To my knowledge, put_in, Map.update etc. all have the one limitation of not automatically creating intermediate keys when needed (for example, put_in(%{a: %{}}, [:a, :b, :c], 42) will fail). But – despite all the risks – this would be useful to have: for instance, I often use reduce to create a nested, aggregate representation of a list of complex objects (say, counting occurrences by categories and sub-categories), which is a one-liner when you don’t have to worry about intermediate keys (think mkdir -p).

The implementation is simple enough (see deep_update example below, which I’m already using all over my projects). I have a nagging feeling this must already be part of Elixir’s standard library and I just don’t know about it? If not, is there a particular reason it isn’t?

  %spec deep_update(map(), [any()], any(), any() -> any()) :: map()
  def deep_update(map, [leaf_key], default, update_func) do
    updated_leaf = if is_map(map) and Map.has_key?(map, leaf_key) do
      update_func.(map[leaf_key])
    else
      default
    end
    Map.put(map, leaf_key, updated_leaf)
  end
  def deep_update(map, [key | key_tail], default, update_func) do
    if is_map(map) and Map.has_key?(map, key) do
      new_branch = deep_update(map[key], key_tail, default, update_func)
      Map.put(map, key, new_branch)
    else
      new_branch = deep_update(%{}, key_tail, default, update_func)
      Map.put(map, key, new_branch)
    end
  end