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59 lines
2.0 KiB
C++
59 lines
2.0 KiB
C++
/*!
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@file
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Forward declares `boost::hana::eval`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_EVAL_HPP
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#define BOOST_HANA_FWD_EVAL_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Evaluate a lazy value and return it.
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//! @relates hana::lazy
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//!
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//! Given a lazy expression `expr`, `eval` evaluates `expr` and returns
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//! the result as a normal value. However, for convenience, `eval` can
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//! also be used with nullary and unary function objects. Specifically,
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//! if `expr` is not a `hana::lazy`, it is called with no arguments at
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//! all and the result of that call (`expr()`) is returned. Otherwise,
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//! if `expr()` is ill-formed, then `expr(hana::id)` is returned instead.
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//! If that expression is ill-formed, then a compile-time error is
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//! triggered.
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//!
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//! The reason for allowing nullary callables in `eval` is because this
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//! allows using nullary lambdas as lazy branches to `eval_if`, which
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//! is convenient. The reason for allowing unary callables and calling
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//! them with `hana::id` is because this allows deferring the
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//! compile-time evaluation of selected expressions inside the callable.
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//! How this can be achieved is documented by `hana::eval_if`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/eval.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto eval = [](auto&& see_documentation) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename T, typename = void>
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struct eval_impl : eval_impl<T, when<true>> { };
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struct eval_t {
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template <typename Expr>
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constexpr decltype(auto) operator()(Expr&& expr) const;
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};
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constexpr eval_t eval{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_EVAL_HPP
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