Hello everyone,

I wrote a small library today called MapDiff.
It returns a map listing the (smallest amount of) changes to get from map_a to map_b:

iex> foo = %{a: 1, b: 2, c: %{d: 3, e: 4, f: 5}}
iex> bar = %{a: 1, b: 42, c: %{d: %{something_else: "entirely"}, f: 10}}
iex> MapDiff.diff(foo, bar)
%{changed: :map_change,
  value: %{a: %{changed: :equal, value: 1},
    b: %{added: 42, changed: :primitive_change, removed: 2},
    c: %{changed: :map_change,
      value: %{d: %{added: %{something_else: "entirely"},
      changed: :primitive_change, removed: 3},
    e: %{changed: :removed, value: 4},
    f: %{added: 10, changed: :primitive_change, removed: 5}}}}}

This is mainly useful if you have two maps describing the state of something, where
replacing all of the old state and then filling in the new is too slow, which means that only
replacing the things that actually are changed is better.

It is similar to what Elm does to represent its output to the HTML Domain Object Model. The reason I wrote MapDiff is that I am planning on writing a library that wraps Erlang’s wxWidgets bindings in a similar way that Elm wraps the HTML DOM, which would allow people to write GUIs in a Functional style (wxWidgets itself is Object-Oriented). Does this sound like a huge undertaking? Yes! Let’s see how far I can get with it …

Anyhow, MapDiff is a very small and basic library, but it might be useful for some other people as well, so I thought I’d publish it on Hex.pm . Feedback is of course greatly appreciated.