Merging Elixir maps

Question

how can I merge those 2 Elixir maps:

foo = %{a: 1, b: 2, c: [%{d: 3, e: 4}, %{d: 5, e: 6}]}
bar = %{a: 1, b: 2, c: [%{d: 7, e: 8}, %{d: 9, e: 0}]}

... to get the following result:

%{a: 1, b: 2, c: [%{d: 3, e: 4}, %{d: 5, e: 6}, %{d: 7, e: 8}, %{d: 9, e: 0}]}

Simple Map.merge(foo,bar) doesn't do it in the way since value of c is a list.

Thank you in advance! Christoph

Answer 1

Use Map.merge/3:

Map.merge(foo, bar, fn
  _k, v1, v2 when is_list(v1) and is_list(v2) -> v1 ++ v2 # lists
  _k, %{} = v1, %{} = v2 -> Map.merge(v1, v2)             # maps
  _k, v1, v1 -> v1                                        # equals
  _k, v1, v2 -> {v1, v2}                                  # non-equals
end)
#⇒ %{a: 1, b: 2,
#    c: [%{d: 3, e: 4}, %{d: 5, e: 6}, %{d: 7, e: 8}, %{d: 9, e: 0}]}

You did not specify the rule to merge anything save for lists, but the above might be easily extended to handle whatever. Now it understands lists, maps, equal values, and non-equal values.