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Diffstat (limited to 'boost/hana/fwd/concept/metafunction.hpp')
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diff --git a/boost/hana/fwd/concept/metafunction.hpp b/boost/hana/fwd/concept/metafunction.hpp new file mode 100644 index 0000000000..00efc049e3 --- /dev/null +++ b/boost/hana/fwd/concept/metafunction.hpp @@ -0,0 +1,99 @@ +/*! +@file +Forward declares `boost::hana::Metafunction`. + +@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_FWD_CONCEPT_METAFUNCTION_HPP +#define BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP + +#include <boost/hana/config.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! @ingroup group-concepts + //! @defgroup group-Metafunction Metafunction + //! A `Metafunction` is a function that takes `hana::type`s as inputs and + //! returns a `hana::type` as output. + //! + //! A `Metafunction` is an object satisfying the [FunctionObject][1] + //! concept, but with additional requirements. First, it must be possible + //! to apply a `Metafunction` to arguments whose tag is `type_tag`, and + //! the result of such an application must be an object whose tag is also + //! `type_tag`. Note that `hana::type` and `hana::basic_type` are the + //! only such types. + //! + //! Secondly, a `Metafunction` must provide a nested `::%apply` template + //! which allows performing the same type-level computation as is done by + //! the call operator. In Boost.MPL parlance, a `Metafunction` `F` is + //! hence a [MetafunctionClass][2] in addition to being a `FunctionObject`. + //! Rigorously, the following must be satisfied by any object `f` of type + //! `F` which is a `Metafunction`, and for arbitrary types `T...`: + //! @code + //! f(hana::type_c<T>...) == hana::type_c<F::apply<T...>::type> + //! @endcode + //! + //! Thirdly, to ease the inter-operation of values and types, + //! `Metafunction`s must also allow being called with arguments that + //! are not `hana::type`s. In that case, the result is equivalent to + //! calling the metafunction on the types of the arguments. Rigorously, + //! this means that for arbitrary objects `x...`, + //! @code + //! f(x...) == f(hana::type_c<decltype(x)>...) + //! @endcode + //! + //! + //! Minimal complete definition + //! --------------------------- + //! The `Metafunction` concept does not have a minimal complete definition + //! in terms of tag-dispatched methods. Instead, the syntactic requirements + //! documented above should be satisfied, and the `Metafunction` struct + //! should be specialized explicitly in Hana's namespace. + //! + //! + //! Concrete models + //! --------------- + //! `hana::metafunction`, `hana::metafunction_class`, `hana::template_` + //! + //! + //! Rationale: Why aren't `Metafunction`s `Comparable`? + //! --------------------------------------------------- + //! When seeing `hana::template_`, a question that naturally arises is + //! whether `Metafunction`s should be made `Comparable`. Indeed, it + //! would seem to make sense to compare two templates `F` and `G` with + //! `template_<F> == template_<G>`. However, in the case where `F` and/or + //! `G` are alias templates, it makes sense to talk about two types of + //! comparisons. The first one is _shallow_ comparison, and it determines + //! that two alias templates are equal if they are the same alias + //! template. The second one is _deep_ comparison, and it determines + //! that two template aliases are equal if they alias the same type for + //! any template argument. For example, given `F` and `G` defined as + //! @code + //! template <typename T> + //! using F = void; + //! + //! template <typename T> + //! using G = void; + //! @endcode + //! + //! shallow comparison would determine that `F` and `G` are different + //! because they are two different template aliases, while deep comparison + //! would determine that `F` and `G` are equal because they always + //! expand to the same type, `void`. Unfortunately, deep comparison is + //! impossible to implement because one would have to check `F` and `G` + //! on all possible types. On the other hand, shallow comparison is not + //! satisfactory because `Metafunction`s are nothing but functions on + //! `type`s, and the equality of two functions is normally defined with + //! deep comparison. Hence, we adopt a conservative stance and avoid + //! providing comparison for `Metafunction`s. + //! + //! [1]: http://en.cppreference.com/w/cpp/concept/FunctionObject + //! [2]: http://www.boost.org/doc/libs/release/libs/mpl/doc/refmanual/metafunction-class.html + template <typename F> + struct Metafunction; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP |