Add mp_map_keys

doc/html/mp11.html

@@ -564,6 +564,7 @@ body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-b
 <li><a href="#mp_map_replace_m_t">mp_map_replace&lt;M, T&gt;</a></li>
 <li><a href="#mp_map_update_m_t_f">mp_map_update&lt;M, T, F&gt;</a></li>
 <li><a href="#mp_map_erase_m_k">mp_map_erase&lt;M, K&gt;</a></li>
+<li><a href="#mp_map_keys_m">mp_map_keys&lt;M&gt;</a></li>
 </ul>
 </li>
 <li><a href="#function">Helper Metafunctions, &lt;boost/mp11/function.hpp&gt;</a>
@@ -3508,6 +3509,17 @@ replaces the existing element <code>L&lt;X, Y&#8230;&#8203;&gt;</code> with <cod
 <p>If the map <code>M</code> contains an element with a key <code>K</code>, removes it.</p>
 </div>
 </div>
+<div class="sect3">
+<h4 id="mp_map_keys_m">mp_map_keys&lt;M&gt;</h4>
+<div class="literalblock">
+<div class="content">
+<pre>template&lt;class M&gt; using mp_map_keys = mp_transform&lt;mp_first, M&gt;;</pre>
+</div>
+</div>
+<div class="paragraph">
+<p><code>mp_map_keys&lt;M&gt;</code> returns a list of the keys of <code>M</code>. When <code>M</code> is a valid map, the keys are unique, so the result is a set.</p>
+</div>
+</div>
 </div>
 <div class="sect2">
 <h3 id="function">Helper Metafunctions, &lt;boost/mp11/function.hpp&gt;</h3>

doc/mp11/map.adoc

@@ -54,3 +54,9 @@ replaces the existing element `L<X, Y...>` with `L<X, F<X, Y...>>`.
     template<class M, class K> using mp_map_erase = /*...*/;
 
 If the map `M` contains an element with a key `K`, removes it.
+
+## mp_map_keys<M>
+
+    template<class M> using mp_map_keys = mp_transform<mp_first, M>;
+
+`mp_map_keys<M>` returns a list of the keys of `M`. When `M` is a valid map, the keys are unique, so the result is a set.

include/boost/mp11/map.hpp

@@ -79,6 +79,9 @@ template<class M, class K> struct mp_map_erase_impl
 
 template<class M, class K> using mp_map_erase = typename detail::mp_map_erase_impl<M, K>::type;
 
+// mp_map_keys<M>
+template<class M> using mp_map_keys = mp_transform<mp_first, M>;
+
 } // namespace mp11
 } // namespace boost
 

test/Jamfile

@@ -123,6 +123,7 @@ run mp_map_insert.cpp : : : $(REQ) ;
 run mp_map_replace.cpp : : : $(REQ) ;
 run mp_map_erase.cpp : : : $(REQ) ;
 run mp_map_update.cpp : : : $(REQ) ;
+run mp_map_keys.cpp : : : $(REQ) ;
 
 # bind
 run mp_bind.cpp : : : $(REQ) ;

test/mp_map_keys.cpp

@@ -0,0 +1,37 @@
+
+// Copyright 2017 Peter Dimov.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+
+#include <boost/mp11/map.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+#include <utility>
+
+int main()
+{
+    using boost::mp11::mp_map_keys;
+    using boost::mp11::mp_list;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<mp_list<>>, mp_list<>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::tuple<>>, std::tuple<>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<mp_list<std::pair<int, int const>>>, mp_list<int>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::tuple<std::pair<int, int const>>>, std::tuple<int>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<mp_list<std::pair<int, int const>, std::pair<long, long const>>>, mp_list<int, long>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::tuple<std::pair<int, int const>, std::pair<long, long const>>>, std::tuple<int, long>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::pair<std::pair<int, int const>, std::pair<long, long const>>>, std::pair<int, long>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<mp_list<mp_list<int>, mp_list<long, long>, mp_list<long long, long long, long long>>>, mp_list<int, long, long long>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::tuple<mp_list<int>, mp_list<long, long>, mp_list<long long, long long, long long>>>, std::tuple<int, long, long long>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_map_keys<std::tuple<std::tuple<int>, std::pair<long, long>, std::tuple<long long, long long, long long>>>, std::tuple<int, long, long long>>));
+
+    return boost::report_errors();
+}