diff --git a/CHANGELOG.md b/CHANGELOG.md
index f11864d6..1bac2016 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,13 +6,20 @@
 * Add basic_streambuf::alloc_size
 * Parser callbacks may not throw
 * Fix Reader concept doc typo
+* Add is_Reader trait
 
 API Changes:
 
 * Added init() to Reader requirements
 * Reader must be nothrow constructible
 * Reader is now constructed right before reading the body
-   - The message passed on construction is filled in
+  - The message passed on construction is filled in
+* Rework HTTP concepts:
+  - Writer uses write instead of operator()
+  - Refactor traits to use void_t
+  - Remove is_ReadableBody, is_WritableBody
+  - Add has_reader, has_writer, is_Reader, is_Writer
+  - More friendly compile errors on failed concept checks
 
 --------------------------------------------------------------------------------
 
diff --git a/doc/quickref.xml b/doc/quickref.xml
index b9c5c647..19ab73c6 100644
--- a/doc/quickref.xml
+++ b/doc/quickref.xml
@@ -50,8 +50,10 @@
           <simplelist type="vert" columns="1">
            <member><link linkend="beast.ref.http__is_Body">is_Body</link></member>
            <member><link linkend="beast.ref.http__is_Parser">is_Parser</link></member>
-           <member><link linkend="beast.ref.http__is_ReadableBody">is_ReadableBody</link></member>
-           <member><link linkend="beast.ref.http__is_WritableBody">is_WritableBody</link></member>
+           <member><link linkend="beast.ref.http__is_Reader">is_Reader</link></member>
+           <member><link linkend="beast.ref.http__is_Writer">is_Reader</link></member>
+           <member><link linkend="beast.ref.http__has_reader">has_reader</link></member>
+           <member><link linkend="beast.ref.http__has_writer">has_writer</link></member>
           </simplelist>
         </entry>
         <entry valign="top">
diff --git a/doc/types/Writer.qbk b/doc/types/Writer.qbk
index 48c91aee..32ea7227 100644
--- a/doc/types/Writer.qbk
+++ b/doc/types/Writer.qbk
@@ -72,7 +72,7 @@ In this table:
     ]
 ]
 [
-    [`a(rc, ec, wf)`]
+    [`a.write(rc, ec, wf)`]
     [`boost::tribool`]
     [
         Called repeatedly after `init` succeeds. `wf` is a function object
@@ -175,10 +175,10 @@ public:
     */
     template<class WriteFunction>
     boost::tribool
-    operator()(
+    write(
         resume_context&&,
         error_code&,
-        WriteFunction&& write) noexcept;
+        WriteFunction&& wf) noexcept;
 };
 ```
 
diff --git a/examples/file_body.hpp b/examples/file_body.hpp
index 6df8dc86..8e7bb55b 100644
--- a/examples/file_body.hpp
+++ b/examples/file_body.hpp
@@ -58,14 +58,15 @@ struct file_body
         }
 
         std::uint64_t
-        content_length() const
+        content_length() const noexcept
         {
             return size_;
         }
 
-        template<class Write>
+        template<class WriteFunction>
         boost::tribool
-        operator()(resume_context&&, error_code&, Write&& write)
+        write(resume_context&&, error_code&,
+            WriteFunction&& wf) noexcept
         {
             if(size_ - offset_ < sizeof(buf_))
                 buf_len_ = static_cast<std::size_t>(
@@ -75,7 +76,7 @@ struct file_body
             auto const nread = fread(buf_, 1, sizeof(buf_), file_);
             (void)nread;
             offset_ += buf_len_;
-            write(boost::asio::buffer(buf_, buf_len_));
+            wf(boost::asio::buffer(buf_, buf_len_));
             return offset_ >= size_;
         }
     };
diff --git a/include/beast/core/detail/type_traits.hpp b/include/beast/core/detail/type_traits.hpp
new file mode 100644
index 00000000..3944e514
--- /dev/null
+++ b/include/beast/core/detail/type_traits.hpp
@@ -0,0 +1,26 @@
+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// 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)
+//
+
+#ifndef BEAST_DETAIL_VOID_T_HPP
+#define BEAST_DETAIL_VOID_T_HPP
+
+namespace beast {
+namespace detail {
+
+template<class... Ts>
+struct make_void
+{
+    using type = void;
+};
+
+template<class... Ts>
+using void_t = typename make_void<Ts...>::type;
+
+} // detail
+} // beast
+
+#endif
diff --git a/include/beast/http/basic_dynabuf_body.hpp b/include/beast/http/basic_dynabuf_body.hpp
index 3a46aed0..f39defc7 100644
--- a/include/beast/http/basic_dynabuf_body.hpp
+++ b/include/beast/http/basic_dynabuf_body.hpp
@@ -62,33 +62,31 @@ private:
         DynamicBuffer const& body_;
 
     public:
-        writer(writer const&) = delete;
