From e7f2e7a45d393907bf9827fb2eda2180ff7697ad Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Fri, 12 Mar 2021 06:35:11 +0200
Subject: [PATCH] Add mp_join

---
 include/boost/mp11/algorithm.hpp |  4 +++
 test/Jamfile                     |  1 +
 test/mp_join.cpp                 | 51 ++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 test/mp_join.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 40a8c55..0ff96d9 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -1296,6 +1296,10 @@ template<class L, class S, class J> struct mp_split_impl
 
 } // namespace detail
 
+// mp_join<L, S>
+
+template<class L, class S> using mp_join = mp_apply<mp_append, mp_intersperse<L, mp_list<S>>>;
+
 } // namespace mp11
 } // namespace boost
 
diff --git a/test/Jamfile b/test/Jamfile
index 531e598..8f4daca 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -126,6 +126,7 @@ run mp_pairwise_fold.cpp ;
 run mp_pairwise_fold_q.cpp ;
 run mp_intersperse.cpp ;
 run mp_split.cpp ;
+run mp_join.cpp ;
 
 # integral
 run integral.cpp ;
diff --git a/test/mp_join.cpp b/test/mp_join.cpp
new file mode 100644
index 0000000..1f4b986
--- /dev/null
+++ b/test/mp_join.cpp
@@ -0,0 +1,51 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+#include <utility>
+
+struct X1 {};
+struct X2 {};
+struct X3 {};
+struct X4 {};
+
+struct S {};
+
+int main()
+{
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_join;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<>>, S>, mp_list<>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<X1>>, S>, mp_list<X1>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<X1, X2>>, S>, mp_list<X1, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<X1, X2, X3>>, S>, mp_list<X1, X2, X3>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<>, mp_list<>>, S>, mp_list<S>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<>, mp_list<>, mp_list<>>, S>, mp_list<S, S>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<>, mp_list<>, mp_list<>, mp_list<>>, S>, mp_list<S, S, S>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<X1>, mp_list<X2>>, S>, mp_list<X1, S, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<X1, X2>, mp_list<X3, X4>>, S>, mp_list<X1, X2, S, X3, X4>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<mp_list<mp_list<>, mp_list<X1, X2>, mp_list<>, mp_list<X3, X4>, mp_list<>>, S>, mp_list<S, X1, X2, S, S, X3, X4, S>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<>>, S>, std::tuple<>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<X1>>, S>, std::tuple<X1>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<X1, X2>>, S>, std::tuple<X1, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<X1, X2, X3>>, S>, std::tuple<X1, X2, X3>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<>, std::tuple<>>, S>, std::tuple<S>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<>, std::tuple<>, std::tuple<>>, S>, std::tuple<S, S>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<>, std::tuple<>, std::tuple<>, std::tuple<>>, S>, std::tuple<S, S, S>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<X1>, std::tuple<X2>>, S>, std::tuple<X1, S, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<X1, X2>, std::tuple<X3, X4>>, S>, std::tuple<X1, X2, S, X3, X4>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_join<std::tuple<std::tuple<>, std::tuple<X1, X2>, std::tuple<>, std::tuple<X3, X4>, std::tuple<>>, S>, std::tuple<S, X1, X2, S, S, X3, X4, S>>));
+
+    return boost::report_errors();
+}