Added thirdparty: boost library

thirdparty/include/boost/hana/fwd/monadic_fold_left.hpp

@@ -0,0 +1,110 @@
+/*!
+@file
+Forward declares `boost::hana::monadic_fold_left`.
+
+Copyright Louis Dionne 2013-2022
+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_MONADIC_FOLD_LEFT_HPP
+#define BOOST_HANA_FWD_MONADIC_FOLD_LEFT_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+namespace boost { namespace hana {
+    //! Monadic left-fold of a structure with a binary operation and an
+    //! optional initial reduction state.
+    //! @ingroup group-Foldable
+    //!
+    //! @note
+    //! This assumes the reader to be accustomed to non-monadic left-folds as
+    //! explained by `hana::fold_left`, and to have read the [primer]
+    //! (@ref monadic-folds) on monadic folds.
+    //!
+    //! `monadic_fold_left<M>` is a left-associative monadic fold. Given a
+    //! `Foldable` with linearization `[x1, ..., xn]`, a function `f` and an
+    //! optional initial state, `monadic_fold_left<M>` applies `f` as follows:
+    //! @code
+    //!     // with state
+    //!     ((((f(state, x1) | f(-, x2)) | f(-, x3)) | ...) | f(-, xn))
+    //!
+    //!     // without state
+    //!     ((((f(x1, x2) | f(-, x3)) | f(-, x4)) | ...) | f(-, xn))
+    //! @endcode
+    //!
+    //! where `f(-, xk)` denotes the partial application of `f` to `xk`, and
+    //! `|` is just the operator version of the monadic `chain`.
+    //!
+    //! When the structure is empty, one of two things may happen. If an
+    //! initial state was provided, it is lifted to the given Monad and
+    //! returned as-is. Otherwise, if the no-state version of the function
+    //! was used, an error is triggered. When the stucture contains a single
+    //! element and the no-state version of the function was used, that
+    //! single element is lifted into the given Monad and returned as is.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a `Monad` `M`, a `Foldable` `F`, an initial state of tag `S`,
+    //! and a function @f$ f : S \times T \to M(S) @f$, the signatures of
+    //! `monadic_fold_left<M>` are
+    //! \f[
+    //!     \mathtt{monadic\_fold\_left}_M :
+    //!         F(T) \times S \times (S \times T \to M(S)) \to M(S)
+    //! \f]
+    //!
+    //! for the version with an initial state, and
+    //! \f[
+    //!     \mathtt{monadic\_fold\_left}_M :
+    //!         F(T) \times (T \times T \to M(T)) \to M(T)
+    //! \f]
+    //!
+    //! for the version without an initial state.
+    //!
+    //! @tparam M
+    //! The Monad representing the monadic context in which the fold happens.
+    //! The return type of `f` must be in that Monad.
+    //!
+    //! @param xs
+    //! The structure to fold.
+    //!
+    //! @param state
+    //! The initial value used for folding. If the structure is empty, this
+    //! value is lifted in to the `M` Monad and then returned as-is.
+    //!
+    //! @param f
+    //! A binary function called as `f(state, x)`, where `state` is the result
+    //! accumulated so far and `x` is an element in the structure. The
+    //! function must return its result inside the `M` Monad.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/monadic_fold_left.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    template <typename M>
+    constexpr auto monadic_fold_left = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename = void>
+    struct monadic_fold_left_impl : monadic_fold_left_impl<T, when<true>> { };
+
+    template <typename M>
+    struct monadic_fold_left_t {
+        template <typename Xs, typename State, typename F>
+        constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const;
+
+        template <typename Xs, typename F>
+        constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
+    };
+
+    template <typename M>
+    BOOST_HANA_INLINE_VARIABLE constexpr monadic_fold_left_t<M> monadic_fold_left{};
+#endif
+}} // end namespace boost::hana
+
+#endif // !BOOST_HANA_FWD_MONADIC_FOLD_LEFT_HPP