Description

The Sequence concept represents generic index-based sequences.

Compared to other abstract concepts, the Sequence concept is very specific. It represents generic index-based sequences. The reason why such a specific concept is provided is because there are a lot of models that behave exactly the same while being implemented in wildly different ways. It is useful to regroup all those data types under the same umbrella for the purpose of generic programming.

In fact, models of this concept are not only similar. They are actually isomorphic, in a sense that we define below, which is a fancy way of rigorously saying that they behave exactly the same to an external observer.

Minimal complete definition

Iterable, Foldable, and make

The Sequence concept does not provide basic methods that could be used as a minimal complete definition; instead, it borrows methods from other concepts and add laws to them. For this reason, it is necessary to specialize the Sequence metafunction in Hana's namespace to tell Hana that a type is indeed a Sequence. Explicitly specializing the Sequence metafunction can be seen like a seal saying "this data type satisfies the additional laws of a `Sequence`", since those can't be checked by Hana automatically.

Laws

The laws for being a Sequence are simple, and their goal is to restrict the semantics that can be associated to the functions provided by other concepts. First, a Sequence must be a finite Iterable (thus a Foldable too). Secondly, for a Sequence tag S, make<S>(x1, ..., xn) must be an object of tag S and whose linearization is [x1, ..., xn]. This basically ensures that objects of tag S are equivalent to their linearization, and that they can be created from such a linearization (with make).

While it would be possible in theory to handle infinite sequences, doing so complicates the implementation of many algorithms. For simplicity, the current version of the library only handles finite sequences. However, note that this does not affect in any way the potential for having infinite Searchables and Iterables.

Refined concepts

Comparable (definition provided automatically)
Two Sequences are equal if and only if they contain the same number of elements and their elements at any given index are equal.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/not_equal.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::make_tuple(1, 2, 3) == hana::make_tuple(1, 2, 3), "");
BOOST_HANA_CONSTANT_CHECK(hana::make_tuple(1, 2, 3) != hana::make_tuple(1, 2, 3, 4));
int main() { }
Orderable (definition provided automatically)
Sequences are ordered using the traditional lexicographical ordering.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/less.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::make_tuple(1, 2, 3) < hana::make_tuple(2, 3, 4), "");
static_assert(hana::make_tuple(1, 2, 3) < hana::make_tuple(1, 2, 3, 4), "");
int main() { }
Functor (definition provided automatically)
Sequences implement transform as the mapping of a function over each element of the sequence. This is somewhat equivalent to what std::transform does to ranges of iterators. Also note that mapping a function over an empty sequence returns an empty sequence and never applies the function, as would be expected.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/transform.hpp>
#include <boost/hana/tuple.hpp>
#include <sstream>
#include <string>
namespace hana = boost::hana;
auto to_string = [](auto x) {
std::ostringstream ss;
ss << x;
return ss.str();
};
int main() {
BOOST_HANA_RUNTIME_CHECK(
hana::transform(hana::make_tuple(1, '2', "345", std::string{"67"}), to_string) ==
hana::make_tuple("1", "2", "345", "67")
);
}
Applicative (definition provided automatically)
First, lifting a value into a Sequence is the same as creating a singleton sequence containing that value. Second, applying a sequence of functions to a sequence of values will apply each function to all the values in the sequence, and then return a list of all the results. In other words,
ap([f1, ..., fN], [x1, ..., xM]) == [
f1(x1), ..., f1(xM),
...
fN(x1), ..., fN(xM)
]
Example:
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/ap.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/ext/std/tuple.hpp>
#include <boost/hana/functional/flip.hpp>
#include <boost/hana/lift.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/tuple.hpp>
#include <tuple>
namespace hana = boost::hana;
static_assert(hana::lift<hana::tuple_tag>('x') == hana::make_tuple('x'), "");
static_assert(hana::equal(hana::lift<hana::ext::std::tuple_tag>('x'), std::make_tuple('x')), "");
constexpr auto f = hana::make_pair;
constexpr auto g = hana::flip(hana::make_pair);
static_assert(
hana::ap(hana::make_tuple(f, g), hana::make_tuple(1, 2, 3), hana::make_tuple('a', 'b'))
==
hana::make_tuple(
f(1, 'a'), f(1, 'b'), f(2, 'a'), f(2, 'b'), f(3, 'a'), f(3, 'b'),
g(1, 'a'), g(1, 'b'), g(2, 'a'), g(2, 'b'), g(3, 'a'), g(3, 'b')
)
, "");
int main() { }
Monad (definition provided automatically)
First, flatenning a Sequence takes a sequence of sequences and concatenates them to get a larger sequence. In other words,
flatten([[a1, ..., aN], ..., [z1, ..., zM]]) == [
a1, ..., aN, ..., z1, ..., zM
]
This acts like a std::tuple_cat function, except it receives a sequence of sequences instead of a variadic pack of sequences to flatten.
Example:
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/flatten.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(
hana::flatten(hana::make_tuple(
hana::make_tuple(1, 2),
hana::make_tuple(3, 4),
hana::make_tuple(hana::make_tuple(5, 6))
))
== hana::make_tuple(1, 2, 3, 4, hana::make_tuple(5, 6))
, "");
int main() { }
Also note that the model of Monad for Sequences can be seen as modeling nondeterminism. A nondeterministic computation can be modeled as a function which returns a sequence of possible results. In this line of thought, chaining a sequence of values into such a function will return a sequence of all the possible output values, i.e. a sequence of all the values applied to all the functions in the sequences.
Example:
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/chain.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/for_each.hpp>
#include <boost/hana/traits.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
#include <type_traits>
#include <utility>
namespace hana = boost::hana;
// Using the `tuple` Monad, we generate all the possible combinations of
// cv-qualifiers and reference qualifiers. Then, we use the `optional`
// Monad to make sure that our generic function can be called with
// arguments of any of those types.
// cv_qualifiers : type -> tuple(type)
auto cv_qualifiers = [](auto t) {
return hana::make_tuple(
t,
hana::traits::add_const(t),
hana::traits::add_volatile(t),
hana::traits::add_volatile(hana::traits::add_const(t))
);
};
// ref_qualifiers : type -> tuple(type)
auto ref_qualifiers = [](auto t) {
return hana::make_tuple(
hana::traits::add_lvalue_reference(t),
hana::traits::add_rvalue_reference(t)
);
};
auto possible_args = cv_qualifiers(hana::type_c<int>) | ref_qualifiers;
BOOST_HANA_CONSTANT_CHECK(
possible_args == hana::make_tuple(
hana::type_c<int&>,
hana::type_c<int&&>,
hana::type_c<int const&>,
hana::type_c<int const&&>,
hana::type_c<int volatile&>,
hana::type_c<int volatile&&>,
hana::type_c<int const volatile&>,
hana::type_c<int const volatile&&>
)
);
struct some_function {
template <typename T>
void operator()(T&&) const { }
};
int main() {
hana::for_each(possible_args, [](auto t) {
using T = typename decltype(t)::type;
static_assert(decltype(hana::is_valid(some_function{})(std::declval<T>())){},
"some_function should be callable with any type of argument");
});
}
MonadPlus (definition provided automatically)
Sequences are models of the MonadPlus concept by considering the empty sequence as the unit of concat, and sequence concatenation as concat.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/append.hpp>
#include <boost/hana/assert.hpp>
#include <boost/hana/concat.hpp>
#include <boost/hana/empty.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/tuple.hpp>
#include <string>
namespace hana = boost::hana;
using namespace std::string_literals;
BOOST_HANA_CONSTANT_CHECK(hana::empty<hana::tuple_tag>() == hana::make_tuple());
static_assert(hana::append(hana::make_tuple(1, '2', 3.3), nullptr)
== hana::make_tuple(1, '2', 3.3, nullptr), "");
int main() {
BOOST_HANA_RUNTIME_CHECK(
hana::concat(hana::make_tuple(1, '2', 3.3), hana::make_tuple("abcdef"s)) ==
hana::make_tuple(1, '2', 3.3, "abcdef"s)
);
}
Foldable
The model of Foldable for Sequences is uniquely determined by the model of Iterable.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/fold_left.hpp>
#include <boost/hana/tuple.hpp>
#include <sstream>
namespace hana = boost::hana;
auto to_string = [](auto x) {
std::ostringstream ss;
ss << x;
return ss.str();
};
auto show = [](auto x, auto y) {
return "(" + to_string(x) + " + " + to_string(y) + ")";
};
int main() {
BOOST_HANA_RUNTIME_CHECK(
hana::fold_left(hana::make_tuple(2, "3", '4'), "1", show) == "(((1 + 2) + 3) + 4)"
);
}
Iterable
The model of Iterable for Sequences corresponds to iteration over each element of the sequence, in order. This model is not provided automatically, and it is in fact part of the minimal complete definition for the Sequence concept.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/drop_front.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/front.hpp>
#include <boost/hana/is_empty.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::front(hana::make_tuple(1, '2', 3.3)) == 1, "");
static_assert(hana::drop_front(hana::make_tuple(1, '2', 3.3)) == hana::make_tuple('2', 3.3), "");
BOOST_HANA_CONSTANT_CHECK(!hana::is_empty(hana::make_tuple(1, '2', 3.3)));
BOOST_HANA_CONSTANT_CHECK(hana::is_empty(hana::make_tuple()));
int main() { }
Searchable (definition provided automatically)
Searching through a Sequence is equivalent to just searching through a list of the values it contains. The keys and the values on which the search is performed are both the elements of the sequence.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/contains.hpp>
#include <boost/hana/core/is_a.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/find_if.hpp>
#include <boost/hana/optional.hpp>
#include <boost/hana/tuple.hpp>
#include <string>
namespace hana = boost::hana;
using namespace std::string_literals;
int main() {
BOOST_HANA_RUNTIME_CHECK(
hana::find_if(hana::make_tuple(1, '2', 3.3, "abc"s), hana::is_a<std::string>) == hana::just("abc"s)
);
BOOST_HANA_RUNTIME_CHECK(
"abc"s ^hana::in^ hana::make_tuple(1, '2', 3.3, "abc"s)
);
}

