@skosch from reading the solutions here i did this (thougt id share for the next person)

  def map_put(data, keys, value) do
    # data = %{} or non empty map
    # keys = [:a, :b, :c]
    # value = 3
    put_in(data, Enum.map(keys, &Access.key(&1, %{})), value)
  end

  def many_map_puts(data, keys_values) do
    # data = %{} or non empty map
    # keys_values = [[keys: [:a, :b, :c], value: 4],[keys: [:z, :y, :x], value: 90]]
    Enum.reduce(keys_values, data, fn x, data ->
      map_put(data, x[:keys], x[:value])
    end)
  end

iex(1)> m = [[keys: [:a, :b, :c], value: 4],[keys: [:z, :y, :x], value: 90]]
[[keys: [:a, :b, :c], value: 4], [keys: [:z, :y, :x], value: 90]]
iex(2)> many_map_puts(%{}, m)
%{a: %{b: %{c: 4}}, z: %{y: %{x: 90}}}

Not entirely sure if this addresses what you are trying to accomplish but have you had a look at Kernel.put_in/3, Kernel.get_and_update_in/3 and the like - and Access behavior?

So if I understand you correctly you want

put_in(%{a: %{}}, [:a, :b, :c], 42)

to result in

%{a: %{b: %{c: 42)} } }

One problem I see is that you are assuming that :b should refer to a Map value i.e. that the nested structure is a homogenous Map. The Access data[key] syntax is right out of the box also supported by Keyword lists - and any other structure that cares to implement the Access behaviour so there is some ambiguity right there - just because there is a key doesn’t automatically mean it should be a Map.

put_in can manipulate anything that Access supports, not just maps. For instance:

iex(1)> a = {1, 2, :x}
{1, 2, :x}
iex(2)> put_in a, [Access.elem(2)], :foo
{1, 2, :foo}