1/*!
2@file
3Forward declares `boost::hana::eval`.
4
5Copyright Louis Dionne 2013-2022
6Distributed under the Boost Software License, Version 1.0.
7(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8 */
9
10#ifndef BOOST_HANA_FWD_EVAL_HPP
11#define BOOST_HANA_FWD_EVAL_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/core/when.hpp>
15
16
17namespace boost { namespace hana {
18 //! Evaluate a lazy value and return it.
19 //! @relates hana::lazy
20 //!
21 //! Given a lazy expression `expr`, `eval` evaluates `expr` and returns
22 //! the result as a normal value. However, for convenience, `eval` can
23 //! also be used with nullary and unary function objects. Specifically,
24 //! if `expr` is not a `hana::lazy`, it is called with no arguments at
25 //! all and the result of that call (`expr()`) is returned. Otherwise,
26 //! if `expr()` is ill-formed, then `expr(hana::id)` is returned instead.
27 //! If that expression is ill-formed, then a compile-time error is
28 //! triggered.
29 //!
30 //! The reason for allowing nullary callables in `eval` is because this
31 //! allows using nullary lambdas as lazy branches to `eval_if`, which
32 //! is convenient. The reason for allowing unary callables and calling
33 //! them with `hana::id` is because this allows deferring the
34 //! compile-time evaluation of selected expressions inside the callable.
35 //! How this can be achieved is documented by `hana::eval_if`.
36 //!
37 //!
38 //! Example
39 //! -------
40 //! @include example/eval.cpp
41#ifdef BOOST_HANA_DOXYGEN_INVOKED
42 constexpr auto eval = [](auto&& see_documentation) -> decltype(auto) {
43 return tag-dispatched;
44 };
45#else
46 template <typename T, typename = void>
47 struct eval_impl : eval_impl<T, when<true>> { };
48
49 struct eval_t {
50 template <typename Expr>
51 constexpr decltype(auto) operator()(Expr&& expr) const;
52 };
53
54 BOOST_HANA_INLINE_VARIABLE constexpr eval_t eval{};
55#endif
56}} // end namespace boost::hana
57
58#endif // !BOOST_HANA_FWD_EVAL_HPP
59

source code of boost/libs/hana/include/boost/hana/fwd/eval.hpp