2001
From: sha512sum <linuxnyasha@tutanota.com>
Date: Thu, 18 Jul 2024 16:28:25 +0000
Subject: [PATCH] Fix LeftFold concept and add example for it

---
 examples/src/left_fold.cpp |  9 +++++++++
 include/utempl/utils.hpp   | 23 ++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 examples/src/left_fold.cpp

diff --git a/examples/src/left_fold.cpp b/examples/src/left_fold.cpp
new file mode 100644
index 0000000..ee67be3
--- /dev/null
+++ b/examples/src/left_fold.cpp
@@ -0,0 +1,9 @@
+#include <utempl/utils.hpp>
+
+auto main() -> int {
+  constexpr auto value = utempl::LeftFold(
+      utempl::GetIndexesTuple<4>(), utempl::kWrapper<std::size_t{}>, []<std::size_t I>(utempl::Wrapper<I>, auto) -> utempl::Wrapper<I + 1> {
+        return {};
+      });
+  static_assert(std::is_same_v<decltype(value), const utempl::Wrapper<static_cast<std::size_t>(4)>>);
+};
diff --git a/include/utempl/utils.hpp b/include/utempl/utils.hpp
index 1f09c06..392a7ca 100644
--- a/include/utempl/utils.hpp
+++ b/include/utempl/utils.hpp
@@ -436,9 +436,30 @@ struct LeftFold<T, F> {
   };
 };
 
+struct LeftFoldIgnorer {
+  static constexpr bool value = false;
+  consteval auto operator|(auto&&) const -> LeftFoldIgnorer {
+    return {};
+  };
+};
+
+template <typename F, typename T>
+struct LeftFoldIsOk {
+  static constexpr bool value = true;
+  template <typename TT>
+  consteval auto operator|(LeftFoldIsOk<F, TT>&& other) -> LeftFoldIsOk<F, std::invoke_result_t<F, T, TT>>
+    requires Function<F, void(T, TT)>
+  {
+    return {};
+  };
+  consteval auto operator|(auto&&) -> LeftFoldIgnorer {
+    return {};
+  };
+};
+
 template <typename F, typename T, typename Tuple>
 concept LeftFoldConcept = decltype(Unpack(std::declval<Tuple>(), []<typename... Ts>(Ts&&...) {
-  return kWrapper<(Function<F, T(T, Ts)> && ...)>;
+  return kWrapper<decltype((LeftFoldIsOk<F, T>{} | ... | LeftFoldIsOk<F, Ts>{}))::value>;
 }))::kValue;
 
 }  // namespace impl