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+CPPromise
+=========
+
+This is a lightweight C++14/17 compatiable implementation of promises (similar
+to Javascript Promise/A+). It allows type-safe polymorphic promises and incurs
+little runtime overhead. The runtime type-checking is enforced and supported by
+the underlying `any` type, an exception will be thrown when the resolved value
+types do not match the types expected in the subsequent computation. See
+`test.cpp` for detailed examples.
+
+API
+===
+
+::
+
+    typename<typename Func> promise_t(Func callback);
+
+Create a new promise object, the ``callback(promise_t pm)`` is invoked
+immediately after the object is constructed, so usually the user registers
+``pm`` to some external logic which triggers ``pm.resolve()`` or
+``pm.reject()`` when the time comes.
+
+::
+
+    template<typename T> resolve(T result) const;
+
+Resolve the promise with value ``result``. This may trigger the other promises
+waiting for the current promise recursively. When a promise is triggered, the
+registered ``on_fulfilled()`` function will be invoked using ``result`` as the argument.
+
+::
+
+    tempalte<typename T> reject(T reason) const;
+
+Reject the promise with value ``result``. This may reject the other promises
+waiting for the current promise recursively. When a promise is rejected, the
+registered ``on_rejected()`` function will be invoked using ``reason`` as the argument.
+
+::
+
+    template<typename FuncFulfilled>
+    promise_t then(FuncFulfilled on_fulfilled) const;
+
+Create a new promise that waits for the resolution of the current promise.
+``on_fulfilled`` will be called with result from the current promise when
+resolved. The rejection will skip the callback and pass on to the promises that
+follow the created promise.
+
+::
+
+    template<typename FuncRejected>
+    promise_t fail(FuncRejected on_rejected) const;
+
+Create a new promise that waits for the rejection of the current promise.
+``on_rejected`` will be called with reason from the current promise when
+rejected. The resolution will skip the callback and pass on to the promises
+that follow the created promise.
+
+::
+
+    template<typename FuncFulfilled, typename FuncRejected>
+    inline promise_t then(FuncFulfilled on_fulfilled,
+                            FuncRejected on_rejected) const;
+
+Create a promise that handles both resolution and rejection of the current promise.