Add map_get/2 guard

[zolrath]

Hello!

Erlang/OTP 21 added both is_map_key/2 and map_get/2, but currently only is_map_key/2 is exposed via Elixir guards.

As most users will likely look through Elixir documentation to discover which guards are available, exposing map_get/2 seems appropriate.

This PR adds a map_get/2 guard, exposing :erlang.map_get/2 to aid in discoverability.
The argument order has been changed to (map, key) for consistency with is_map_key/2 and the Map module.

I've defined map_get/2 as a macro, as if I simply def and immediately delegate to :erlang.map_get/2 tests fail with:

error: cannot invoke remote function Kernel.map_get/2 inside guards
  lib/elixir/test/elixir/kernel_test.exs:1521: KernelTest."test map_get/2"/1

Thanks for everything!

[zolrath]

Ah, just found a discussion from 2018 on ElixirForum about this! Am I correct reading "We are going with #1" and the fact that is_map_keys exists that this is intended to be added as well?

The code that triggered the discussion around this guard/this pull request was during Advent of Code answer discussions:

  @winners %{rock: :scissors, paper: :rock, scissors: :paper}

  def battle(same, same), do: :draw
  def battle(them, you) when :erlang.map_get(you, @winners) == them, do: :win
  def battle(_, _), do: :lose

As it's one of the few erlang guards not listed in the docs, there was some "oh hey that guard exists?" discussion.

[josevalim]

Correct. We know it exists, we know it is missing, but we are skeptical about adding a fourth way of reading map keys (map.key, map[key], Map.get). We may consider adding other ways in the future, such as pattern matching.

[zolrath]

Correct. We know it exists, we know it is missing, but we are skeptical about adding a fourth way of reading map keys (map.key, map[key], Map.get). We may consider adding other ways in the future, such as pattern matching.

Sounds good to me! If the above map_get could be expressed with map[key] that would definitely feel natural.

def battle(them, you) when @winners[you] == them, do: :win