[string] Add a model of the Monoid concept

Fixes #117

example/misc/printf.cpp

@@ -4,17 +4,17 @@
 
 #include <boost/hana/adjust_if.hpp>
 #include <boost/hana/at_key.hpp>
-#include <boost/hana/core/to.hpp>
 #include <boost/hana/core/is_a.hpp>
+#include <boost/hana/core/to.hpp>
 #include <boost/hana/filter.hpp>
-#include <boost/hana/fold_left.hpp>
 #include <boost/hana/functional/compose.hpp>
 #include <boost/hana/functional/partial.hpp>
 #include <boost/hana/map.hpp>
 #include <boost/hana/not.hpp>
 #include <boost/hana/pair.hpp>
 #include <boost/hana/prepend.hpp>
 #include <boost/hana/string.hpp>
+#include <boost/hana/sum.hpp>
 #include <boost/hana/tuple.hpp>
 #include <boost/hana/type.hpp>
 #include <boost/hana/unpack.hpp>
@@ -29,12 +29,6 @@ constexpr auto formats = hana::make_map(
     hana::make_pair(hana::type_c<char const*>, hana::string_c<'%', 's'>)
 );
 
-struct concat_strings {
-    template <char ...s1, char ...s2>
-    constexpr auto operator()(hana::string<s1...>, hana::string<s2...>) const
-    { return hana::string_c<s1..., s2...>; }
-};
-
 template <typename ...Tokens>
 constexpr auto format(Tokens ...tokens_) {
     auto tokens = hana::make_tuple(tokens_...);
@@ -50,7 +44,7 @@ constexpr auto format(Tokens ...tokens_) {
         hana::compose(hana::partial(hana::at_key, formats), hana::typeid_)
     );
 
-    auto format_string = hana::fold_left(format_string_tokens, hana::string_c<>, concat_strings{});
+    auto format_string = hana::sum<hana::string_tag>(format_string_tokens);
     auto variables = hana::filter(tokens, hana::compose(hana::not_, hana::is_a<hana::string_tag>));
     return hana::prepend(variables, format_string);
 }

example/string/monoid.cpp

@@ -0,0 +1,15 @@
+// Copyright Louis Dionne 2013-2016
+// 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/equal.hpp>
+#include <boost/hana/plus.hpp>
+#include <boost/hana/string.hpp>
+namespace hana = boost::hana;
+
+
+auto hello_world = BOOST_HANA_STRING("Hello ") + BOOST_HANA_STRING("world!");
+BOOST_HANA_CONSTANT_CHECK(hello_world == BOOST_HANA_STRING("Hello world!"));
+
+int main() { }

include/boost/hana/fwd/string.hpp

@@ -38,8 +38,8 @@ BOOST_HANA_NAMESPACE_BEGIN
     //! The representation of `hana::string` is implementation-defined.
     //! In particular, one should not take for granted that the template
     //! parameters are `char`s. The proper way to access the contents of
-    //! a `hana::string` as character constants is to use `hana::unpack`
-    //! or `hana::to<char const*>`, as documented below.
+    //! a `hana::string` as character constants is to use `hana::unpack`,
+    //! `.c_str()` or `hana::to<char const*>`, as documented below.
     //!
     //!
     //! Modeled concepts
@@ -57,23 +57,28 @@ BOOST_HANA_NAMESPACE_BEGIN
     //! lexicographical comparison of strings.
     //! @include example/string/orderable.cpp
     //!
-    //! 3. `Foldable`\n
+    //! 3. `Monoid`\n
+    //! Strings form a monoid under concatenation, with the neutral element
+    //! being the empty string.
+    //! @include example/string/monoid.cpp
+    //!
+    //! 4. `Foldable`\n
     //! Folding a string is equivalent to folding the sequence of its
     //! characters.
     //! @include example/string/foldable.cpp
     //!
-    //! 4. `Iterable`\n
+    //! 5. `Iterable`\n
     //! Iterating over a string is equivalent to iterating over the sequence
     //! of its characters. Also note that `operator[]` can be used instead of
     //! the `at` function.
     //! @include example/string/iterable.cpp
     //!
-    //! 5. `Searchable`\n
+    //! 6. `Searchable`\n
     //! Searching through a string is equivalent to searching through the
     //! sequence of its characters.
     //! @include example/string/searchable.cpp
     //!
-    //! 6. `Hashable`\n
+    //! 7. `Hashable`\n
     //! The hash of a compile-time string is a type uniquely representing
     //! that string.
     //! @include example/string/hashable.cpp
@@ -125,6 +130,10 @@ BOOST_HANA_NAMESPACE_BEGIN
         template <typename X, typename Y>
         friend constexpr auto operator>=(X&& x, Y&& y);
 
