/*!
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@file
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Forward declares `boost::hana::eval_if`.
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@copyright Louis Dionne 2013-2017
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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_IF_HPP
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#define BOOST_HANA_FWD_EVAL_IF_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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//! Conditionally execute one of two branches based on a condition.
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//! @ingroup group-Logical
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//!
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//! Given a condition and two branches in the form of lambdas or
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//! `hana::lazy`s, `eval_if` will evaluate the branch selected by the
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//! condition with `eval` and return the result. The exact requirements
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//! for what the branches may be are the same requirements as those for
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//! the `eval` function.
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//!
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//!
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//! Deferring compile-time evaluation inside `eval_if`
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//! --------------------------------------------------
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//! By passing a unary callable to `eval_if`, it is possible to defer
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//! the compile-time evaluation of selected expressions inside the
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//! lambda. This is useful when instantiating a branch would trigger
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//! a compile-time error; we only want the branch to be instantiated
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//! when that branch is selected. Here's how it can be achieved.
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//!
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//! For simplicity, we'll use a unary lambda as our unary callable.
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//! Our lambda must accept a parameter (usually called `_`), which
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//! can be used to defer the compile-time evaluation of expressions
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//! as required. For example,
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//! @code
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//! template <typename N>
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//! auto fact(N n) {
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//! return hana::eval_if(n == hana::int_c<0>,
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//! [] { return hana::int_c<1>; },
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//! [=](auto _) { return n * fact(_(n) - hana::int_c<1>); }
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//! );
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//! }
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//! @endcode
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//!
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//! What happens here is that `eval_if` will call `eval` on the selected
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//! branch. In turn, `eval` will call the selected branch either with
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//! nothing -- for the _then_ branch -- or with `hana::id` -- for the
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//! _else_ branch. Hence, `_(x)` is always the same as `x`, but the
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//! compiler can't tell until the lambda has been called! Hence, the
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//! compiler has to wait before it instantiates the body of the lambda
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//! and no infinite recursion happens. However, this trick to delay the
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//! instantiation of the lambda's body can only be used when the condition
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//! is known at compile-time, because otherwise both branches have to be
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//! instantiated inside the `eval_if` anyway.
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//!
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//! There are several caveats to note with this approach to lazy branching.
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//! First, because we're using lambdas, it means that the function's
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//! result can't be used in a constant expression. This is a limitation
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//! of the current language.
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//!
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//! The second caveat is that compilers currently have several bugs
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//! regarding deeply nested lambdas with captures. So you always risk
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//! crashing the compiler, but this is a question of time before it is
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//! not a problem anymore.
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//!
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//! Finally, it means that conditionals can't be written directly inside
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//! unevaluated contexts. The reason is that a lambda can't appear in an
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//! unevaluated context, for example in `decltype`. One way to workaround
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//! this is to completely lift your type computations into variable
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//! templates instead. For example, instead of writing
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//! @code
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//! template <typename T>
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//! struct pointerize : decltype(
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//! hana::eval_if(hana::traits::is_pointer(hana::type_c<T>),
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//! [] { return hana::type_c<T>; },
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//! [](auto _) { return _(hana::traits::add_pointer)(hana::type_c<T>); }
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//! ))
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//! { };
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//! @endcode
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//!
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//! you could instead write
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//!
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//! @code
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//! template <typename T>
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//! auto pointerize_impl(T t) {
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//! return hana::eval_if(hana::traits::is_pointer(t),
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//! [] { return hana::type_c<T>; },
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//! [](auto _) { return _(hana::traits::add_pointer)(hana::type_c<T>); }
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//! );
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//! }
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//!
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//! template <typename T>
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//! using pointerize = decltype(pointerize_impl(hana::type_c<T>));
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//! @endcode
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//!
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//! > __Note__: This example would actually be implemented more easily
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//! > with partial specializations, but my bag of good examples is empty
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//! > at the time of writing this.
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//!
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//! Now, this hoop-jumping only has to be done in one place, because
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//! you should use normal function notation everywhere else in your
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//! metaprogram to perform type computations. So the syntactic
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//! cost is amortized over the whole program.
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//!
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//! Another way to work around this limitation of the language would be
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//! to use `hana::lazy` for the branches. However, this is only suitable
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//! when the branches are not too complicated. With `hana::lazy`, you
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//! could write the previous example as
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//! @code
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//! template <typename T>
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//! struct pointerize : decltype(
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//! hana::eval_if(hana::traits::is_pointer(hana::type_c<T>),
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//! hana::make_lazy(hana::type_c<T>),
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//! hana::make_lazy(hana::traits::add_pointer)(hana::type_c<T>)
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//! ))
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//! { };
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//! @endcode
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//!
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//!
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//! @param cond
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//! The condition determining which of the two branches is selected.
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//!
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//! @param then
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//! An expression called as `eval(then)` if `cond` is true-valued.
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//!
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//! @param else_
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//! A function called as `eval(else_)` if `cond` is false-valued.
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//!
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//!
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//! Example
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//! -------
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//! @include example/eval_if.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto eval_if = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename L, typename = void>
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struct eval_if_impl : eval_if_impl<L, when<true>> { };
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struct eval_if_t {
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template <typename Cond, typename Then, typename Else>
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constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const;
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};
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constexpr eval_if_t eval_if{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_EVAL_IF_HPP
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