-        writer& operator=(writer const&) = delete;
-
         template<bool isRequest, class Headers>
         explicit
         writer(message<
-                isRequest, basic_dynabuf_body, Headers> const& m)
+                isRequest, basic_dynabuf_body, Headers> const& m) noexcept
             : body_(m.body)
         {
         }
 
         void
-        init(error_code& ec)
+        init(error_code& ec) noexcept
         {
         }
 
         std::uint64_t
-        content_length() const
+        content_length() const noexcept
         {
             return body_.size();
         }
 
-        template<class Write>
+        template<class WriteFunction>
         boost::tribool
-        operator()(resume_context&&, error_code&, Write&& write)
+        write(resume_context&&, error_code&,
+            WriteFunction&& wf) noexcept
         {
-            write(body_.data());
+            wf(body_.data());
             return true;
         }
     };
diff --git a/include/beast/http/concepts.hpp b/include/beast/http/concepts.hpp
index fc457a3f..d490196d 100644
--- a/include/beast/http/concepts.hpp
+++ b/include/beast/http/concepts.hpp
@@ -9,7 +9,10 @@
 #define BEAST_HTTP_TYPE_CHECK_HPP
 
 #include <beast/core/error.hpp>
+#include <beast/core/detail/type_traits.hpp>
+#include <beast/http/resume_context.hpp>
 #include <boost/asio/buffer.hpp>
+#include <boost/logic/tribool.hpp>
 #include <type_traits>
 #include <utility>
 
@@ -18,63 +21,107 @@ namespace http {
 
 namespace detail {
 
-template<class T>
-class has_value_type
+struct write_function
 {
-    template<class U, class R =
-        typename U::value_type>
-    static std::true_type check(int);
+    template<class ConstBufferSequence>
+    void
+    operator()(ConstBufferSequence const&);
+};
+
+template<class T, class = beast::detail::void_t<>>
+struct has_value_type : std::false_type {};
+
+template<class T>
+struct has_value_type<T, beast::detail::void_t<
+    typename T::value_type
+        > > : std::true_type {};
+
+template<class T, class = beast::detail::void_t<>>
+struct has_content_length : std::false_type {};
+
+template<class T>
+struct has_content_length<T, beast::detail::void_t<decltype(
+    std::declval<T>().content_length()
+        )> > : std::true_type
+{
+    static_assert(std::is_convertible<
+        decltype(std::declval<T>().content_length()),
+            std::uint64_t>::value,
+        "Writer::content_length requirements not met");
+};
+
+#if 0
+template<class T, class M, class = beast::detail::void_t<>>
+struct is_Writer : std::false_type {};
+
+template<class T, class M>
+struct is_Writer<T, M, beast::detail::void_t<decltype(
+    std::declval<T>().init(
+        std::declval<error_code&>())
+    // VFALCO This is unfortunate, we have to provide the template
+    //        argument type because this is not a deduced context?
+    //
+    ,std::declval<T>().template write<detail::write_function>(
+        std::declval<resume_context>(),
+        std::declval<error_code&>(),
+        std::declval<detail::write_function>())
+            )> > : std::integral_constant<bool,
+    std::is_nothrow_constructible<T, M const&>::value &&
+    std::is_convertible<decltype(
+        std::declval<T>().template write<detail::write_function>(
+            std::declval<resume_context>(),
+            std::declval<error_code&>(),
+            std::declval<detail::write_function>())),
+                boost::tribool>::value
+        >
+{
+    static_assert(std::is_same<
+        typename M::body_type::writer, T>::value,
+            "Mismatched writer and message");
+};
+
+#else
+
+template<class T, class M>
+class is_Writer
+{
+    template<class U, class R = decltype(
+        std::declval<U>().init(std::declval<error_code&>()),
+            std::true_type{})>
+    static R check1(int);
     template<class>
-    static std::false_type check(...);
-    using type = decltype(check<T>(0));
-public:
-    static bool constexpr value = type::value;
-};
+    static std::false_type check1(...);
+    using type1 = decltype(check1<T>(0));
 
-template<class T, bool B = has_value_type<T>::value>
-struct extract_value_type
-{
-    using type = void;
-};
-
-template<class T>
-struct extract_value_type<T, true>
-{
-    using type = typename T::value_type;
-};
-
-template<class T>
-class has_reader
-{
+    // VFALCO This is unfortunate, we have to provide the template
+    //        argument type because this is not a deduced context?
