Elixir - 如何深度合并 map ？

Question

与 Map.merge我有:

Map.merge(%{ a: %{ b: 1 }}, %{ a: %{ c: 3 }}) # => %{ a: %{ c: 3 }}

但实际上我想:

Map.merge(%{ a: %{ b: 1 }}, %{ a: %{ c: 3 }}) # => %{ a: %{ b: 1, c: 3 }}

在这种情况下，是否有没有编写递归样板函数的本地方法？

Answer 1

正如@Dogbert 建议的那样，您可以编写一个函数来递归合并 map 。

defmodule MapUtils do
  def deep_merge(left, right) do
    Map.merge(left, right, &deep_resolve/3)
  end

  # Key exists in both maps, and both values are maps as well.
  # These can be merged recursively.
  defp deep_resolve(_key, left = %{}, right = %{}) do
    deep_merge(left, right)
  end

  # Key exists in both maps, but at least one of the values is
  # NOT a map. We fall back to standard merge behavior, preferring
  # the value on the right.
  defp deep_resolve(_key, _left, right) do
    right
  end
end

以下是一些测试用例，可让您了解如何解决冲突:

ExUnit.start

defmodule MapUtils.Test do
  use ExUnit.Case

  test 'one level of maps without conflict' do
    result = MapUtils.deep_merge(%{a: 1}, %{b: 2})
    assert result == %{a: 1, b: 2}
  end

  test 'two levels of maps without conflict' do
    result = MapUtils.deep_merge(%{a: %{b: 1}}, %{a: %{c: 3}})
    assert result == %{a: %{b: 1, c: 3}}
  end

  test 'three levels of maps without conflict' do
    result = MapUtils.deep_merge(%{a: %{b: %{c: 1}}}, %{a: %{b: %{d: 2}}})
    assert result == %{a: %{b: %{c: 1, d: 2}}}
  end

  test 'non-map value in left' do
    result = MapUtils.deep_merge(%{a: 1}, %{a: %{b: 2}})
    assert result == %{a: %{b:  2}}
  end

  test 'non-map value in right' do
    result = MapUtils.deep_merge(%{a: %{b: 1}}, %{a: 2})
    assert result == %{a: 2}
  end

  test 'non-map value in both' do
    result = MapUtils.deep_merge(%{a: 1}, %{a: 2})
    assert result == %{a: 2}
  end
end