1 files changed, 39 insertions, 0 deletions
diff --git a/src/boost/libs/hana/example/map/foldable.cpp b/src/boost/libs/hana/example/map/foldable.cpp
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+++ b/src/boost/libs/hana/example/map/foldable.cpp
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+// Copyright Louis Dionne 2013-2017
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+
+#include <boost/hana/assert.hpp>
+#include <boost/hana/first.hpp>
+#include <boost/hana/fold_left.hpp>
+#include <boost/hana/insert.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/map.hpp>
+#include <boost/hana/pair.hpp>
+#include <boost/hana/second.hpp>
+#include <boost/hana/type.hpp>
+namespace hana = boost::hana;
+
+
+int main() {
+    // Given a map of (key, value) pairs, returns a map of (value, key) pairs.
+    // This requires both the keys and the values to be compile-time Comparable.
+    auto invert = [](auto map) {
+        return hana::fold_left(map, hana::make_map(), [](auto map, auto pair) {
+            return hana::insert(map, hana::make_pair(hana::second(pair), hana::first(pair)));
+        });
+    };
+
+    auto m = hana::make_map(
+        hana::make_pair(hana::type_c<int>, hana::int_c<1>),
+        hana::make_pair(hana::type_c<float>, hana::int_c<2>),
+        hana::make_pair(hana::int_c<3>, hana::type_c<void>)
+    );
+
+    BOOST_HANA_CONSTANT_CHECK(invert(m) ==
+        hana::make_map(
+            hana::make_pair(hana::int_c<1>, hana::type_c<int>),
+            hana::make_pair(hana::int_c<2>, hana::type_c<float>),
+            hana::make_pair(hana::type_c<void>, hana::int_c<3>)
+        )
+    );
+}