elixir - Pattern match function against empty map

Question

I'm playing around with pattern match and I found out, that it's not quite easy to pattern match parameters of a method against an empty map. I thought it would go something like this:

defmodule PatternMatch do
  def modify(%{}) do
    %{}
  end

  def modify(map) do
    # expensive operation
    %{ modified: "map" }
  end
end

But it seems like the first function clause matches arbitrary maps:

iex> PatternMatch.modify(%{a: "map"})
==> %{}

Is there another way to check for empty maps?

Answer 1

It works this way by design, but admittedly it can be a bit confusing at first glance. This feature allows you to destructure maps using pattern matching, without having to specify all keys. For example:

iex> %{b: value} = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}

iex> value
2

Consequently, %{} will match any map. If you want to match an empty map in a function, you have to use a guard clause:

defmodule PatternMatch do
  def modify(map) when map == %{} do
    %{}
  end

  def modify(map) do
    # ...
  end
end