+    //
     template<class U, class R =
-        typename U::reader>
-    static std::true_type check(int);
+        std::is_convertible<decltype(
+            std::declval<U>().template write<detail::write_function>(
+                std::declval<resume_context>(),
+                std::declval<error_code&>(),
+                std::declval<detail::write_function>()))
+            , boost::tribool>>
+    static R check2(int);
     template<class>
-    static std::false_type check(...);
-public:
-    using type = decltype(check<T>(0));
-};
+    static std::false_type check2(...);
+    using type2 = decltype(check2<T>(0));
 
-template<class T>
-class has_writer
-{
-    template<class U, class R =
-        typename U::writer>
-    static std::true_type check(int);
-    template<class>
-    static std::false_type check(...);
 public:
-    using type = decltype(check<T>(0));
-};
+    static_assert(std::is_same<
+        typename M::body_type::writer, T>::value,
+            "Mismatched writer and message");
 
-template<class T>
-struct is_Body
-{
     using type = std::integral_constant<bool,
-        has_value_type<T>::value 
+        std::is_nothrow_constructible<T, M const&>::value
+        && type1::value
+        && type2::value
     >;
 };
 
+#endif
+
 template<class T>
 class is_Parser
 {
@@ -116,31 +163,97 @@ public:
 
 } // detail
 
-/// Determine if `T` meets the requirements of `Body`.
+/// Determine if `T` meets the requirements of @b Body.
 template<class T>
 #if GENERATING_DOCS
 struct is_Body : std::integral_constant<bool, ...>{};
 #else
-using is_Body = typename detail::is_Body<T>::type;
+using is_Body = detail::has_value_type<T>;
 #endif
 
-/// Determine if `T` meets the requirements of `ReadableBody`.
-template<class T>
+/** Determine if a @ref Body has a nested type `reader`.
+
+    @tparam T The type to check, which must meet the
+    requirements of @b Body.
+*/
 #if GENERATING_DOCS
-struct is_ReadableBody : std::integral_constant<bool, ...>{};
-#else
-using is_ReadableBody = typename detail::has_reader<T>::type;
-#endif
-
-/// Determine if `T` meets the requirements of `WritableBody`.
 template<class T>
-#if GENERATING_DOCS
-struct is_WritableBody : std::integral_constant<bool, ...>{};
+struct has_reader : std::integral_constant<bool, ...>{};
 #else
-using is_WritableBody = typename detail::has_writer<T>::type;
+template<class T, class = beast::detail::void_t<>>
+struct has_reader : std::false_type {};
+
+template<class T>
+struct has_reader<T, beast::detail::void_t<
+    typename T::reader
+        > > : std::true_type {};
 #endif
 
-/// Determine if `T` meets the requirements of `Parser`.
+/** Determine if a @ref Body has a nested type `writer`.
+
+    @tparam T The type to check, which must meet the
+    requirements of @b Body.
+*/
+#if GENERATING_DOCS
+template<class T>
+struct has_writer : std::integral_constant<bool, ...>{};
+#else
+template<class T, class = beast::detail::void_t<>>
+struct has_writer : std::false_type {};
+
+template<class T>
+struct has_writer<T, beast::detail::void_t<
+    typename T::writer
+        > > : std::true_type {};
+#endif
+
+/** Determine if `T` meets the requirements of @b Reader for `M`.
+
+    @tparam T The type to test.
+
+    @tparam M The message type to test with, which must be of
+    type `message`.
