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/*!
@file
Defines `boost::hana::curry`.
@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_FUNCTIONAL_CURRY_HPP
#define BOOST_HANA_FUNCTIONAL_CURRY_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/detail/decay.hpp>
#include <boost/hana/functional/apply.hpp>
#include <boost/hana/functional/partial.hpp>
#include <cstddef>
#include <type_traits>
#include <utility>
BOOST_HANA_NAMESPACE_BEGIN
//! @ingroup group-functional
//! Curry a function up to the given number of arguments.
//!
//! [Currying][Wikipedia.currying] is a technique in which we consider a
//! function taking multiple arguments (or, equivalently, a tuple of
//! arguments), and turn it into a function which takes a single argument
//! and returns a function to handle the remaining arguments. To help
//! visualize, let's denote the type of a function `f` which takes
//! arguments of types `X1, ..., Xn` and returns a `R` as
//! @code
//! (X1, ..., Xn) -> R
//! @endcode
//!
//! Then, currying is the process of taking `f` and turning it into an
//! equivalent function (call it `g`) of type
//! @code
//! X1 -> (X2 -> (... -> (Xn -> R)))
//! @endcode
//!
//! This gives us the following equivalence, where `x1`, ..., `xn` are
//! objects of type `X1`, ..., `Xn` respectively:
//! @code
//! f(x1, ..., xn) == g(x1)...(xn)
//! @endcode
//!
//! Currying can be useful in several situations, especially when working
//! with higher-order functions.
//!
//! This `curry` utility is an implementation of currying in C++.
//! Specifically, `curry<n>(f)` is a function such that
//! @code
//! curry<n>(f)(x1)...(xn) == f(x1, ..., xn)
//! @endcode
//!
//! Note that the `n` has to be specified explicitly because the existence
//! of functions with variadic arguments in C++ make it impossible to know
//! when currying should stop.
//!
//! Unlike usual currying, this implementation also allows a curried
//! function to be called with several arguments at a time. Hence, the
//! following always holds
//! @code
//! curry<n>(f)(x1, ..., xk) == curry<n - k>(f)(x1)...(xk)
//! @endcode
//!
//! Of course, this requires `k` to be less than or equal to `n`; failure
//! to satisfy this will trigger a static assertion. This syntax is
//! supported because it makes curried functions usable where normal
//! functions are expected.
//!
//! Another "extension" is that `curry<0>(f)` is supported: `curry<0>(f)`
//! is a nullary function; whereas the classical definition for currying
//! seems to leave this case undefined, as nullary functions don't make
//! much sense in purely functional languages.
//!
//!
//! Example
//! -------
//! @include example/functional/curry.cpp
//!
//!
//! [Wikipedia.currying]: http://en.wikipedia.org/wiki/Currying
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <std::size_t n>
constexpr auto curry = [](auto&& f) {
return [perfect-capture](auto&& x1) {
return [perfect-capture](auto&& x2) {
...
return [perfect-capture](auto&& xn) -> decltype(auto) {
return forwarded(f)(
forwarded(x1), forwarded(x2), ..., forwarded(xn)
);
};
};
};
};
#else
template <std::size_t n, typename F>
struct curry_t;
template <std::size_t n>
struct make_curry_t {
template <typename F>
constexpr curry_t<n, typename detail::decay<F>::type>
operator()(F&& f) const { return {static_cast<F&&>(f)}; }
};
template <std::size_t n>
constexpr make_curry_t<n> curry{};
namespace curry_detail {
template <std::size_t n>
constexpr make_curry_t<n> curry_or_call{};
template <>
constexpr auto curry_or_call<0> = apply;
}
template <std::size_t n, typename F>
struct curry_t {
F f;
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) const& {
static_assert(sizeof...(x) <= n,
"too many arguments provided to boost::hana::curry");
return curry_detail::curry_or_call<n - sizeof...(x)>(
partial(f, static_cast<X&&>(x)...)
);
}
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) & {
static_assert(sizeof...(x) <= n,
"too many arguments provided to boost::hana::curry");
return curry_detail::curry_or_call<n - sizeof...(x)>(
partial(f, static_cast<X&&>(x)...)
);
}
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) && {
static_assert(sizeof...(x) <= n,
"too many arguments provided to boost::hana::curry");
return curry_detail::curry_or_call<n - sizeof...(x)>(
partial(std::move(f), static_cast<X&&>(x)...)
);
}
};
template <typename F>
struct curry_t<0, F> {
F f;
constexpr decltype(auto) operator()() const&
{ return f(); }
constexpr decltype(auto) operator()() &
{ return f(); }
constexpr decltype(auto) operator()() &&
{ return std::move(f)(); }
};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FUNCTIONAL_CURRY_HPP
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