Concrete models

hana::tuple

Variables
constexpr auto	boost::hana::cartesian_product
	Computes the cartesian product of a sequence of sequences.Given a sequence of sequences, `cartesian_product` returns a new sequence of sequences containing the cartesian product of the original sequences. For this method to finish, a finite number of finite sequences must be provided. More...

constexpr auto	boost::hana::drop_back
	Drop the last `n` elements of a finite sequence, and return the rest.Given a finite `Sequence` `xs` with a linearization of `[x1, ..., xm]` and a non-negative `IntegralConstant` `n`, `drop_back(xs, n)` is a sequence with the same tag as `xs` whose linearization is `[x1, ..., xm-n]`. If `n` is not given, it defaults to an `IntegralConstant` with a value equal to `1`. More...

constexpr auto	boost::hana::group
	Group adjacent elements of a sequence that all respect a binary predicate, by default equality.Given a finite Sequence and an optional predicate (by default `equal`), `group` returns a sequence of subsequences representing groups of adjacent elements that are "equal" with respect to the predicate. In other words, the groups are such that the predicate is satisfied when it is applied to any two adjacent elements in that group. The sequence returned by `group` is such that the concatenation of its elements is equal to the original sequence, which is equivalent to saying that the order of the elements is not changed. More...

constexpr insert_t	boost::hana::insert {}
	Insert a value at a given index in a sequence.Given a sequence, an index and an element to insert, `insert` inserts the element at the given index. More...

constexpr auto	boost::hana::insert_range
	Insert several values at a given index in a sequence.Given a sequence, an index and any `Foldable` containing elements to insert, `insert_range` inserts the elements in the `Foldable` at the given index of the sequence. More...

constexpr auto	boost::hana::intersperse
	Insert a value between each pair of elements in a finite sequence.Given a finite `Sequence` `xs` with a linearization of `[x1, x2, ..., xn]`, `intersperse(xs, z)` is a new sequence with a linearization of `[x1, z, x2, z, x3, ..., xn-1, z, xn]`. In other words, it inserts the `z` element between every pair of elements of the original sequence. If the sequence is empty or has a single element, `intersperse` returns the sequence as-is. In all cases, the sequence must be finite. More...

constexpr auto	boost::hana::partition
	Partition a sequence based on a `predicate`.Specifically, returns an unspecified `Product` whose first element is a sequence of the elements satisfying the predicate, and whose second element is a sequence of the elements that do not satisfy the predicate. More...

constexpr auto	boost::hana::permutations
	Return a sequence of all the permutations of the given sequence.Specifically, `permutations(xs)` is a sequence whose elements are permutations of the original sequence `xs`. The permutations are not guaranteed to be in any specific order. Also note that the number of permutations grows very rapidly as the length of the original sequence increases. The growth rate is `O(length(xs)!)`; with a sequence `xs` of length only 8, `permutations(xs)` contains over 40 000 elements! More...

constexpr auto	boost::hana::remove_at
	Remove the element at a given index from a sequence.`remove_at` returns a new sequence identical to the original, except that the element at the given index is removed. Specifically, `remove_at([x0, ..., xn-1, xn, xn+1, ..., xm], n)` is a new sequence equivalent to `[x0, ..., xn-1, xn+1, ..., xm]`. More...

template<std::size_t n>
constexpr auto	boost::hana::remove_at_c
	Equivalent to `remove_at`; provided for convenience. More...

constexpr auto	boost::hana::remove_range
	Remove the elements inside a given range of indices from a sequence.`remove_range` returns a new sequence identical to the original, except that elements at indices in the provided range are removed. Specifically, `remove_range([x0, ..., xn], from, to)` is a new sequence equivalent to `[x0, ..., x_from-1, x_to, ..., xn]`. More...

template<std::size_t from, std::size_t to>
constexpr auto	boost::hana::remove_range_c
	Equivalent to `remove_range`; provided for convenience. More...

constexpr auto	boost::hana::reverse
	Reverse a sequence.Specifically, `reverse(xs)` is a new sequence containing the same elements as `xs`, except in reverse order. More...

constexpr auto	boost::hana::scan_left
	Fold a Sequence to the left and return a list containing the successive reduction states.Like `fold_left`, `scan_left` reduces a sequence to a single value using a binary operation. However, unlike `fold_left`, it builds up a sequence of the intermediary results computed along the way and returns that instead of only the final reduction state. Like `fold_left`, `scan_left` can be used with or without an initial reduction state. More...

constexpr auto	boost::hana::scan_right
	Fold a Sequence to the right and return a list containing the successive reduction states.Like `fold_right`, `scan_right` reduces a sequence to a single value using a binary operation. However, unlike `fold_right`, it builds up a sequence of the intermediary results computed along the way and returns that instead of only the final reduction state. Like `fold_right`, `scan_right` can be used with or without an initial reduction state. More...

constexpr auto	boost::hana::slice
	Extract the elements of a `Sequence` at the given indices.Given an arbitrary sequence of `indices`, `slice` returns a new sequence of the elements of the original sequence that appear at those indices. In other words,. More...

template<std::size_t from, std::size_t to>
constexpr auto	boost::hana::slice_c
	Shorthand to `slice` a contiguous range of elements.`slice_c` is simply a shorthand to slice a contiguous range of elements. In particular, `slice_c<from, to>(xs)` is equivalent to `slice(xs, range_c<std::size_t, from, to>)`, which simply slices all the elements of `xs` contained in the half-open interval delimited by `[from, to)`. Like for `slice`, the indices used with `slice_c` are 0-based and they must be in the bounds of the sequence being sliced. More...

constexpr auto	boost::hana::sort
	Sort a sequence, optionally based on a custom `predicate`.Given a Sequence and an optional predicate (by default `less`), `sort` returns a new sequence containing the same elements as the original, except they are ordered in such a way that if `x` comes before `y` in the sequence, then either `predicate(x, y)` is true, or both `predicate(x, y)` and `predicate(y, x)` are false. More...

constexpr auto	boost::hana::span
	Returns a `Product` containing the longest prefix of a sequence satisfying a predicate, and the rest of the sequence.The first component of the returned `Product` is a sequence for which all elements satisfy the given predicate. The second component of the returned `Product` is a sequence containing the remainder of the argument. Both or either sequences may be empty, depending on the input argument. More specifically,. More...

constexpr auto	boost::hana::take_back
	Returns the last `n` elements of a sequence, or the whole sequence if the sequence has less than `n` elements.Given a `Sequence` `xs` and an `IntegralConstant` `n`, `take_back(xs, n)` is a new sequence containing the last `n` elements of `xs`, in the same order. If `length(xs) <= n`, the whole sequence is returned and no error is triggered. More...

template<std::size_t n>
constexpr auto	boost::hana::take_back_c
	Equivalent to `take_back`; provided for convenience. More...

constexpr auto	boost::hana::take_front
	Returns the first `n` elements of a sequence, or the whole sequence if the sequence has less than `n` elements.Given a `Sequence` `xs` and an `IntegralConstant` `n`, `take_front(xs, n)` is a new sequence containing the first `n` elements of `xs`, in the same order. If `length(xs) <= n`, the whole sequence is returned and no error is triggered. More...

template<std::size_t n>
constexpr auto	boost::hana::take_front_c
	Equivalent to `take_front`; provided for convenience. More...

constexpr auto	boost::hana::take_while
	Take elements from a sequence while the `predicate` is satisfied.Specifically, `take_while` returns a new sequence containing the longest prefix of `xs` in which all the elements satisfy the given predicate. More...