+*/
+#if GENERATING_DOCS
+template<class T, class M>
+struct is_Reader : std::integral_constant<bool, ...> {};
+#else
+template<class T, class M, class = beast::detail::void_t<>>
+struct is_Reader : std::false_type {};
+
+template<class T, class M>
+struct is_Reader<T, M, beast::detail::void_t<decltype(
+    std::declval<T>().init(
+        std::declval<error_code&>()),
+    std::declval<T>().write(
+        std::declval<void const*>(),
+        std::declval<std::size_t>(),
+        std::declval<error_code&>())
+            )> > : std::integral_constant<bool,
+    std::is_nothrow_constructible<T, M&>::value
+        >
+{
+    static_assert(std::is_same<
+        typename M::body_type::reader, T>::value,
+            "Mismatched reader and message");
+};
+#endif
+
+/** Determine if `T` meets the requirements of @b Writer for `M`.
+
+    @tparam T The type to test.
+
+    @tparam M The message type to test with, which must be of
+    type `message`.
+*/
+template<class T, class M>
+#if GENERATING_DOCS
+struct is_Writer : std::integral_constant<bool, ...> {};
+#else
+using is_Writer = typename detail::is_Writer<T, M>::type;
+#endif
+
+/// Determine if `T` meets the requirements of @b Parser.
 template<class T>
 #if GENERATING_DOCS
 struct is_Parser : std::integral_constant<bool, ...>{};
diff --git a/include/beast/http/detail/has_content_length.hpp b/include/beast/http/detail/has_content_length.hpp
deleted file mode 100644
index 09584a5e..00000000
--- a/include/beast/http/detail/has_content_length.hpp
+++ /dev/null
@@ -1,43 +0,0 @@
-//
-// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com)
-//
-// 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)
-//
-
-#ifndef BEAST_HTTP_DETAIL_HAS_CONTENT_LENGTH_HPP
-#define BEAST_HTTP_DETAIL_HAS_CONTENT_LENGTH_HPP
-
-#include <cstdint>
-#include <type_traits>
-
-namespace beast {
-namespace http {
-namespace detail {
-
-template<class T>
-class has_content_length_value
-{
-    template<class U, class R = typename std::is_convertible<
-        decltype(std::declval<U>().content_length()),
-            std::uint64_t>>
-    static R check(int);
-    template<class>
-    static std::false_type check(...);
-    using type = decltype(check<T>(0));
-public:
-    // `true` if `T` meets the requirements.
-    static bool const value = type::value;
-};
-
-// Determines if the writer can provide the content length
-template<class T>
-using has_content_length =
-    std::integral_constant<bool,
-        has_content_length_value<T>::value>;
-
-} // detail
-} // http
-} // beast
-
-#endif
diff --git a/include/beast/http/empty_body.hpp b/include/beast/http/empty_body.hpp
index 152a71fe..1c1d45a7 100644
--- a/include/beast/http/empty_body.hpp
+++ b/include/beast/http/empty_body.hpp
@@ -35,31 +35,29 @@ private:
 
     struct writer
     {
-        writer(writer const&) = delete;
-        writer& operator=(writer const&) = delete;
-
         template<bool isRequest, class Headers>
         explicit
-        writer(message<isRequest, empty_body, Headers> const& m)
+        writer(message<isRequest, empty_body, Headers> const& m) noexcept
         {
         }
 
         void
-        init(error_code& ec)
+        init(error_code& ec) noexcept
         {
         }
 
         std::uint64_t
-        content_length() const
+        content_length() const noexcept
         {
             return 0;
         }
 
-        template<class Write>
+        template<class WriteFunction>
         boost::tribool
-        operator()(resume_context&&, error_code&, Write&& write)
+        write(resume_context&&, error_code&,
+            WriteFunction&& wf) noexcept
         {
-            write(boost::asio::null_buffers{});
+            wf(boost::asio::null_buffers{});
             return true;
         }
     };
diff --git a/include/beast/http/impl/message_v1.ipp b/include/beast/http/impl/message_v1.ipp
index dafe19d6..2cc21203 100644
--- a/include/beast/http/impl/message_v1.ipp
+++ b/include/beast/http/impl/message_v1.ipp
@@ -9,9 +9,9 @@
 #define BEAST_HTTP_IMPL_MESSAGE_V1_IPP
 
 #include <beast/core/error.hpp>
+#include <beast/http/concepts.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/core/detail/ci_char_traits.hpp>
-#include <beast/http/detail/has_content_length.hpp>
 #include <boost/optional.hpp>
 #include <stdexcept>
 
