From f4b3414e621a1613b3152a3e3d6357121ccf9a55 Mon Sep 17 00:00:00 2001
From: Dmitry Arkhipov <grisumbras@gmail.com>
Date: Mon, 26 Jul 2021 21:10:05 +0300
Subject: [PATCH] Add support for n-ary functions to mp_compose

---
 doc/mp11/utility.adoc          |  2 +-
 include/boost/mp11/utility.hpp |  8 +++++---
 test/mp_compose.cpp            | 13 +++++++++++++
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/doc/mp11/utility.adoc b/doc/mp11/utility.adoc
index d5bb622..7a9777a 100644
--- a/doc/mp11/utility.adoc
+++ b/doc/mp11/utility.adoc
@@ -295,7 +295,7 @@ As `mp_not_fn`, but takes a quoted metafunction.
 
 `mp_compose<F1, F2, ..., Fn>` is a quoted metafunction that applies
 `F1`, `F2`, ..., `Fn` to its argument, in sequence. That is,
-`mp_compose<F1, F2, ..., Fn>::fn<T>` is `Fn<...F2<F1<T>>...>`.
+`mp_compose<F1, F2, ..., Fn>::fn<T...>` is `Fn<...F2<F1<T...>>...>`.
 
 ## mp_compose_q<Q...>
 
diff --git a/include/boost/mp11/utility.hpp b/include/boost/mp11/utility.hpp
index 533009a..b3fa7a9 100644
--- a/include/boost/mp11/utility.hpp
+++ b/include/boost/mp11/utility.hpp
@@ -11,6 +11,8 @@
 #include <boost/mp11/integral.hpp>
 #include <boost/mp11/detail/mp_list.hpp>
 #include <boost/mp11/detail/mp_fold.hpp>
+#include <boost/mp11/detail/mp_front.hpp>
+#include <boost/mp11/detail/mp_rename.hpp>
 #include <boost/mp11/detail/config.hpp>
 
 namespace boost
@@ -233,7 +235,7 @@ template<class Q> using mp_not_fn_q = mp_not_fn<Q::template fn>;
 namespace detail
 {
 
-template<class T, class Q> using mp_reverse_invoke_q = mp_invoke_q<Q, T>;
+template<class L, class Q> using mp_compose_helper = mp_list< mp_apply_q<Q, L> >;
 
 } // namespace detail
 
@@ -241,14 +243,14 @@ template<class T, class Q> using mp_reverse_invoke_q = mp_invoke_q<Q, T>;
 
 template<template<class...> class... F> struct mp_compose
 {
-    template<class T> using fn = mp_fold<mp_list<mp_quote<F>...>, T, detail::mp_reverse_invoke_q>;
+    template<class... T> using fn = mp_front< mp_fold<mp_list<mp_quote<F>...>, mp_list<T...>, detail::mp_compose_helper> >;
 };
 
 #endif
 
 template<class... Q> struct mp_compose_q
 {
-    template<class T> using fn = mp_fold<mp_list<Q...>, T, detail::mp_reverse_invoke_q>;
+    template<class... T> using fn = mp_front< mp_fold<mp_list<Q...>, mp_list<T...>, detail::mp_compose_helper> >;
 };
 
 } // namespace mp11
diff --git a/test/mp_compose.cpp b/test/mp_compose.cpp
index 46184a6..57a86ed 100644
--- a/test/mp_compose.cpp
+++ b/test/mp_compose.cpp
@@ -4,6 +4,7 @@
 // https://www.boost.org/LICENSE_1_0.txt
 
 
+#include <boost/mp11/list.hpp>
 #include <boost/mp11/utility.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 
@@ -15,6 +16,8 @@ template<class T> using G1 = F1<T>;
 template<class T> using G2 = F2<T>;
 template<class T> using G3 = F3<T>;
 
+template<class... T> struct H {};
+
 int main()
 {
     using namespace boost::mp11;
@@ -32,6 +35,10 @@ int main()
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose<F1, F2, F3>::fn<void>, F3<F2<F1<void>>>>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose<G1, G2, G3>::fn<void>, F3<F2<F1<void>>>>));
 
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose<H>::fn<int, char>, H<int, char>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose<H, F1>::fn<void, float>, F1<H<void, float>>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose<H, F1, G1>::fn<void, float>, G1<F1<H<void, float>>>>));
+
 #endif
 
     using QF1 = mp_quote<F1>;
@@ -42,6 +49,8 @@ int main()
     using QG2 = mp_quote<G2>;
     using QG3 = mp_quote<G3>;
 
+    using QH = mp_quote<H>;
+
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<>::fn<void>, void>));
 
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QF1>::fn<void>, F1<void>>));
@@ -53,6 +62,10 @@ int main()
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QF1, QF2, QF3>::fn<void>, F3<F2<F1<void>>>>));
     BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QG1, QG2, QG3>::fn<void>, F3<F2<F1<void>>>>));
 
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QH>::fn<int, char>, H<int, char>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QH, QF1>::fn<void, float>, F1<H<void, float>>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_compose_q<QH, QF1, QG1>::fn<void, float>, G1<F1<H<void, float>>>>));
+
     //
 
     return boost::report_errors();