template<typename S >
constexpr auto	boost::hana::unfold_left
	Dual operation to `fold_left` for sequences.While `fold_left` reduces a structure to a summary value from the left, `unfold_left` builds a sequence from a seed value and a function, starting from the left. More...

template<typename S >
constexpr auto	boost::hana::unfold_right
	Dual operation to `fold_right` for sequences.While `fold_right` reduces a structure to a summary value from the right, `unfold_right` builds a sequence from a seed value and a function, starting from the right. More...

constexpr auto	boost::hana::unique
	Removes all consecutive duplicate elements from a Sequence.Given a `Sequence` and an optional binary predicate, `unique` returns a new sequence containing only the first element of every subrange of the original sequence whose elements are all equal. In other words, it turns a sequence of the form `[a, a, b, c, c, c, d, d, d, a]` into a sequence `[a, b, c, d, a]`. The equality of two elements is determined by the provided `predicate`, or by `equal` if no `predicate` is provided. More...

constexpr auto	boost::hana::zip
	Zip one sequence or more.Given `n` sequences `s1, ..., sn`, `zip` produces a sequence whose `i`-th element is a tuple of `(s1[i], ..., sn[i])`, where `sk[i]` denotes the `i`-th element of the `k`-th sequence. In other words, `zip` produces a sequence of the form. More...

constexpr auto	boost::hana::zip_shortest
	Zip one sequence or more.Given `n` sequences `s1, ..., sn`, `zip_shortest` produces a sequence whose `i`-th element is a tuple of `(s1[i], ..., sn[i])`, where `sk[i]` denotes the `i`-th element of the `k`-th sequence. In other words, `zip_shortest` produces a sequence of the form. More...

constexpr auto	boost::hana::zip_shortest_with
	Zip one sequence or more with a given function.Given a `n`-ary function `f` and `n` sequences `s1, ..., sn`, `zip_shortest_with` produces a sequence whose `i`-th element is `f(s1[i], ..., sn[i])`, where `sk[i]` denotes the `i`-th element of the `k`-th sequence. In other words, `zip_shortest_with` produces a sequence of the form. More...

constexpr auto	boost::hana::zip_with
	Zip one sequence or more with a given function.Given a `n`-ary function `f` and `n` sequences `s1, ..., sn`, `zip_with` produces a sequence whose `i`-th element is `f(s1[i], ..., sn[i])`, where `sk[i]` denotes the `i`-th element of the `k`-th sequence. In other words, `zip_with` produces a sequence of the form. More...

Variable Documentation

◆ cartesian_product

constexpr auto boost::hana::cartesian_product

#include <boost/hana/fwd/cartesian_product.hpp>

Initial value:

= [](auto&& xs) {
        return tag-dispatched;
    }

Computes the cartesian product of a sequence of sequences.Given a sequence of sequences, cartesian_product returns a new sequence of sequences containing the cartesian product of the original sequences. For this method to finish, a finite number of finite sequences must be provided.

Note: All the sequences must have the same tag, and that tag must also match that of the top-level sequence.

Signature

Given a Sequence S(T), the signature is

\[ \mathtt{cartesian\_product} : S(S(T)) \to S(S(T)) \]

Parameters

xs	A sequence of sequences of which the cartesian product is computed.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/cartesian_product.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;
constexpr auto tuples = hana::make_tuple(
    hana::make_tuple(1, 2, 3),
    hana::make_tuple('a', 'b'),
    hana::make_tuple(hana::type_c<int>, hana::type_c<char>)
);
constexpr auto prod = hana::make_tuple(
    hana::make_tuple(1, 'a', hana::type_c<int>),
    hana::make_tuple(1, 'a', hana::type_c<char>),
    hana::make_tuple(1, 'b', hana::type_c<int>),
    hana::make_tuple(1, 'b', hana::type_c<char>),
    hana::make_tuple(2, 'a', hana::type_c<int>),
    hana::make_tuple(2, 'a', hana::type_c<char>),
    hana::make_tuple(2, 'b', hana::type_c<int>),
    hana::make_tuple(2, 'b', hana::type_c<char>),
    hana::make_tuple(3, 'a', hana::type_c<int>),
    hana::make_tuple(3, 'a', hana::type_c<char>),
    hana::make_tuple(3, 'b', hana::type_c<int>),
    hana::make_tuple(3, 'b', hana::type_c<char>)
);
static_assert(hana::cartesian_product(tuples) == prod, "");
int main() { }

◆ drop_back

constexpr auto boost::hana::drop_back

#include <boost/hana/fwd/drop_back.hpp>

Initial value:

= [](auto&& xs[, auto const& n]) {
        return tag-dispatched;
    }

Drop the last n elements of a finite sequence, and return the rest.Given a finite Sequence xs with a linearization of [x1, ..., xm] and a non-negative IntegralConstant n, drop_back(xs, n) is a sequence with the same tag as xs whose linearization is [x1, ..., xm-n]. If n is not given, it defaults to an IntegralConstant with a value equal to 1.

In case length(xs) <= n, drop_back will simply drop the whole sequence without failing, thus returning an empty sequence.

Parameters

xs	The sequence from which elements are dropped.
n	A non-negative `IntegralConstant` representing the number of elements to be dropped from the end of the sequence. If `n` is not given, it defaults to an `IntegralConstant` with a value equal to `1`.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/drop_back.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(0, '1', 2.0);
static_assert(hana::drop_back(xs, hana::size_c<0>) == xs, "");
static_assert(hana::drop_back(xs, hana::size_c<1>) == hana::make_tuple(0, '1'), "");
static_assert(hana::drop_back(xs, hana::size_c<2>) == hana::make_tuple(0), "");
BOOST_HANA_CONSTANT_CHECK(hana::drop_back(xs, hana::size_c<3>) == hana::make_tuple());
BOOST_HANA_CONSTANT_CHECK(hana::drop_back(xs, hana::size_c<4>) == hana::make_tuple());
static_assert(hana::drop_back(xs) == hana::make_tuple(0, '1'), "");
int main() { }

◆ group

constexpr auto boost::hana::group

#include <boost/hana/fwd/group.hpp>

Initial value:

= [](auto&& xs[, auto&& predicate]) {
        return tag-dispatched;
    }

Group adjacent elements of a sequence that all respect a binary predicate, by default equality.Given a finite Sequence and an optional predicate (by default equal), group returns a sequence of subsequences representing groups of adjacent elements that are "equal" with respect to the predicate. In other words, the groups are such that the predicate is satisfied when it is applied to any two adjacent elements in that group. The sequence returned by group is such that the concatenation of its elements is equal to the original sequence, which is equivalent to saying that the order of the elements is not changed.

If no predicate is provided, adjacent elements in the sequence must all be compile-time Comparable.

Signature

Given a Sequence s with tag S(T), an IntegralConstant Bool holding a value of type bool, and a predicate \( pred : T \times T \to Bool \), group has the following signatures. For the variant with a provided predicate,

\[ \mathtt{group} : S(T) \times (T \times T \to Bool) \to S(S(T)) \]

for the variant without a custom predicate, T is required to be Comparable. The signature is then

\[ \mathtt{group} : S(T) \to S(S(T)) \]

Parameters

xs The sequence to split into groups.

predicate A binary function called as predicate(x, y), where x and y are adjacent elements in the sequence, whether both elements should be in the same group (subsequence) of the result. In the current version of the library, the result returned by predicate must be an IntegralConstant holding a value of a type convertible to bool. Also, predicate has to define an equivalence relation as defined by the Comparable concept. When this predicate is not provided, it defaults to equal, which requires the comparison of any two adjacent elements in the sequence to return a boolean IntegralConstant.

Syntactic sugar (`group.by`)

group can be called in a third way, which provides a nice syntax especially when working with the comparing combinator:

group.by(predicate, xs) == group(xs, predicate)

group.by(predicate) == group(-, predicate)

where group(-, predicate) denotes the partial application of group to predicate.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/comparing.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/group.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/length.hpp>
#include <boost/hana/range.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;
// group without a predicate
BOOST_HANA_CONSTANT_CHECK(
    hana::group(hana::make_tuple(hana::int_c<1>, hana::long_c<1>, hana::type_c<int>, hana::char_c<'x'>, hana::char_c<'x'>))
        == hana::make_tuple(
            hana::make_tuple(hana::int_c<1>, hana::long_c<1>),
            hana::make_tuple(hana::type_c<int>),
            hana::make_tuple(hana::char_c<'x'>, hana::char_c<'x'>)
        )
);
// group with a predicate
constexpr auto tuples = hana::make_tuple(
    hana::range_c<int, 0, 1>,
    hana::range_c<int, 0, 2>,
    hana::range_c<int, 1, 3>,
    hana::range_c<int, 2, 6>
);
BOOST_HANA_CONSTANT_CHECK(
    hana::group(tuples, hana::comparing(hana::length))
        == hana::make_tuple(
            hana::make_tuple(
                hana::range_c<int, 0, 1>
            ),
            hana::make_tuple(
                hana::range_c<int, 0, 2>,
                hana::range_c<int, 1, 3>
            ),
            hana::make_tuple(
                hana::range_c<int, 2, 6>
            )
        )
);
// group.by is syntactic sugar
static_assert(
    hana::group.by(hana::comparing(hana::typeid_),
                   hana::make_tuple(1, 2, 3, 'x', 'y', 4.4, 5.5))
        == hana::make_tuple(
            hana::make_tuple(1, 2, 3),
            hana::make_tuple('x', 'y'),
            hana::make_tuple(4.4, 5.5)
        )
, "");
int main() { }

◆ insert

constexpr auto boost::hana::insert {}

Insert a value at a given index in a sequence.Given a sequence, an index and an element to insert, insert inserts the element at the given index.

