1/*!
2@file
3Defines `boost::hana::curry`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_FUNCTIONAL_CURRY_HPP
11#define BOOST_HANA_FUNCTIONAL_CURRY_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/detail/decay.hpp>
15#include <boost/hana/functional/apply.hpp>
16#include <boost/hana/functional/partial.hpp>
17
18#include <cstddef>
19#include <type_traits>
20#include <utility>
21
22
23namespace boost { namespace hana {
24 //! @ingroup group-functional
25 //! Curry a function up to the given number of arguments.
26 //!
27 //! [Currying][Wikipedia.currying] is a technique in which we consider a
28 //! function taking multiple arguments (or, equivalently, a tuple of
29 //! arguments), and turn it into a function which takes a single argument
30 //! and returns a function to handle the remaining arguments. To help
31 //! visualize, let's denote the type of a function `f` which takes
32 //! arguments of types `X1, ..., Xn` and returns a `R` as
33 //! @code
34 //! (X1, ..., Xn) -> R
35 //! @endcode
36 //!
37 //! Then, currying is the process of taking `f` and turning it into an
38 //! equivalent function (call it `g`) of type
39 //! @code
40 //! X1 -> (X2 -> (... -> (Xn -> R)))
41 //! @endcode
42 //!
43 //! This gives us the following equivalence, where `x1`, ..., `xn` are
44 //! objects of type `X1`, ..., `Xn` respectively:
45 //! @code
46 //! f(x1, ..., xn) == g(x1)...(xn)
47 //! @endcode
48 //!
49 //! Currying can be useful in several situations, especially when working
50 //! with higher-order functions.
51 //!
52 //! This `curry` utility is an implementation of currying in C++.
53 //! Specifically, `curry<n>(f)` is a function such that
54 //! @code
55 //! curry<n>(f)(x1)...(xn) == f(x1, ..., xn)
56 //! @endcode
57 //!
58 //! Note that the `n` has to be specified explicitly because the existence
59 //! of functions with variadic arguments in C++ make it impossible to know
60 //! when currying should stop.
61 //!
62 //! Unlike usual currying, this implementation also allows a curried
63 //! function to be called with several arguments at a time. Hence, the
64 //! following always holds
65 //! @code
66 //! curry<n>(f)(x1, ..., xk) == curry<n - k>(f)(x1)...(xk)
67 //! @endcode
68 //!
69 //! Of course, this requires `k` to be less than or equal to `n`; failure
70 //! to satisfy this will trigger a static assertion. This syntax is
71 //! supported because it makes curried functions usable where normal
72 //! functions are expected.
73 //!
74 //! Another "extension" is that `curry<0>(f)` is supported: `curry<0>(f)`
75 //! is a nullary function; whereas the classical definition for currying
76 //! seems to leave this case undefined, as nullary functions don't make
77 //! much sense in purely functional languages.
78 //!
79 //!
80 //! Example
81 //! -------
82 //! @include example/functional/curry.cpp
83 //!
84 //!
85 //! [Wikipedia.currying]: http://en.wikipedia.org/wiki/Currying
86#ifdef BOOST_HANA_DOXYGEN_INVOKED
87 template <std::size_t n>
88 constexpr auto curry = [](auto&& f) {
89 return [perfect-capture](auto&& x1) {
90 return [perfect-capture](auto&& x2) {
91 ...
92 return [perfect-capture](auto&& xn) -> decltype(auto) {
93 return forwarded(f)(
94 forwarded(x1), forwarded(x2), ..., forwarded(xn)
95 );
96 };
97 };
98 };
99 };
100#else
101 template <std::size_t n, typename F>
102 struct curry_t;
103
104 template <std::size_t n>
105 struct make_curry_t {
106 template <typename F>
107 constexpr curry_t<n, typename detail::decay<F>::type>
108 operator()(F&& f) const { return {static_cast<F&&>(f)}; }
109 };
110
111 template <std::size_t n>
112 BOOST_HANA_INLINE_VARIABLE constexpr make_curry_t<n> curry{};
113
114 namespace curry_detail { namespace {
115 template <std::size_t n>
116 constexpr make_curry_t<n> curry_or_call{};
117
118 template <>
119 constexpr auto curry_or_call<0> = apply;
120 }}
121
122 template <std::size_t n, typename F>
123 struct curry_t {
124 F f;
125
126 template <typename ...X>
127 constexpr decltype(auto) operator()(X&& ...x) const& {
128 static_assert(sizeof...(x) <= n,
129 "too many arguments provided to boost::hana::curry");
130 return curry_detail::curry_or_call<n - sizeof...(x)>(
131 partial(f, static_cast<X&&>(x)...)
132 );
133 }
134
135 template <typename ...X>
136 constexpr decltype(auto) operator()(X&& ...x) & {
137 static_assert(sizeof...(x) <= n,
138 "too many arguments provided to boost::hana::curry");
139 return curry_detail::curry_or_call<n - sizeof...(x)>(
140 partial(f, static_cast<X&&>(x)...)
141 );
142 }
143
144 template <typename ...X>
145 constexpr decltype(auto) operator()(X&& ...x) && {
146 static_assert(sizeof...(x) <= n,
147 "too many arguments provided to boost::hana::curry");
148 return curry_detail::curry_or_call<n - sizeof...(x)>(
149 partial(std::move(f), static_cast<X&&>(x)...)
150 );
151 }
152 };
153
154 template <typename F>
155 struct curry_t<0, F> {
156 F f;
157
158 constexpr decltype(auto) operator()() const&
159 { return f(); }
160
161 constexpr decltype(auto) operator()() &
162 { return f(); }
163
164 constexpr decltype(auto) operator()() &&
165 { return std::move(f)(); }
166 };
167#endif
168}} // end namespace boost::hana
169
170#endif // !BOOST_HANA_FUNCTIONAL_CURRY_HPP
171

source code of boost/libs/hana/include/boost/hana/functional/curry.hpp