+        //! Performs concatenation; equivalent to `hana::plus`
+        template <typename X, typename Y>
+        friend constexpr auto operator+(X&& x, Y&& y);
+
         //! Equivalent to `hana::at`
         template <typename N>
         constexpr decltype(auto) operator[](N&& n);

include/boost/hana/string.hpp

@@ -32,7 +32,9 @@ Distributed under the Boost Software License, Version 1.0.
 #include <boost/hana/fwd/is_empty.hpp>
 #include <boost/hana/fwd/length.hpp>
 #include <boost/hana/fwd/less.hpp>
+#include <boost/hana/fwd/plus.hpp>
 #include <boost/hana/fwd/unpack.hpp>
+#include <boost/hana/fwd/zero.hpp>
 #include <boost/hana/if.hpp>
 #include <boost/hana/integral_constant.hpp>
 #include <boost/hana/optional.hpp>
@@ -177,6 +179,30 @@ BOOST_HANA_NAMESPACE_BEGIN
         }
     };
 
+    //////////////////////////////////////////////////////////////////////////
+    // Monoid
+    //////////////////////////////////////////////////////////////////////////
+    template <>
+    struct plus_impl<string_tag, string_tag> {
+        template <char ...s1, char ...s2>
+        static constexpr auto
+        apply(string<s1...> const&, string<s2...> const&) {
+            return string<s1..., s2...>{};
+        }
+    };
+
+    template <>
+    struct zero_impl<string_tag> {
+        static constexpr auto apply() {
+            return string<>{};
+        }
+    };
+
+    template <char ...s1, char ...s2>
+    constexpr auto operator+(string<s1...> const&, string<s2...> const&) {
+        return hana::string<s1..., s2...>{};
+    }
+
     //////////////////////////////////////////////////////////////////////////
     // Foldable
     //////////////////////////////////////////////////////////////////////////

test/string/laws.cpp

@@ -9,6 +9,7 @@
 #include <laws/foldable.hpp>
 #include <laws/hashable.hpp>
 #include <laws/iterable.hpp>
+#include <laws/monoid.hpp>
 #include <laws/orderable.hpp>
 #include <laws/searchable.hpp>
 namespace hana = boost::hana;
@@ -31,6 +32,21 @@ int main() {
         hana::test::TestHashable<hana::string_tag>{strings};
     }
 
+    // Monoid
+    {
+        auto strings = hana::make_tuple(
+            BOOST_HANA_STRING(""),
+            BOOST_HANA_STRING("a"),
+            BOOST_HANA_STRING("ab"),
+            BOOST_HANA_STRING("abc"),
+            BOOST_HANA_STRING("abcd"),
+            BOOST_HANA_STRING("abcde"),
+            BOOST_HANA_STRING("ba")
+        );
+
+        hana::test::TestMonoid<hana::string_tag>{strings};
+    }
+
     // Foldable and Iterable
     {
         auto strings = hana::make_tuple(

test/string/plus.cpp

@@ -0,0 +1,35 @@
+// Copyright Louis Dionne 2013-2016
+// 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/equal.hpp>
+#include <boost/hana/plus.hpp>
+#include <boost/hana/string.hpp>
+namespace hana = boost::hana;
+
+
+int main() {
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        hana::plus(BOOST_HANA_STRING(""), BOOST_HANA_STRING("")),
+        BOOST_HANA_STRING("")
+    ));
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        hana::plus(BOOST_HANA_STRING("abcd"), BOOST_HANA_STRING("")),
+        BOOST_HANA_STRING("abcd")
+    ));
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        hana::plus(BOOST_HANA_STRING(""), BOOST_HANA_STRING("abcd")),
+        BOOST_HANA_STRING("abcd")
+    ));
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        hana::plus(BOOST_HANA_STRING("abcd"), BOOST_HANA_STRING("efg")),
+        BOOST_HANA_STRING("abcdefg")
+    ));
+
+    // check operator
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        BOOST_HANA_STRING("abc") + BOOST_HANA_STRING("def"),
+        BOOST_HANA_STRING("abcdef")
+    ));
+}

test/string/zero.cpp

@@ -0,0 +1,17 @@
+// Copyright Louis Dionne 2013-2016
+// 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/equal.hpp>
+#include <boost/hana/string.hpp>
+#include <boost/hana/zero.hpp>
+namespace hana = boost::hana;
+
+
+int main() {
+    BOOST_HANA_CONSTANT_CHECK(hana::equal(
+        hana::zero<hana::string_tag>(),
+        BOOST_HANA_STRING("")
+    ));
+}