hana/fwd/eval.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::eval`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_EVAL_HPP
0011 #define BOOST_HANA_FWD_EVAL_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Evaluate a lazy value and return it.
0019     //! @relates hana::lazy
0020     //!
0021     //! Given a lazy expression `expr`, `eval` evaluates `expr` and returns
0022     //! the result as a normal value. However, for convenience, `eval` can
0023     //! also be used with nullary and unary function objects. Specifically,
0024     //! if `expr` is not a `hana::lazy`, it is called with no arguments at
0025     //! all and the result of that call (`expr()`) is returned. Otherwise,
0026     //! if `expr()` is ill-formed, then `expr(hana::id)` is returned instead.
0027     //! If that expression is ill-formed, then a compile-time error is
0028     //! triggered.
0029     //!
0030     //! The reason for allowing nullary callables in `eval` is because this
0031     //! allows using nullary lambdas as lazy branches to `eval_if`, which
0032     //! is convenient. The reason for allowing unary callables and calling
0033     //! them with `hana::id` is because this allows deferring the
0034     //! compile-time evaluation of selected expressions inside the callable.
0035     //! How this can be achieved is documented by `hana::eval_if`.
0036     //!
0037     //!
0038     //! Example
0039     //! -------
0040     //! @include example/eval.cpp
0041 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0042     constexpr auto eval = [](auto&& see_documentation) -> decltype(auto) {
0043         return tag-dispatched;
0044     };
0045 #else
0046     template <typename T, typename = void>
0047     struct eval_impl : eval_impl<T, when<true>> { };
0048
0049     struct eval_t {
0050         template <typename Expr>
0051         constexpr decltype(auto) operator()(Expr&& expr) const;
0052     };
0053
0054     BOOST_HANA_INLINE_VARIABLE constexpr eval_t eval{};
0055 #endif
0056 }} // end namespace boost::hana
0057
0058 #endif // !BOOST_HANA_FWD_EVAL_HPP