Canonical way of updating/replacing a map value in `boost::hana::map`

Question

What is the canonical way of updating a value (given a key and a new value) inside a boost::hana::map?

I tried using boost::hana::replace_if but it does not work on map as it is not a Functor - I can get it to work by converting the map to a tuple and then back to a map, but it sounds inefficient.

The alternative I'm currently using is calling map::erase_key followed by map::insert.

Is there any replace or update function designed for this purpose that I might be missing? Or is this the "canonical" way of updating a value?

Answer 1

I don't think there is currently a canonical way to do this. If there are valid use cases for this, perhaps we could get a function in there to support it. The problem with hana::erase_key is that you will be creating a new map and then again with hana::insert. For the time being, using hana::unpack and then creating a new map is probably your best bet.

#include <boost/hana.hpp>

namespace hana = boost::hana;

template <typename NewPair>
struct replace_helper_t
{
  NewPair const& new_pair;

  template <typename Pair>
  constexpr decltype(auto) operator()(Pair&& p) const
  {
    return hana::if_(
      hana::equal(hana::first(new_pair), hana::first(p)),
      new_pair,
      std::forward<Pair>(p)
    );
  }
};

struct replace_t
{
  template <typename Map, typename NewPair>
  constexpr auto operator()(Map&& m, NewPair&& new_pair) const
  {
    return hana::unpack(std::forward<Map>(m),
      hana::on(
        hana::make_map,
        replace_helper_t<NewPair>{std::forward<NewPair>(new_pair)}
      )
    );
  }
};

constexpr replace_t replace{};

int main()
{
  auto my_map = hana::make_map(
    hana::make_pair(hana::int_c<7>, 7),
    hana::make_pair(hana::int_c<13>, 13),
    hana::make_pair(hana::int_c<23>, 23)
  );

  auto new_map = replace(my_map, hana::make_pair(hana::int_c<13>, 14.0f));

  BOOST_HANA_RUNTIME_ASSERT(new_map ==
    hana::make_map(
      hana::make_pair(hana::int_c<7>, 7),
      hana::make_pair(hana::int_c<13>, 14.0f),
      hana::make_pair(hana::int_c<23>, 23)
    )
  );
}

On another note, perhaps hana::map should be a Functor.