Parameters

xs	The sequence in which a value should be inserted.
n	The index at which an element should be inserted. This must be a non-negative `Constant` of an integral type, and it must also be true that `n < length(xs)` if `xs` is a finite sequence.
element	The element to insert in the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/insert.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/tuple.hpp>
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
using namespace std::literals;
int main() {
    auto xs = hana::make_tuple("Hello"s, "world!"s);
    BOOST_HANA_RUNTIME_CHECK(
        hana::insert(xs, 1_c, " "s) == hana::make_tuple("Hello"s, " "s, "world!"s)
    );
}

◆ insert_range

constexpr auto boost::hana::insert_range

#include <boost/hana/fwd/insert_range.hpp>

Initial value:

= [](auto&& xs, auto&& n, auto&& elements) {
        return tag-dispatched;
    }

Insert several values at a given index in a sequence.Given a sequence, an index and any Foldable containing elements to insert, insert_range inserts the elements in the Foldable at the given index of the sequence.

Parameters

xs	The sequence in which values should be inserted.
n	The index at which elements should be inserted. This must be a non-negative `Constant` of an integral type, and it must also be true that `n < length(xs)` if `xs` is a finite sequence.
elements	A `Foldable` containing elements to insert in the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/insert_range.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/tuple.hpp>
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
using namespace std::literals;
int main() {
    auto xs = hana::make_tuple("Hello"s, "world!"s);
    BOOST_HANA_RUNTIME_CHECK(
        hana::insert_range(xs, 1_c, hana::make_tuple(1, 2, 3)) == hana::make_tuple("Hello"s, 1, 2, 3, "world!"s)
    );
}

◆ intersperse

constexpr auto boost::hana::intersperse

#include <boost/hana/fwd/intersperse.hpp>

Initial value:

= [](auto&& xs, auto&& z) {
        return tag-dispatched;
    }

Insert a value between each pair of elements in a finite sequence.Given a finite Sequence xs with a linearization of [x1, x2, ..., xn], intersperse(xs, z) is a new sequence with a linearization of [x1, z, x2, z, x3, ..., xn-1, z, xn]. In other words, it inserts the z element between every pair of elements of the original sequence. If the sequence is empty or has a single element, intersperse returns the sequence as-is. In all cases, the sequence must be finite.

Parameters

xs	The sequence in which a value is interspersed.
z	The value to be inserted between every pair of elements of the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/intersperse.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::intersperse(hana::make_tuple(1, '2', 3.3), 'x') == hana::make_tuple(1, 'x', '2', 'x', 3.3), "");
BOOST_HANA_CONSTANT_CHECK(hana::intersperse(hana::make_tuple(), 'x') == hana::make_tuple());
int main() { }

◆ partition

constexpr auto boost::hana::partition

#include <boost/hana/fwd/partition.hpp>

Initial value:

= [](auto&& xs, auto&& predicate) {
        return tag-dispatched;
    }

Partition a sequence based on a predicate.Specifically, returns an unspecified Product whose first element is a sequence of the elements satisfying the predicate, and whose second element is a sequence of the elements that do not satisfy the predicate.

Signature

Given a Sequence S(T), an IntegralConstant Bool holding a value of type bool, and a predicate \( T \to Bool \), partition has the following signature:

\[ \mathtt{partition} : S(T) \times (T \to Bool) \to S(T) \times S(T) \]

Parameters

xs	The sequence to be partitioned.
predicate	A function called as `predicate(x)` for each element `x` in the sequence, and returning whether `x` should be added to the sequence in the first component or in the second component of the resulting pair. In the current version of the library, `predicate` must return an `IntegralConstant` holding a value convertible to `bool`.

Syntactic sugar (`partition.by`)

partition can be called in an alternate way, which provides a nice syntax in some cases where the predicate is short:

partition.by(predicate, xs) == partition(xs, predicate)

partition.by(predicate) == partition(-, predicate)

where partition(-, predicate) denotes the partial application of partition to predicate.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/ext/std/integral_constant.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/mod.hpp>
#include <boost/hana/not_equal.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/partition.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
#include <type_traits>
namespace hana = boost::hana;
BOOST_HANA_CONSTANT_CHECK(
    hana::partition(hana::tuple_c<int, 1, 2, 3, 4, 5, 6, 7>, [](auto x) {
        return x % hana::int_c<2> != hana::int_c<0>;
    })
    ==
    hana::make_pair(
        hana::tuple_c<int, 1, 3, 5, 7>,
        hana::tuple_c<int, 2, 4, 6>
    )
);
BOOST_HANA_CONSTANT_CHECK(
    hana::partition(hana::tuple_t<void, int, float, char, double>, hana::trait<std::is_floating_point>)
    ==
    hana::make_pair(
        hana::tuple_t<float, double>,
        hana::tuple_t<void, int, char>
    )
);
// partition.by is syntactic sugar
BOOST_HANA_CONSTANT_CHECK(
    hana::partition.by(hana::trait<std::is_floating_point>,
                       hana::tuple_t<void, int, float, char, double>)
    ==
    hana::make_pair(
        hana::tuple_t<float, double>,
        hana::tuple_t<void, int, char>
    )
);
int main() { }

◆ permutations

constexpr auto boost::hana::permutations

#include <boost/hana/fwd/permutations.hpp>

Initial value:

= [](auto&& xs) {
        return tag-dispatched;
    }

Return a sequence of all the permutations of the given sequence.Specifically, permutations(xs) is a sequence whose elements are permutations of the original sequence xs. The permutations are not guaranteed to be in any specific order. Also note that the number of permutations grows very rapidly as the length of the original sequence increases. The growth rate is O(length(xs)!); with a sequence xs of length only 8, permutations(xs) contains over 40 000 elements!

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/all_of.hpp>
#include <boost/hana/assert.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/contains.hpp>
#include <boost/hana/functional/curry.hpp>
#include <boost/hana/permutations.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTEXPR_LAMBDA auto is_permutation_of = hana::curry<2>([](auto xs, auto perm) {
    return hana::contains(hana::permutations(xs), perm);
});
int main() {
    BOOST_HANA_CONSTEXPR_CHECK(
        hana::all_of(
            hana::make_tuple(
                hana::make_tuple('1', 2, 3.0),
                hana::make_tuple('1', 3.0, 2),
                hana::make_tuple(2, '1', 3.0),
                hana::make_tuple(2, 3.0, '1'),
                hana::make_tuple(3.0, '1', 2),
                hana::make_tuple(3.0, 2, '1')
            ),
            is_permutation_of(hana::make_tuple('1', 2, 3.0))
        )
    );
}

◆ remove_at

constexpr auto boost::hana::remove_at

#include <boost/hana/fwd/remove_at.hpp>

Initial value:

= [](auto&& xs, auto const& n) {
        return tag-dispatched;
    }

Remove the element at a given index from a sequence.remove_at returns a new sequence identical to the original, except that the element at the given index is removed. Specifically, remove_at([x0, ..., xn-1, xn, xn+1, ..., xm], n) is a new sequence equivalent to [x0, ..., xn-1, xn+1, ..., xm].

Note: The behavior is undefined if the index is out of the bounds of the sequence.

Parameters

xs	A sequence from which an element is to be removed.
n	An non-negative `IntegralConstant` representing the index of the element to be removed from the sequence. The behavior is undefined if that index is not in the bounds of the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/remove_at.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(0, '1', 2.2, 3u);
static_assert(hana::remove_at(xs, hana::size_c<2>) == hana::make_tuple(0, '1', 3u), "");
int main() { }

◆ remove_at_c

template<std::size_t n>

constexpr auto boost::hana::remove_at_c

#include <boost/hana/fwd/remove_at.hpp>

Initial value:

= [](auto&& xs) {
        return hana::remove_at(forwarded(xs), hana::size_c<n>);
    }

Equivalent to remove_at; provided for convenience.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/remove_at.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(0, '1', 2.2, 3u);
static_assert(hana::remove_at_c<2>(xs) == hana::make_tuple(0, '1', 3u), "");
int main() { }

◆ remove_range

constexpr auto boost::hana::remove_range

#include <boost/hana/fwd/remove_range.hpp>

Initial value:

= [](auto&& xs, auto const& from, auto const& to) {
        return tag-dispatched;
    }

Remove the elements inside a given range of indices from a sequence.remove_range returns a new sequence identical to the original, except that elements at indices in the provided range are removed. Specifically, remove_range([x0, ..., xn], from, to) is a new sequence equivalent to [x0, ..., x_from-1, x_to, ..., xn].

