deep_fetch.exs

opts = [with: %{name: "George"}]
acc = %{
  foo: %{
    columns: %{
      name: "Will"
    }
  }
}

with {:ok, override} <- Keyword.fetch(opts, :with),
     {:ok, row} <- Map.fetch(acc, row_name),
     {:ok, columns} <- Map.fetch(row, :columns) do
       columns = Map.merge(columns, override)
       put_in(acc, [row_name, :columns], columns)
else
  :error -> acc

with {:ok, override} <- Keyword.fetch(opts, :with),
     %{} = cols <- get_in(acc, [row_name, :columns]),
     columns = Map.merge(cols, override) do

  put_in(acc, [row_name, :columns], columns)
else
  _ -> acc
end