ecency-mobile/ios/Pods/boost-for-react-native/boost/hana/fwd/eval_if.hpp

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/*!
@file
Forward declares `boost::hana::eval_if`.
@copyright Louis Dionne 2013-2016
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_EVAL_IF_HPP
#define BOOST_HANA_FWD_EVAL_IF_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Conditionally execute one of two branches based on a condition.
//! @ingroup group-Logical
//!
//! Given a condition and two branches in the form of lambdas or
//! `hana::lazy`s, `eval_if` will evaluate the branch selected by the
//! condition with `eval` and return the result. The exact requirements
//! for what the branches may be are the same requirements as those for
//! the `eval` function.
//!
//!
//! Deferring compile-time evaluation inside `eval_if`
//! --------------------------------------------------
//! By passing a unary callable to `eval_if`, it is possible to defer
//! the compile-time evaluation of selected expressions inside the
//! lambda. This is useful when instantiating a branch would trigger
//! a compile-time error; we only want the branch to be instantiated
//! when that branch is selected. Here's how it can be achieved.
//!
//! For simplicity, we'll use a unary lambda as our unary callable.
//! Our lambda must accept a parameter (usually called `_`), which
//! can be used to defer the compile-time evaluation of expressions
//! as required. For example,
//! @code
//! template <typename N>
//! auto fact(N n) {
//! return hana::eval_if(n == hana::int_c<0>,
//! [] { return hana::int_c<1>; },
//! [=](auto _) { return n * fact(_(n) - hana::int_c<1>); }
//! );
//! }
//! @endcode
//!
//! What happens here is that `eval_if` will call `eval` on the selected
//! branch. In turn, `eval` will call the selected branch either with
//! nothing -- for the _then_ branch -- or with `hana::id` -- for the
//! _else_ branch. Hence, `_(x)` is always the same as `x`, but the
//! compiler can't tell until the lambda has been called! Hence, the
//! compiler has to wait before it instantiates the body of the lambda
//! and no infinite recursion happens. However, this trick to delay the
//! instantiation of the lambda's body can only be used when the condition
//! is known at compile-time, because otherwise both branches have to be
//! instantiated inside the `eval_if` anyway.
//!
//! There are several caveats to note with this approach to lazy branching.
//! First, because we're using lambdas, it means that the function's
//! result can't be used in a constant expression. This is a limitation
//! of the current language.
//!
//! The second caveat is that compilers currently have several bugs
//! regarding deeply nested lambdas with captures. So you always risk
//! crashing the compiler, but this is a question of time before it is
//! not a problem anymore.
//!
//! Finally, it means that conditionals can't be written directly inside
//! unevaluated contexts. The reason is that a lambda can't appear in an
//! unevaluated context, for example in `decltype`. One way to workaround
//! this is to completely lift your type computations into variable
//! templates instead. For example, instead of writing
//! @code
//! template <typename T>
//! struct pointerize : decltype(
//! hana::eval_if(hana::traits::is_pointer(hana::type_c<T>),
//! [] { return hana::type_c<T>; },
//! [](auto _) { return _(hana::traits::add_pointer)(hana::type_c<T>); }
//! ))
//! { };
//! @endcode
//!
//! you could instead write
//!
//! @code
//! template <typename T>
//! auto pointerize_impl(T t) {
//! return hana::eval_if(hana::traits::is_pointer(t),
//! [] { return hana::type_c<T>; },
//! [](auto _) { return _(hana::traits::add_pointer)(hana::type_c<T>); }
//! );
//! }
//!
//! template <typename T>
//! using pointerize = decltype(pointerize_impl(hana::type_c<T>));
//! @endcode
//!
//! > __Note__: This example would actually be implemented more easily
//! > with partial specializations, but my bag of good examples is empty
//! > at the time of writing this.
//!
//! Now, this hoop-jumping only has to be done in one place, because
//! you should use normal function notation everywhere else in your
//! metaprogram to perform type computations. So the syntactic
//! cost is amortized over the whole program.
//!
//! Another way to work around this limitation of the language would be
//! to use `hana::lazy` for the branches. However, this is only suitable
//! when the branches are not too complicated. With `hana::lazy`, you
//! could write the previous example as
//! @code
//! template <typename T>
//! struct pointerize : decltype(
//! hana::eval_if(hana::traits::is_pointer(hana::type_c<T>),
//! hana::make_lazy(hana::type_c<T>),
//! hana::make_lazy(hana::traits::add_pointer)(hana::type_c<T>)
//! ))
//! { };
//! @endcode
//!
//!
//! @param cond
//! The condition determining which of the two branches is selected.
//!
//! @param then
//! An expression called as `eval(then)` if `cond` is true-valued.
//!
//! @param else_
//! A function called as `eval(else_)` if `cond` is false-valued.
//!
//!
//! Example
//! -------
//! @include example/eval_if.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto eval_if = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename L, typename = void>
struct eval_if_impl : eval_if_impl<L, when<true>> { };
struct eval_if_t {
template <typename Cond, typename Then, typename Else>
constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const;
};
constexpr eval_if_t eval_if{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_EVAL_IF_HPP