ceph/src/boost/boost/hana/fwd/fold_left.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::fold_left`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed 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_FOLD_LEFT_HPP
  11 #define BOOST_HANA_FWD_FOLD_LEFT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Left-fold of a structure using a binary operation and an optional
  19     //! initial reduction state.
  20     //! @ingroup group-Foldable
  21     //!
  22     //! `fold_left` is a left-associative fold using a binary operation.
  23     //! Given a structure containing `x1, ..., xn`, a function `f` and
  24     //! an optional initial state, `fold_left` applies `f` as follows
  25     //! @code
  26     //!     f(... f(f(f(x1, x2), x3), x4) ..., xn) // without state
  27     //!     f(... f(f(f(f(state, x1), x2), x3), x4) ..., xn) // with state
  28     //! @endcode
  29     //!
  30     //! When the structure is empty, two things may arise. If an initial
  31     //! state was provided, it is returned as-is. Otherwise, if the no-state
  32     //! version of the function was used, an error is triggered. When the
  33     //! stucture contains a single element and the no-state version of the
  34     //! function was used, that single element is returned as is.
  35     //!
  36     //!
  37     //! Signature
  38     //! ---------
  39     //! Given a `Foldable` `F` and an optional initial state of tag `S`,
  40     //! the signatures for `fold_left` are
  41     //! \f[
  42     //!     \mathtt{fold\_left} : F(T) \times S \times (S \times T \to S) \to S
  43     //! \f]
  44     //!
  45     //! for the variant with an initial state, and
  46     //! \f[
  47     //!     \mathtt{fold\_left} : F(T) \times (T \times T \to T) \to T
  48     //! \f]
  49     //!
  50     //! for the variant without an initial state.
  51     //!
  52     //! @param xs
  53     //! The structure to fold.
  54     //!
  55     //! @param state
  56     //! The initial value used for folding.
  57     //!
  58     //! @param f
  59     //! A binary function called as `f(state, x)`, where `state` is the
  60     //! result accumulated so far and `x` is an element in the structure.
  61     //! For left folds without an initial state, the function is called as
  62     //! `f(x1, x2)`, where `x1` and `x2` are elements of the structure.
  63     //!
  64     //!
  65     //! Example
  66     //! -------
  67     //! @include example/fold_left.cpp
  68 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  69     constexpr auto fold_left = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
  70         return tag-dispatched;
  71     };
  72 #else
  73     template <typename T, typename = void>
  74     struct fold_left_impl : fold_left_impl<T, when<true>> { };
  75
  76     struct fold_left_t {
  77         template <typename Xs, typename State, typename F>
  78         constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const;
  79
  80         template <typename Xs, typename F>
  81         constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
  82     };
  83
  84     constexpr fold_left_t fold_left{};
  85 #endif
  86 BOOST_HANA_NAMESPACE_END
  87
  88 #endif // !BOOST_HANA_FWD_FOLD_LEFT_HPP