Maybe.hpp

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#ifndef NONE_HPP_
#define NONE_HPP_
 
#include "capymeta/concepts/Trait.hpp"
 
namespace capy::meta
{
 
namespace implementation_details_
{
    struct Nil {};
}
 
template<typename T>
struct Maybe;
 
using None = Maybe<implementation_details_::Nil>;
 
template<typename T>
    requires (!std::same_as<implementation_details_::Nil, T>)
using Some = Maybe<T>;
 
template<typename T>
struct Maybe
{
    static constexpr bool HAS_VALUE = true;
 
    using Value = T;
 
    template<t_trait<MetaArity::N1> Mapper>
    using Map = Maybe<call_t<Mapper, T>>;
 
    template<v_trait<MetaArity::N1> Predicate>
    using Filter = std::conditional_t<
        call_v<Predicate, T>,
        Some<T>,
        None
    >;
 
    template<t_trait<MetaArity::N1> Mapper>
    using AndThen = call_t<Mapper, T>;
 
    template<typename OtherValue>
    using Or = Maybe<T>;
};
 
template<>
struct Maybe<implementation_details_::Nil>
{
    static constexpr bool HAS_VALUE = false;
 
    using Value = implementation_details_::Nil;
 
    template<t_trait<MetaArity::N1> Mapper>
    using Map = None;
 
    template<v_trait<MetaArity::N1> Predicate>
    using Filter = None;
 
    template<t_trait<MetaArity::N1> Mapper>
    using AndThen = None;
 
    template<typename OtherValue>
    using Or = Maybe<OtherValue>;
};
 
}
 
#endif // NONE_HPP_