[Proposal] Map.take_in/2

[Samuel Manzanera]

Hello everybody,

I propose the support of Map.take_in/2 to extract values using nested keys (recursively).

I think it may be useful when you only want a part of the map -- mostly when its deep nested map (i.e KV stores, DB engines, etc..) to reduce memory footprint with copy and message passing.

Example:

iex> Map.take_in(%{a: "hello", b: %{c: "hi", d: "hola"}}, [:a, b: [:d]])

%{a: "hello", b: %{d: "hola"}}

As discussed in the #1029 the feature should be extended to Access  for both Map and Keyword implementation.

To analyse the efficiency regarding pattern matching.

But it's a way to resolve the duplication from pattern matching as in this example.

iex> %{a: a, b: %{d: d}} = %{a: "hello", b: %{c: "hi", d: "hola"}}
iex> %{a: a, b: %{d: d}}
%{a: "hello", b: %{d: "hola"}}

Thanks :)

[Samuel Manzanera]

Another advantage it is the support of dynamic key pattern matching.
If you are supporting function which accept paramete as the keys to filter the map, so it can be dynamically filtered out.

Example

def get_data(fields) do

   get_map_from_state_or_db()

   |> Map.take_in(fields) # or take_in (with Access implementation)

end

[Fernando Tapia Rico]

I think the syntax might be ambiguous in some edge gases because the keys on a map can be anything. For example:

iex(1)> map = %{:a => "hello", {:b, [:d]} => 1, :b => %{d: 3}}

%{:a => "hello", :b => %{d: 3}, {:b, [:d]} => 1}

iex(2)> Map.take(map, [:a, b: [:d]])

%{:a => "hello", {:b, [:d]} => 1}