Note: The behavior is undefined if the range contains any index out of the bounds of the sequence.

Parameters

xs	A sequence from which elements are removed.
[from,to)	An half-open interval of `IntegralConstant`s representing the indices of the elements to be removed from the sequence. The `IntegralConstant`s in the half-open interval must be non-negative and in the bounds of the sequence. The half-open interval must also be valid, meaning that `from <= to`.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/remove_range.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(0, '1', 2.2, 3u, 4, 5.5);
static_assert(hana::remove_range(xs, hana::size_c<2>, hana::size_c<4>) == hana::make_tuple(0, '1', 4, 5.5), "");
int main() { }

◆ remove_range_c

template<std::size_t from, std::size_t to>

constexpr auto boost::hana::remove_range_c

#include <boost/hana/fwd/remove_range.hpp>

Initial value:

= [](auto&& xs) {
        return hana::remove_range(forwarded(xs), hana::size_c<from>, hana::size_c<to>);
    }

Equivalent to remove_range; provided for convenience.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/remove_range.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(0, '1', 2.2, 3u, 4, 5.5);
static_assert(hana::remove_range_c<2, 4>(xs) == hana::make_tuple(0, '1', 4, 5.5), "");
int main() { }

◆ reverse

constexpr auto boost::hana::reverse

#include <boost/hana/fwd/reverse.hpp>

Initial value:

= [](auto&& xs) {
        return tag-dispatched;
    }

Reverse a sequence.Specifically, reverse(xs) is a new sequence containing the same elements as xs, except in reverse order.

Parameters

xs	The sequence to reverse.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/reverse.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::reverse(hana::make_tuple(1, '2', 3.3)) == hana::make_tuple(3.3, '2', 1), "");
int main() { }

◆ scan_left

constexpr auto boost::hana::scan_left

#include <boost/hana/fwd/scan_left.hpp>

Initial value:

= [](auto&& xs[, auto&& state], auto const& f) {
        return tag-dispatched;
    }

Fold a Sequence to the left and return a list containing the successive reduction states.Like fold_left, scan_left reduces a sequence to a single value using a binary operation. However, unlike fold_left, it builds up a sequence of the intermediary results computed along the way and returns that instead of only the final reduction state. Like fold_left, scan_left can be used with or without an initial reduction state.

When the sequence is empty, two things may arise. If an initial state was provided, a singleton list containing that state is returned. Otherwise, if no initial state was provided, an empty list is returned. In particular, unlike for fold_left, using scan_left on an empty sequence without an initial state is not an error.

More specifically, scan_left([x1, ..., xn], state, f) is a sequence whose ith element is equivalent to fold_left([x1, ..., xi], state, f). The no-state variant is handled in an analogous way. For illustration, consider this left fold on a short sequence:

fold_left([x1, x2, x3], state, f) == f(f(f(state, x1), x2), x3)

The analogous sequence generated with scan_left will be

scan_left([x1, x2, x3], state, f) == [
    state,
    f(state, x1),
    f(f(state, x1), x2),
    f(f(f(state, x1), x2), x3)
]

Similarly, consider this left fold (without an initial state) on a short sequence:

fold_left([x1, x2, x3, x4], f) == f(f(f(x1, x2), x3), x4)

The analogous sequence generated with scan_left will be

scan_left([x1, x2, x3, x4], f) == [
    x1,
    f(x1, x2),
    f(f(x1, x2), x3),
    f(f(f(x1, x2), x3), x4)
]

Parameters

xs	The sequence to scan from the left.
state	The (optional) initial reduction state.
f	A binary function called as `f(state, x)`, where `state` is the result accumulated so far and `x` is an element in the sequence. If no initial state is provided, `f` is called as `f(x1, x2)`, where `x1` and `x2` are both elements of the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/scan_left.hpp>
#include <boost/hana/tuple.hpp>
#include <sstream>
namespace hana = boost::hana;
auto to_string = [](auto x) {
    std::ostringstream ss;
    ss << x;
    return ss.str();
};
auto f = [](auto state, auto element) {
    return "f(" + to_string(state) + ", " + to_string(element) + ")";
};
int main() {
    // with initial state
    BOOST_HANA_RUNTIME_CHECK(hana::scan_left(hana::make_tuple(2, "3", '4'), 1, f) == hana::make_tuple(
        1,
        "f(1, 2)",
        "f(f(1, 2), 3)",
        "f(f(f(1, 2), 3), 4)"
    ));
    // without initial state
    BOOST_HANA_RUNTIME_CHECK(hana::scan_left(hana::make_tuple(1, "2", '3'), f) == hana::make_tuple(
        1,
        "f(1, 2)",
        "f(f(1, 2), 3)"
    ));
}

◆ scan_right

constexpr auto boost::hana::scan_right

#include <boost/hana/fwd/scan_right.hpp>

Initial value:

= [](auto&& xs[, auto&& state], auto const& f) {
        return tag-dispatched;
    }

Fold a Sequence to the right and return a list containing the successive reduction states.Like fold_right, scan_right reduces a sequence to a single value using a binary operation. However, unlike fold_right, it builds up a sequence of the intermediary results computed along the way and returns that instead of only the final reduction state. Like fold_right, scan_right can be used with or without an initial reduction state.

When the sequence is empty, two things may arise. If an initial state was provided, a singleton list containing that state is returned. Otherwise, if no initial state was provided, an empty list is returned. In particular, unlike for fold_right, using scan_right on an empty sequence without an initial state is not an error.

More specifically, scan_right([x1, ..., xn], state, f) is a sequence whose ith element is equivalent to fold_right([x1, ..., xi], state, f). The no-state variant is handled in an analogous way. For illustration, consider this right fold on a short sequence:

fold_right([x1, x2, x3], state, f) == f(x1, f(x2, f(x3, state)))

The analogous sequence generated with scan_right will be

scan_right([x1, x2, x3], state, f) == [
    f(x1, f(x2, f(x3, state))),
          f(x2, f(x3, state)),
                f(x3, state),
                      state
]

Similarly, consider this right fold (without an initial state) on a short sequence:

fold_right([x1, x2, x3, x4], f) == f(x1, f(x2, f(x3, x4)))

The analogous sequence generated with scan_right will be

scan_right([x1, x2, x3, x4], f) == [
    f(x1, f(x2, f(x3, x4))),
          f(x2, f(x3, x4)),
                f(x3, x4),
                      x4
]

Parameters

xs	The sequence to scan from the right.
state	The (optional) initial reduction state.
f	A binary function called as `f(x, state)`, where `state` is the result accumulated so far and `x` is an element in the sequence. When no initial state is provided, `f` is called as `f(x1, x2)`, where `x1` and `x2` are elements of the sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/scan_right.hpp>
#include <boost/hana/tuple.hpp>
#include <sstream>
namespace hana = boost::hana;
auto to_string = [](auto x) {
    std::ostringstream ss;
    ss << x;
    return ss.str();
};
auto f = [](auto element, auto state) {
    return "f(" + to_string(element) + ", " + to_string(state) + ")";
};
int main() {
    // with initial state
    BOOST_HANA_RUNTIME_CHECK(hana::scan_right(hana::make_tuple(1, "2", '3'), 4, f) == hana::make_tuple(
        "f(1, f(2, f(3, 4)))",
        "f(2, f(3, 4))",
        "f(3, 4)",
        4
    ));
    // without initial state
    BOOST_HANA_RUNTIME_CHECK(hana::scan_right(hana::make_tuple(1, "2", '3'), f) == hana::make_tuple(
        "f(1, f(2, 3))",
        "f(2, 3)",
        '3'
    ));
}

◆ slice

constexpr auto boost::hana::slice

#include <boost/hana/fwd/slice.hpp>

Initial value:

= [](auto&& xs, auto&& indices) {
        return tag-dispatched;
    }

Extract the elements of a Sequence at the given indices.Given an arbitrary sequence of indices, slice returns a new sequence of the elements of the original sequence that appear at those indices. In other words,.

slice([x1, ..., xn], [i1, ..., ik]) == [x_i1, ..., x_ik]

The indices do not have to be ordered or contiguous in any particular way, but they must not be out of the bounds of the sequence. It is also possible to specify the same index multiple times, in which case the element at this index will be repeatedly included in the resulting sequence.

Parameters

xs	The sequence from which a subsequence is extracted.
indices	A compile-time `Foldable` containing non-negative `IntegralConstant`s representing the indices. The indices are 0-based, and they must all be in bounds of the `xs` sequence. Note that any `Foldable` will really do (no need for an `Iterable`, for example); the linearization of the `indices` is used to determine the order of the elements included in the slice.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/core/to.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/filter.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/length.hpp>
#include <boost/hana/mod.hpp>
#include <boost/hana/range.hpp>
#include <boost/hana/slice.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;
using namespace hana::literals;
// Slice a contiguous range
constexpr auto xs = hana::make_tuple(0, '1', 2.2, 3_c, hana::type_c<float>);
static_assert(
    hana::slice(xs, hana::tuple_c<std::size_t, 1, 2, 3>) ==
    hana::make_tuple('1', 2.2, 3_c)
, "");
// A more complex example with a non-contiguous range
constexpr auto letters = hana::to_tuple(hana::range_c<char, 'a', 'z'>);
constexpr auto indices = hana::to_tuple(hana::make_range(hana::size_c<0>, hana::length(letters)));
auto even_indices = hana::filter(indices, [](auto n) {
    return n % hana::size_c<2> == hana::size_c<0>;
});
BOOST_HANA_CONSTANT_CHECK(
    hana::slice(letters, even_indices) == hana::tuple_c<char,
        'a', 'c', 'e', 'g', 'i', 'k', 'm', 'o', 'q', 's', 'u', 'w', 'y'
    >
);
int main() { }

◆ slice_c

template<std::size_t from, std::size_t to>

constexpr auto boost::hana::slice_c

#include <boost/hana/fwd/slice.hpp>

Initial value:

= [](auto&& xs) {
        return hana::slice(forwarded(xs), hana::range_c<std::size_t, from, to>);
    }

Shorthand to slice a contiguous range of elements.slice_c is simply a shorthand to slice a contiguous range of elements. In particular, slice_c<from, to>(xs) is equivalent to slice(xs, range_c<std::size_t, from, to>), which simply slices all the elements of xs contained in the half-open interval delimited by [from, to). Like for slice, the indices used with slice_c are 0-based and they must be in the bounds of the sequence being sliced.

Template Parameters

from	The index of the first element in the slice.
to	One-past the index of the last element in the slice. It must hold that `from <= to`.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/slice.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;
static_assert(
    hana::slice_c<1, 3>(hana::make_tuple(1, '2', 3.3, hana::type_c<float>))
            ==
    hana::make_tuple('2', 3.3)
, "");
int main() { }

◆ sort

constexpr auto boost::hana::sort

#include <boost/hana/fwd/sort.hpp>

Initial value:

= [](auto&& xs[, auto&& predicate]) {
        return tag-dispatched;
    }

Sort a sequence, optionally based on a custom predicate.Given a Sequence and an optional predicate (by default less), sort returns a new sequence containing the same elements as the original, except they are ordered in such a way that if x comes before y in the sequence, then either predicate(x, y) is true, or both predicate(x, y) and predicate(y, x) are false.

Also note that the sort is guaranteed to be stable. Hence, if x comes before y in the original sequence and both predicate(x, y) and predicate(y, x) are false, then x will come before y in the resulting sequence.

If no predicate is provided, the elements in the sequence must all be compile-time Orderable.

Signature

Given a Sequence S(T), a boolean IntegralConstant Bool and a binary predicate \( T \times T \to Bool \), sort has the following signatures. For the variant with a provided predicate,

\[ \mathtt{sort} : S(T) \times (T \times T \to Bool) \to S(T) \]

for the variant without a custom predicate, T is required to be Orderable. The signature is then

\[ \mathtt{sort} : S(T) \to S(T) \]

Parameters

xs The sequence to sort.

predicate A function called as predicate(x, y) for two elements x and y of the sequence, and returning a boolean IntegralConstant representing whether x is to be considered less than y, i.e. whether x should appear before y in the resulting sequence. More specifically, predicate must define a strict weak ordering on the elements of the sequence. When the predicate is not specified, this defaults to less. In the current version of the library, the predicate has to return an IntegralConstant holding a value convertible to a bool.

Syntactic sugar (`sort.by`)

sort can be called in a third way, which provides a nice syntax especially when working with the ordering combinator:

sort.by(predicate, xs) == sort(xs, predicate)

sort.by(predicate) == sort(-, predicate)

where sort(-, predicate) denotes the partial application of sort to predicate.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/front.hpp>
#include <boost/hana/greater.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/negate.hpp>
#include <boost/hana/ordering.hpp>
#include <boost/hana/sort.hpp>
#include <boost/hana/tuple.hpp>
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
using namespace std::literals;
// sort without a predicate
BOOST_HANA_CONSTANT_CHECK(
    hana::sort(hana::make_tuple(1_c, -2_c, 3_c, 0_c)) ==
               hana::make_tuple(-2_c, 0_c, 1_c, 3_c)
);
// sort with a predicate
BOOST_HANA_CONSTANT_CHECK(
    hana::sort(hana::make_tuple(1_c, -2_c, 3_c, 0_c), hana::greater) ==
               hana::make_tuple(3_c, 1_c, 0_c, -2_c)
);
int main() {
    // sort.by is syntactic sugar
    auto tuples = hana::make_tuple(
        hana::make_tuple(2_c, 'x', nullptr),
        hana::make_tuple(1_c, "foobar"s, hana::int_c<4>)
    );
    BOOST_HANA_RUNTIME_CHECK(
        hana::sort.by(hana::ordering(hana::front), tuples)
            == hana::make_tuple(
                hana::make_tuple(1_c, "foobar"s, hana::int_c<4>),
                hana::make_tuple(2_c, 'x', nullptr)
            )
    );
}

◆ span

constexpr auto boost::hana::span

#include <boost/hana/fwd/span.hpp>

Initial value:

= [](auto&& xs, auto&& predicate) {
        return tag-dispatched;
    }

Returns a Product containing the longest prefix of a sequence satisfying a predicate, and the rest of the sequence.The first component of the returned Product is a sequence for which all elements satisfy the given predicate. The second component of the returned Product is a sequence containing the remainder of the argument. Both or either sequences may be empty, depending on the input argument. More specifically,.

span(xs, predicate) == make_pair(take_while(xs, predicate),

drop_while(xs, predicate))

except that make_pair may be an arbitrary Product.

Signature

Given a Sequence S(T), a Logical Bool and a predicate \( T \to Bool \), span has the following signature:

\[ \mathtt{span} : S(T) \times (T \to Bool) \to S(T) \times S(T) \]

Parameters

xs	The sequence to break into two parts.
predicate	A function called as `predicate(x)`, where `x` is an element of the sequence, and returning a `Logical. In the current implementation of the library,`predicate`has to return a compile-time`Logical`.

Syntactic sugar (`span.by`)

span can be called in an alternate way, which provides a nice syntax in some cases where the predicate is short:

span.by(predicate, xs) == span(xs, predicate)

span.by(predicate) == span(-, predicate)

where span(-, predicate) denotes the partial application of span to predicate.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/less.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/span.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
constexpr auto xs = hana::make_tuple(hana::int_c<1>, hana::int_c<2>, hana::int_c<3>, hana::int_c<4>);
BOOST_HANA_CONSTANT_CHECK(
    hana::span(xs, hana::less.than(hana::int_c<3>))
    ==
    hana::make_pair(hana::make_tuple(hana::int_c<1>, hana::int_c<2>),
                    hana::make_tuple(hana::int_c<3>, hana::int_c<4>))
);
BOOST_HANA_CONSTANT_CHECK(
    hana::span(xs, hana::less.than(hana::int_c<0>))
    ==
    hana::make_pair(hana::make_tuple(), xs)
);
BOOST_HANA_CONSTANT_CHECK(
    hana::span(xs, hana::less.than(hana::int_c<5>))
    ==
    hana::make_pair(xs, hana::make_tuple())
);
// span.by is syntactic sugar
BOOST_HANA_CONSTANT_CHECK(
    hana::span.by(hana::less.than(hana::int_c<3>), xs)
    ==
    hana::make_pair(hana::make_tuple(hana::int_c<1>, hana::int_c<2>),
                    hana::make_tuple(hana::int_c<3>, hana::int_c<4>))
);
int main() { }

◆ take_back

constexpr auto boost::hana::take_back

#include <boost/hana/fwd/take_back.hpp>

Initial value:

= [](auto&& xs, auto const& n) {
        return tag-dispatched;
    }

Returns the last n elements of a sequence, or the whole sequence if the sequence has less than n elements.Given a Sequence xs and an IntegralConstant n, take_back(xs, n) is a new sequence containing the last n elements of xs, in the same order. If length(xs) <= n, the whole sequence is returned and no error is triggered.

