C++ map, filter, fold

container.hpp

#ifndef xtd_container_hpp
#define xtd_container_hpp

#include <cstddef>
#include <initializer_list>
#include <vector>

#include "prelude.hpp"

template<typename Cr, typename Fn>
Cr map(const Cr& source, const Fn& mapper)
{
    constrain_as_container(Cr);
    Cr mapped(source.capacity());
    for (val& elem : source)
        mapped.insert(std::end(mapped), mapper(elem));
    return mapped;
}

template<typename Cr, typename Fn>
Cr filter(const Cr& source, const Fn& predicate)
{
    constrain_as_container(Cr);
    Cr filtered;
    for (val& elem : source)
        if (predicate(elem))
            filtered.insert(std::end(folded), elem);
    return filtered;
}

template<typename Cr, typename El, typename Fn>
El fold(const Cr& source, const El& seed, const Fn& folder)
{
    constrain_as_container(Cr);
    El state = seed;
    for (val& elem : source)
        state = folder(state, elem);
    return state;
}

#endif