| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::Metafunction`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP |
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| 11 | #define BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | |
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| 15 | |
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| 16 | namespace boost { namespace hana { |
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| 17 | //! @ingroup group-concepts |
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| 18 | //! @defgroup group-Metafunction Metafunction |
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| 19 | //! A `Metafunction` is a function that takes `hana::type`s as inputs and |
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| 20 | //! returns a `hana::type` as output. |
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| 21 | //! |
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| 22 | //! A `Metafunction` is an object satisfying the [FunctionObject][1] |
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| 23 | //! concept, but with additional requirements. First, it must be possible |
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| 24 | //! to apply a `Metafunction` to arguments whose tag is `type_tag`, and |
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| 25 | //! the result of such an application must be an object whose tag is also |
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| 26 | //! `type_tag`. Note that `hana::type` and `hana::basic_type` are the |
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| 27 | //! only such types. |
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| 28 | //! |
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| 29 | //! Secondly, a `Metafunction` must provide a nested `::%apply` template |
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| 30 | //! which allows performing the same type-level computation as is done by |
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| 31 | //! the call operator. In Boost.MPL parlance, a `Metafunction` `F` is |
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| 32 | //! hence a [MetafunctionClass][2] in addition to being a `FunctionObject`. |
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| 33 | //! Rigorously, the following must be satisfied by any object `f` of type |
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| 34 | //! `F` which is a `Metafunction`, and for arbitrary types `T...`: |
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| 35 | //! @code |
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| 36 | //! f(hana::type_c<T>...) == hana::type_c<F::apply<T...>::type> |
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| 37 | //! @endcode |
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| 38 | //! |
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| 39 | //! Thirdly, to ease the inter-operation of values and types, |
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| 40 | //! `Metafunction`s must also allow being called with arguments that |
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| 41 | //! are not `hana::type`s. In that case, the result is equivalent to |
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| 42 | //! calling the metafunction on the types of the arguments. Rigorously, |
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| 43 | //! this means that for arbitrary objects `x...`, |
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| 44 | //! @code |
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| 45 | //! f(x...) == f(hana::type_c<decltype(x)>...) |
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| 46 | //! @endcode |
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| 47 | //! |
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| 48 | //! |
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| 49 | //! Minimal complete definition |
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| 50 | //! --------------------------- |
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| 51 | //! The `Metafunction` concept does not have a minimal complete definition |
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| 52 | //! in terms of tag-dispatched methods. Instead, the syntactic requirements |
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| 53 | //! documented above should be satisfied, and the `Metafunction` struct |
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| 54 | //! should be specialized explicitly in Hana's namespace. |
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| 55 | //! |
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| 56 | //! |
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| 57 | //! Concrete models |
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| 58 | //! --------------- |
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| 59 | //! `hana::metafunction`, `hana::metafunction_class`, `hana::template_` |
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| 60 | //! |
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| 61 | //! |
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| 62 | //! Rationale: Why aren't `Metafunction`s `Comparable`? |
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| 63 | //! --------------------------------------------------- |
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| 64 | //! When seeing `hana::template_`, a question that naturally arises is |
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| 65 | //! whether `Metafunction`s should be made `Comparable`. Indeed, it |
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| 66 | //! would seem to make sense to compare two templates `F` and `G` with |
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| 67 | //! `template_<F> == template_<G>`. However, in the case where `F` and/or |
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| 68 | //! `G` are alias templates, it makes sense to talk about two types of |
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| 69 | //! comparisons. The first one is _shallow_ comparison, and it determines |
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| 70 | //! that two alias templates are equal if they are the same alias |
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| 71 | //! template. The second one is _deep_ comparison, and it determines |
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| 72 | //! that two template aliases are equal if they alias the same type for |
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| 73 | //! any template argument. For example, given `F` and `G` defined as |
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| 74 | //! @code |
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| 75 | //! template <typename T> |
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| 76 | //! using F = void; |
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| 77 | //! |
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| 78 | //! template <typename T> |
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| 79 | //! using G = void; |
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| 80 | //! @endcode |
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| 81 | //! |
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| 82 | //! shallow comparison would determine that `F` and `G` are different |
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| 83 | //! because they are two different template aliases, while deep comparison |
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| 84 | //! would determine that `F` and `G` are equal because they always |
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| 85 | //! expand to the same type, `void`. Unfortunately, deep comparison is |
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| 86 | //! impossible to implement because one would have to check `F` and `G` |
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| 87 | //! on all possible types. On the other hand, shallow comparison is not |
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| 88 | //! satisfactory because `Metafunction`s are nothing but functions on |
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| 89 | //! `type`s, and the equality of two functions is normally defined with |
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| 90 | //! deep comparison. Hence, we adopt a conservative stance and avoid |
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| 91 | //! providing comparison for `Metafunction`s. |
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| 92 | //! |
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| 93 | //! [1]: http://en.cppreference.com/w/cpp/named_req/FunctionObject |
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| 94 | //! [2]: http://www.boost.org/doc/libs/release/libs/mpl/doc/refmanual/metafunction-class.html |
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| 95 | template <typename F> |
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| 96 | struct Metafunction; |
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| 97 | }} // end namespace boost::hana |
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| 98 | |
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| 99 | #endif // !BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP |
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| 100 | |
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