Parameters

xs	The sequence to take the elements from.
n	A non-negative `IntegralConstant` representing the number of elements to keep in the resulting sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/take_back.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTANT_CHECK(hana::take_back(hana::make_tuple(1, '2', 3.3), hana::size_c<0>) == hana::make_tuple());
static_assert(hana::take_back(hana::make_tuple(1, '2', 3.3), hana::size_c<1>) == hana::make_tuple(3.3), "");
static_assert(hana::take_back(hana::make_tuple(1, '2', 3.3), hana::size_c<2>) == hana::make_tuple('2', 3.3), "");
static_assert(hana::take_back(hana::make_tuple(1, '2', 3.3), hana::size_c<3>) == hana::make_tuple(1, '2', 3.3), "");
static_assert(hana::take_back(hana::make_tuple(1, '2', 3.3), hana::size_c<4>) == hana::make_tuple(1, '2', 3.3), "");
int main() { }

◆ take_back_c

template<std::size_t n>

constexpr auto boost::hana::take_back_c

#include <boost/hana/fwd/take_back.hpp>

Initial value:

= [](auto&& xs) {
        return hana::take_back(forwarded(xs), hana::size_c<n>);
    }

Equivalent to take_back; provided for convenience.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/take_back.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::take_back_c<2>(hana::make_tuple(1, '2', 3.3)) == hana::make_tuple('2', 3.3), "");
int main() { }

◆ take_front

constexpr auto boost::hana::take_front

#include <boost/hana/fwd/take_front.hpp>

Initial value:

= [](auto&& xs, auto const& n) {
        return tag-dispatched;
    }

Returns the first n elements of a sequence, or the whole sequence if the sequence has less than n elements.Given a Sequence xs and an IntegralConstant n, take_front(xs, n) is a new sequence containing the first n elements of xs, in the same order. If length(xs) <= n, the whole sequence is returned and no error is triggered.

Parameters

xs	The sequence to take the elements from.
n	A non-negative `IntegralConstant` representing the number of elements to keep in the resulting sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/take_front.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTANT_CHECK(hana::take_front(hana::make_tuple(1, '2', 3.3), hana::size_c<0>) == hana::make_tuple());
static_assert(hana::take_front(hana::make_tuple(1, '2', 3.3), hana::size_c<1>) == hana::make_tuple(1), "");
static_assert(hana::take_front(hana::make_tuple(1, '2', 3.3), hana::size_c<2>) == hana::make_tuple(1, '2'), "");
static_assert(hana::take_front(hana::make_tuple(1, '2', 3.3), hana::size_c<3>) == hana::make_tuple(1, '2', 3.3), "");
static_assert(hana::take_front(hana::make_tuple(1, '2', 3.3), hana::size_c<4>) == hana::make_tuple(1, '2', 3.3), "");
int main() { }

◆ take_front_c

template<std::size_t n>

constexpr auto boost::hana::take_front_c

#include <boost/hana/fwd/take_front.hpp>

Initial value:

= [](auto&& xs) {
        return hana::take_front(forwarded(xs), hana::size_c<n>);
    }

Equivalent to take_front; provided for convenience.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/take_front.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
static_assert(hana::take_front_c<2>(hana::make_tuple(1, '2', 3.3)) == hana::make_tuple(1, '2'), "");
int main() { }

◆ take_while

constexpr auto boost::hana::take_while

#include <boost/hana/fwd/take_while.hpp>

Initial value:

= [](auto&& xs, auto&& predicate) {
        return tag-dispatched;
    }

Take elements from a sequence while the predicate is satisfied.Specifically, take_while returns a new sequence containing the longest prefix of xs in which all the elements satisfy the given predicate.

Parameters

xs	The sequence to take elements from.
predicate	A function called as `predicate(x)`, where `x` is an element of the sequence, and returning a `Logical` representing whether `x` should be included in the resulting sequence. In the current version of the library, `predicate` has to return a `Constant Logical`.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/less.hpp>
#include <boost/hana/take_while.hpp>
#include <boost/hana/tuple.hpp>
namespace hana = boost::hana;
using namespace hana::literals;
BOOST_HANA_CONSTANT_CHECK(
    hana::take_while(hana::tuple_c<int, 0, 1, 2, 3>, hana::less.than(2_c))
    ==
    hana::tuple_c<int, 0, 1>
);
int main() { }

◆ unfold_left

template<typename S >

constexpr auto boost::hana::unfold_left

#include <boost/hana/fwd/unfold_left.hpp>

Initial value:

= [](auto&& state, auto&& f) {
        return tag-dispatched;
    }

Dual operation to fold_left for sequences.While fold_left reduces a structure to a summary value from the left, unfold_left builds a sequence from a seed value and a function, starting from the left.

Signature

Given a Sequence S, an initial value state of tag I, an arbitrary Product P and a function \( f : I \to P(I, T) \), unfold_left<S> has the following signature:

\[ \mathtt{unfold\_left}_S : I \times (I \to P(I, T)) \to S(T) \]

Template Parameters

S	The tag of the sequence to build up.

Parameters

state	An initial value to build the sequence from.
f	A function called as `f(state)`, where `state` is an initial value, and returning `nothing` if it is done producing the sequence. otherwise, `just(make<P>(state, x))`, where `state` is the new initial value used in the next call to `f`, `x` is an element to be appended to the resulting sequence, and `P` is an arbitrary `Product`.

Fun fact

In some cases, unfold_left can undo a fold_left operation:

unfold_left<S>(fold_left(xs, state, f), g) == xs

if the following holds

g(f(x, y)) == just(make_pair(x, y))

g(state) == nothing

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/if.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/minus.hpp>
#include <boost/hana/optional.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/unfold_left.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTANT_CHECK(
    hana::unfold_left<hana::tuple_tag>(hana::int_c<10>, [](auto x) {
        return hana::if_(x == hana::int_c<0>,
            hana::nothing,
            hana::just(hana::make_pair(x - hana::int_c<1>, x))
        );
    })
    ==
    hana::tuple_c<int, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10>
);
int main() { }

◆ unfold_right

template<typename S >

constexpr auto boost::hana::unfold_right

#include <boost/hana/fwd/unfold_right.hpp>

Initial value:

= [](auto&& state, auto&& f) {
        return tag-dispatched;
    }

Dual operation to fold_right for sequences.While fold_right reduces a structure to a summary value from the right, unfold_right builds a sequence from a seed value and a function, starting from the right.

Signature

Given a Sequence S, an initial value state of tag I, an arbitrary Product P and a function \( f : I \to P(T, I) \), unfold_right<S> has the following signature:

\[ \mathtt{unfold\_right}_S : I \times (I \to P(T, I)) \to S(T) \]

Template Parameters

S	The tag of the sequence to build up.

Parameters

state	An initial value to build the sequence from.
f	A function called as `f(state)`, where `state` is an initial value, and returning `nothing` if it is done producing the sequence. otherwise, `just(make<P>(x, state))`, where `state` is the new initial value used in the next call to `f`, `x` is an element to be prepended to the resulting sequence, and `P` is an arbitrary `Product`.

Fun fact

In some cases, unfold_right can undo a fold_right operation:

unfold_right<S>(fold_right(xs, state, f), g) == xs

if the following holds

g(f(x, y)) == just(make_pair(x, y))

g(state) == nothing

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/if.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/minus.hpp>
#include <boost/hana/optional.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/unfold_right.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTANT_CHECK(
    hana::unfold_right<hana::tuple_tag>(hana::int_c<10>, [](auto x) {
        return hana::if_(x == hana::int_c<0>,
            hana::nothing,
            hana::just(hana::make_pair(x, x - hana::int_c<1>))
        );
    })
    ==
    hana::tuple_c<int, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1>
);
int main() { }

◆ unique

constexpr auto boost::hana::unique

#include <boost/hana/fwd/unique.hpp>

Initial value:

= [](auto&& xs[, auto&& predicate]) {
        return tag-dispatched;
    }

Removes all consecutive duplicate elements from a Sequence.Given a Sequence and an optional binary predicate, unique returns a new sequence containing only the first element of every subrange of the original sequence whose elements are all equal. In other words, it turns a sequence of the form [a, a, b, c, c, c, d, d, d, a] into a sequence [a, b, c, d, a]. The equality of two elements is determined by the provided predicate, or by equal if no predicate is provided.

Signature

Given a Sequence S(T), a Logical Bool and a binary predicate \( T \times T \to Bool \), unique has the following signature:

\[ \mathtt{unique} : S(T) \times (T \times T \to Bool) \to S(T) \]

Parameters

xs The sequence from which to remove consecutive duplicates.

predicate A function called as predicate(x, y), where x and y are adjacent elements of the sequence, and returning a Logical representing whether x and y should be considered equal. predicate should define an equivalence relation over the elements of the sequence. In the current implementation of the library, predicate has to return a compile-time Logical. This parameter is optional; it defaults to equal if it is not provided, which then requires the elements of the sequence to be compile-time Comparable.

Syntactic sugar (`unique.by`)

unique can be called in an alternate way, which provides a nice syntax, especially in conjunction with the comparing combinator:

unique.by(predicate, xs) == unique(xs, predicate)

unique.by(predicate) == unique(-, predicate)

where unique(-, predicate) denotes the partial application of unique to predicate.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/comparing.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
#include <boost/hana/unique.hpp>
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
// unique without a predicate
constexpr auto types = hana::tuple_t<int, float, float, char, int, int, int, double>;
BOOST_HANA_CONSTANT_CHECK(
    hana::unique(types) == hana::tuple_t<int, float, char, int, double>
);
int main() {
    // unique with a predicate
    auto objects = hana::make_tuple(1, 2, "abc"s, 'd', "efg"s, "hij"s, 3.4f);
    BOOST_HANA_RUNTIME_CHECK(
        hana::unique(objects, [](auto const& t, auto const& u) {
            return hana::typeid_(t) == hana::typeid_(u);
        })
        == hana::make_tuple(1, "abc"s, 'd', "efg"s, 3.4f)
    );
    // unique.by is syntactic sugar
    BOOST_HANA_RUNTIME_CHECK(
        hana::unique.by(hana::comparing(hana::typeid_), objects) ==
            hana::make_tuple(1, "abc"s, 'd', "efg"s, 3.4f)
    );
}

◆ zip

constexpr auto boost::hana::zip

#include <boost/hana/fwd/zip.hpp>

Initial value:

= [](auto&& x1, ..., auto&& xn) {
        return tag-dispatched;
    }

Zip one sequence or more.Given n sequences s1, ..., sn, zip produces a sequence whose i-th element is a tuple of (s1[i], ..., sn[i]), where sk[i] denotes the i-th element of the k-th sequence. In other words, zip produces a sequence of the form.

[
    make_tuple(s1[0], ..., sn[0]),
    make_tuple(s1[1], ..., sn[1]),
    ...
    make_tuple(s1[M], ..., sn[M])
]

where M is the length of the sequences, which are all assumed to have the same length. Assuming the sequences to all have the same size allows the library to perform some optimizations. To zip sequences that may have different lengths, zip_shortest should be used instead. Also note that it is an error to provide no sequence at all, i.e. zip expects at least one sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/zip.hpp>
namespace hana = boost::hana;
static_assert(
    hana::zip(hana::make_tuple(1, 'a'), hana::make_tuple(2, 3.3))
    ==
    hana::make_tuple(hana::make_tuple(1, 2), hana::make_tuple('a', 3.3))
, "");
int main() { }

◆ zip_shortest

constexpr auto boost::hana::zip_shortest

#include <boost/hana/fwd/zip_shortest.hpp>

Initial value:

= [](auto&& x1, ..., auto&& xn) {
        return tag-dispatched;
    }

Zip one sequence or more.Given n sequences s1, ..., sn, zip_shortest produces a sequence whose i-th element is a tuple of (s1[i], ..., sn[i]), where sk[i] denotes the i-th element of the k-th sequence. In other words, zip_shortest produces a sequence of the form.

[
    make_tuple(s1[0], ..., sn[0]),
    make_tuple(s1[1], ..., sn[1]),
    ...
    make_tuple(s1[M], ..., sn[M])
]

where M is the length of the shortest sequence. Hence, the returned sequence stops when the shortest input sequence is exhausted. If you know that all the sequences you are about to zip have the same length, you should use zip instead, since it can be more optimized. Also note that it is an error to provide no sequence at all, i.e. zip_shortest expects at least one sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/zip_shortest.hpp>
namespace hana = boost::hana;
static_assert(
    hana::zip_shortest(hana::make_tuple(1, 'a'), hana::make_tuple(2, 3.3), hana::make_tuple(3, 'c', "ignored"))
    ==
    hana::make_tuple(hana::make_tuple(1, 2, 3), hana::make_tuple('a', 3.3, 'c'))
, "");
int main() { }

◆ zip_shortest_with

constexpr auto boost::hana::zip_shortest_with

#include <boost/hana/fwd/zip_shortest_with.hpp>

Initial value:

= [](auto&& f, auto&& x1, ..., auto&& xn) {
        return tag-dispatched;
    }

Zip one sequence or more with a given function.Given a n-ary function f and n sequences s1, ..., sn, zip_shortest_with produces a sequence whose i-th element is f(s1[i], ..., sn[i]), where sk[i] denotes the i-th element of the k-th sequence. In other words, zip_shortest_with produces a sequence of the form.

[
    f(s1[0], ..., sn[0]),
    f(s1[1], ..., sn[1]),
    ...
    f(s1[M], ..., sn[M])
]

where M is the length of the shortest sequence. Hence, the returned sequence stops when the shortest input sequence is exhausted. If you know that all the sequences you are about to zip have the same length, you should use zip_with instead, since it can be more optimized. Also note that it is an error to provide no sequence at all, i.e. zip_shortest_with expects at least one sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/mult.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/zip_shortest_with.hpp>
namespace hana = boost::hana;
static_assert(
    hana::zip_shortest_with(hana::mult, hana::make_tuple(1, 2, 3, 4), hana::make_tuple(5, 6, 7, 8, "ignored"))
    ==
    hana::make_tuple(5, 12, 21, 32)
, "");
int main() { }

◆ zip_with

constexpr auto boost::hana::zip_with

#include <boost/hana/fwd/zip_with.hpp>

Initial value:

= [](auto&& f, auto&& x1, ..., auto&& xn) {
        return tag-dispatched;
    }

Zip one sequence or more with a given function.Given a n-ary function f and n sequences s1, ..., sn, zip_with produces a sequence whose i-th element is f(s1[i], ..., sn[i]), where sk[i] denotes the i-th element of the k-th sequence. In other words, zip_with produces a sequence of the form.

[
    f(s1[0], ..., sn[0]),
    f(s1[1], ..., sn[1]),
    ...
    f(s1[M], ..., sn[M])
]

where M is the length of the sequences, which are all assumed to have the same length. Assuming the sequences to all have the same size allows the library to perform some optimizations. To zip sequences that may have different lengths, zip_shortest_with should be used instead. Also note that it is an error to provide no sequence at all, i.e. zip_with expects at least one sequence.

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/equal.hpp>
#include <boost/hana/ext/std/tuple.hpp>
#include <boost/hana/mult.hpp>
#include <boost/hana/transform.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
#include <boost/hana/unpack.hpp>
#include <boost/hana/zip_with.hpp>
#include <tuple>
#include <type_traits>
#include <utility>
namespace hana = boost::hana;
// Basic usage:
static_assert(
    hana::zip_with(hana::mult, hana::make_tuple(1, 2, 3, 4), hana::make_tuple(5, 6, 7, 8))
    ==
    hana::make_tuple(5, 12, 21, 32)
, "");
// Example of computing a tuple of all the common types of several tuples:
template<typename... Ts>
using common_tuple_t = typename decltype(
    hana::unpack(
        hana::zip_with(
            hana::metafunction<std::common_type>,
            hana::transform(std::declval<Ts>(), hana::decltype_)...
        ),
        hana::template_<std::tuple>
    )
)::type;
static_assert(std::is_same<
    common_tuple_t<
        std::tuple<bool, int, unsigned>,
        std::tuple<char, long, long>,
        std::tuple<int, long long, double>
    >,
    std::tuple<int, long long, double>
>::value, "");
int main() { }

Description

Minimal complete definition

Laws

Refined concepts

Concrete models

Variables

Variable Documentation

◆ cartesian_product

Signature

Example

◆ drop_back

Example

◆ group

Signature

Syntactic sugar (group.by)

Example

◆ insert

Example

◆ insert_range

Example

◆ intersperse

Example

◆ partition

Signature

Syntactic sugar (partition.by)

Example

◆ permutations

Example

◆ remove_at

Example

◆ remove_at_c

Example

◆ remove_range

Example

◆ remove_range_c

Example

◆ reverse

Example

◆ scan_left

Example

◆ scan_right

Example

◆ slice

Example

◆ slice_c

Example

◆ sort

Signature

Syntactic sugar (sort.by)

Example

◆ span

Signature

Syntactic sugar (span.by)

Example

◆ take_back

Example

◆ take_back_c

Example

◆ take_front

Example

◆ take_front_c

Example

◆ take_while

Example

◆ unfold_left

Signature

Fun fact

Example

◆ unfold_right

Signature

Fun fact

Example

◆ unique

Signature

Syntactic sugar (unique.by)

Example

◆ zip

Example

◆ zip_shortest

Example

Syntactic sugar (`group.by`)

Syntactic sugar (`partition.by`)

Syntactic sugar (`sort.by`)

Syntactic sugar (`span.by`)

Syntactic sugar (`unique.by`)