Deeply update a map with key-value pairs

Questions / Help
1

Let’s say we have:

game = %App.Games.Game{
  id: "owieruoiewurwer",
  players: %{
    "jwuiyeywi_oiwueroiu" => %App.Games.Player{
      admin: true,
      card: "4"
    },
    "oiwueroiquw_ncjaioq" => %App.Games.Player{
      admin: false,
      card: "6"
    }
  }
}

What it the best way to put nil in each player card ?
I used update_in/3 with Access.all/0 but can’t make it work with key-value pairs in :players.

The only solution I found it is to convert my string keys into atoms, but that would force me to iterate through the player “list” two times.

2

You need to replace values to build the result.

  • At the top level, we’re placing players
  • And then we need to iterate the map (as key-value pair), and for each value we need to put nil to card key.

Quick solution:

v = %{
  id: "owieruoiewurwer",
  players: %{
    "jwuiyeywi_oiwueroiu" => %{
      admin: true,
      card: "4"
    },
    "oiwueroiquw_ncjaioq" => %{
      admin: false,
      card: "6"
    }
  }
}

%{
  v
  | players:
      Map.new(
        v.players,
        fn {k, player} -> {k, %{player | card: nil}} end
      )
}
|> IO.inspect()

Note that it would be better to make functions to capture “domain logic” properly. e.g. if reset_game/1 or empty_card/1.

1 Like
3

That worked just fine, thanks !

I’ve never used Map.new before. Just to be sure, update_in/3 can’t be used in those circumstances ?

4

Since update_in needs keys to look at for a value, you cannot use it to transform all values.

If you need to change a value under a known path, you can use that for example.

1 Like
5

It can be used, but you need to write your own lens (Access.access_fun/2 type):

all_map_values = fn
  :get, data, next ->
    for {_, v} <- data, do: next.(v)

  :get_and_update, data, next ->
    {gets, updates} =
      Enum.reduce(data, {[], []}, fn {k, v}, {gets, updates} ->
        case next.(v) do
          {get, update} -> {[get | gets], [{k, update} | updates]}
          :pop -> {gets, updates}
        end
      end)

    {:lists.reverse(gets), Map.new(updates)}
end

game = %{
  id: "owieruoiewurwer",
  players: %{
    "jwuiyeywi_oiwueroiu" => %{
      admin: true,
      card: "4"
    },
    "oiwueroiquw_ncjaioq" => %{
      admin: false,
      card: "6"
    }
  }
}

IO.inspect(put_in(game, [:players, all_map_values, :card], nil))
4 Likes
6

I thought the Access module could help me bypass that kind of function definition. But it makes a lot of sense to simply explain how to access the value like this in the end.

Thanks !