@@ -116,8 +116,13 @@ prepare(message_v1<isRequest, Body, Headers>& msg,
     Options&&... options)
 {
     // VFALCO TODO
-    //static_assert(is_WritableBody<Body>::value,
-    //  "WritableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_writer<Body>::value,
+        "Body has no writer");
+    static_assert(is_Writer<typename Body::writer,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Writer requirements not met");
     detail::prepare_info pi;
     detail::prepare_content_length(pi, msg,
         detail::has_content_length<typename Body::writer>{});
diff --git a/include/beast/http/impl/read.ipp b/include/beast/http/impl/read.ipp
index 1c3c6bb3..7d5d1c65 100644
--- a/include/beast/http/impl/read.ipp
+++ b/include/beast/http/impl/read.ipp
@@ -150,8 +150,13 @@ read(SyncReadStream& stream, DynamicBuffer& dynabuf,
         "SyncReadStream requirements not met");
     static_assert(is_DynamicBuffer<DynamicBuffer>::value,
         "DynamicBuffer requirements not met");
-    static_assert(is_ReadableBody<Body>::value,
-        "ReadableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_reader<Body>::value,
+        "Body has no reader");
+    static_assert(is_Reader<typename Body::reader,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Reader requirements not met");
     error_code ec;
     beast::http::read(stream, dynabuf, msg, ec);
     if(ec)
@@ -169,8 +174,13 @@ read(SyncReadStream& stream, DynamicBuffer& dynabuf,
         "SyncReadStream requirements not met");
     static_assert(is_DynamicBuffer<DynamicBuffer>::value,
         "DynamicBuffer requirements not met");
-    static_assert(is_ReadableBody<Body>::value,
-        "ReadableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_reader<Body>::value,
+        "Body has no reader");
+    static_assert(is_Reader<typename Body::reader,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Reader requirements not met");
     parser_v1<isRequest, Body, Headers> p;
     beast::http::parse(stream, dynabuf, p, ec);
     if(ec)
@@ -192,8 +202,13 @@ async_read(AsyncReadStream& stream, DynamicBuffer& dynabuf,
         "AsyncReadStream requirements not met");
     static_assert(is_DynamicBuffer<DynamicBuffer>::value,
         "DynamicBuffer requirements not met");
-    static_assert(is_ReadableBody<Body>::value,
-        "ReadableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_reader<Body>::value,
+        "Body has no reader");
+    static_assert(is_Reader<typename Body::reader,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Reader requirements not met");
     beast::async_completion<ReadHandler,
         void(error_code)> completion(handler);
     detail::read_op<AsyncReadStream, DynamicBuffer,
diff --git a/include/beast/http/impl/write.ipp b/include/beast/http/impl/write.ipp
index 02628f4b..039b434a 100644
--- a/include/beast/http/impl/write.ipp
+++ b/include/beast/http/impl/write.ipp
@@ -11,7 +11,6 @@
 #include <beast/http/concepts.hpp>
 #include <beast/http/resume_context.hpp>
 #include <beast/http/detail/chunk_encode.hpp>
-#include <beast/http/detail/has_content_length.hpp>
 #include <beast/core/buffer_cat.hpp>
 #include <beast/core/bind_handler.hpp>
 #include <beast/core/buffer_concepts.hpp>
@@ -184,7 +183,7 @@ class write_op
         }
 
         template<class ConstBufferSequence>
-        void operator()(ConstBufferSequence const& buffers)
+        void operator()(ConstBufferSequence const& buffers) const
         {
             auto& d = *self_.d_;
             // write headers and body
@@ -212,7 +211,7 @@ class write_op
         }
 
         template<class ConstBufferSequence>
-        void operator()(ConstBufferSequence const& buffers)
+        void operator()(ConstBufferSequence const& buffers) const
         {
             auto& d = *self_.d_;
             // write body
@@ -323,7 +322,7 @@ operator()(error_code ec, std::size_t, bool again)
 
         case 1:
         {
-            auto const result = d.wp.w(
+            boost::tribool const result = d.wp.w.write(
                 std::move(d.copy), ec, writef0_lambda{*this});
             if(ec)
             {
@@ -354,7 +353,7 @@ operator()(error_code ec, std::size_t, bool again)
 
         case 3:
         {
-            auto const result = d.wp.w(
+            boost::tribool result = d.wp.w.write(
                 std::move(d.copy), ec, writef_lambda{*this});
             if(ec)
             {
@@ -420,7 +419,7 @@ public:
     }
 
     template<class ConstBufferSequence>
-    void operator()(ConstBufferSequence const& buffers)
+    void operator()(ConstBufferSequence const& buffers) const
     {
         // write headers and body
         if(chunked_)
@@ -449,7 +448,7 @@ public:
     }
 
     template<class ConstBufferSequence>
-    void operator()(ConstBufferSequence const& buffers)
+    void operator()(ConstBufferSequence const& buffers) const
     {
         // write body
         if(chunked_)
@@ -472,8 +471,13 @@ write(SyncWriteStream& stream,
 {
     static_assert(is_SyncWriteStream<SyncWriteStream>::value,
         "SyncWriteStream requirements not met");
-    static_assert(is_WritableBody<Body>::value,
-        "WritableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_writer<Body>::value,
+        "Body has no writer");
+    static_assert(is_Writer<typename Body::writer,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Writer requirements not met");
     error_code ec;
     write(stream, msg, ec);
     if(ec)
@@ -489,8 +493,13 @@ write(SyncWriteStream& stream,
 {
     static_assert(is_SyncWriteStream<SyncWriteStream>::value,
         "SyncWriteStream requirements not met");
-    static_assert(is_WritableBody<Body>::value,
-        "WritableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_writer<Body>::value,
+        "Body has no writer");
+    static_assert(is_Writer<typename Body::writer,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Writer requirements not met");
     detail::write_preparation<isRequest, Body, Headers> wp(msg);
     wp.init(ec);
     if(ec)
@@ -506,9 +515,11 @@ write(SyncWriteStream& stream,
             cv.notify_one();
         }};
     auto copy = resume;
-    boost::tribool result = wp.w(std::move(copy),
-        ec, detail::writef0_lambda<SyncWriteStream,
-            decltype(wp.sb)>{stream, wp.sb, wp.chunked, ec});
+    boost::tribool result =
+        wp.w.write(std::move(copy), ec,
+            detail::writef0_lambda<SyncWriteStream,
+                decltype(wp.sb)>{stream,
+                    wp.sb, wp.chunked, ec});
     if(ec)
         return;
     if(boost::indeterminate(result))
@@ -527,11 +538,16 @@ write(SyncWriteStream& stream,
     wp.sb.consume(wp.sb.size());
     if(! result)
     {
+        detail::writef_lambda<SyncWriteStream> wf{
+            stream, wp.chunked, ec};
         for(;;)
         {
-            result = wp.w(std::move(copy), ec,
-                detail::writef_lambda<SyncWriteStream>{
-                    stream, wp.chunked, ec});
+#if 0
+            result = wp.w.write(std::move(copy), ec, wf);
+#else
+            result = wp.w.write(std::move(copy), ec,
+                detail::writef_lambda<SyncWriteStream>{stream, wp.chunked, ec});            
+#endif
             if(ec)
                 return;
             if(result)
@@ -573,8 +589,13 @@ async_write(AsyncWriteStream& stream,
 {
     static_assert(is_AsyncWriteStream<AsyncWriteStream>::value,
         "AsyncWriteStream requirements not met");
-    static_assert(is_WritableBody<Body>::value,
-        "WritableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_writer<Body>::value,
+        "Body has no writer");
+    static_assert(is_Writer<typename Body::writer,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Writer requirements not met");
     beast::async_completion<WriteHandler,
         void(error_code)> completion(handler);
     detail::write_op<AsyncWriteStream, decltype(completion.handler),
@@ -636,8 +657,13 @@ std::ostream&
 operator<<(std::ostream& os,
     message_v1<isRequest, Body, Headers> const& msg)
 {
-    static_assert(is_WritableBody<Body>::value,
-        "WritableBody requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_writer<Body>::value,
+        "Body has no writer");
+    static_assert(is_Writer<typename Body::writer,
+        message_v1<isRequest, Body, Headers>>::value,
+            "Writer requirements not met");
     detail::ostream_SyncStream oss(os);
     error_code ec;
     write(oss, msg, ec);
diff --git a/include/beast/http/parser_v1.hpp b/include/beast/http/parser_v1.hpp
index dc1f4f99..ea610b3b 100644
--- a/include/beast/http/parser_v1.hpp
+++ b/include/beast/http/parser_v1.hpp
@@ -91,13 +91,12 @@ private:
     using reader =
         typename message_type::body_type::reader;
 
-    static_assert(is_ReadableBody<Body>::value,
-        "ReadableBody requirements not met");
-
-    // Reader must be nothrow constructible
-    static_assert(std::is_nothrow_constructible<
-        reader, message_type&>::value,
-            "Reader requirements not met");
+    static_assert(is_Body<Body>::value,
+        "Body requirements not met");
+    static_assert(has_reader<Body>::value,
+        "Body has no reader");
+    static_assert(is_Reader<reader, message_type>::value,
+        "Reader requirements not met");
 
     std::string field_;
     std::string value_;
diff --git a/include/beast/http/string_body.hpp b/include/beast/http/string_body.hpp
index 1148c67f..25790487 100644
--- a/include/beast/http/string_body.hpp
+++ b/include/beast/http/string_body.hpp
@@ -62,33 +62,31 @@ private:
         value_type const& body_;
 
     public:
-        writer(writer const&) = delete;
-        writer& operator=(writer const&) = delete;
-
         template<bool isRequest, class Headers>
         explicit
         writer(message<
-                isRequest, string_body, Headers> const& msg)
+                isRequest, string_body, Headers> const& msg) noexcept
             : body_(msg.body)
         {
         }
 
         void
-        init(error_code& ec)
+        init(error_code& ec) noexcept
         {
         }
 
         std::uint64_t
-        content_length() const
+        content_length() const noexcept
         {
             return body_.size();
         }
 
-        template<class Write>
+        template<class WriteFunction>
         boost::tribool
-        operator()(resume_context&&, error_code&, Write&& write)
+        write(resume_context&&, error_code&,
+            WriteFunction&& wf) noexcept
         {
-            write(boost::asio::buffer(body_));
+            wf(boost::asio::buffer(body_));
             return true;
         }
     };
diff --git a/test/http/write.cpp b/test/http/write.cpp
index f08c6b62..e1041703 100644
--- a/test/http/write.cpp
+++ b/test/http/write.cpp
@@ -104,21 +104,22 @@ public:
         public:
             template<bool isRequest, class Allocator>
             explicit
-            writer(message<isRequest, unsized_body, Allocator> const& msg)
+            writer(message<isRequest, unsized_body, Allocator> const& msg) noexcept
                 : body_(msg.body)
             {
             }
 
             void
-            init(error_code& ec)
+            init(error_code& ec) noexcept
             {
             }
 
-            template<class Write>
+            template<class WriteFunction>
             boost::tribool
-            operator()(resume_context&&, error_code&, Write&& write)
+            write(resume_context&&, error_code&,
+                WriteFunction&& wf) noexcept
             {
-                write(boost::asio::buffer(body_));
+                wf(boost::asio::buffer(body_));
                 return true;
             }
         };
@@ -168,13 +169,13 @@ public:
         public:
             template<bool isRequest, class Allocator>
             explicit
-            writer(message<isRequest, fail_body, Allocator> const& msg)
+            writer(message<isRequest, fail_body, Allocator> const& msg) noexcept
                 : body_(msg.body)
             {
             }
 
             void
-            init(error_code& ec)
+            init(error_code& ec) noexcept
             {
                 body_.fc_.fail(ec);
             }
@@ -197,9 +198,10 @@ public:
                 }
             };
 
-            template<class Write>
+            template<class WriteFunction>
             boost::tribool
-            operator()(resume_context&& rc, error_code& ec, Write&& write)
+            write(resume_context&& rc, error_code& ec,
+                WriteFunction&& wf) noexcept
             {
                 if(body_.fc_.fail(ec))
                     return false;
@@ -211,7 +213,7 @@ public:
                 }
                 if(n_ >= body_.s_.size())
                     return true;
-                write(boost::asio::buffer(body_.s_.data() + n_, 1));
+                wf(boost::asio::buffer(body_.s_.data() + n_, 1));
                 ++n_;
                 return n_ == body_.s_.size();
             }