This function is invoked for every key/value pair in the document, in a depth-first, bottom-up, traversal of the document.

The transformer must return one of two values:

• {:error, term} - an error occurred in the transformer, and decoding should fail
• term - replace the value for the given key with the value returned from the transformer in the final document

Example

An example transformation would be the conversion of tables of a certain shape to a known struct value.

The struct:

defmodule Server do
  defstruct [:name, :ip, :ports]
end

TOML which contains a table of this shape:

[servers.alpha]
ip = "192.168.1.1"
ports = [8080, 8081]

[servers.beta]
ip = "192.168.1.2"
ports = [8082, 8083]

And finally, the transforer implementation:

defmodule ServerTransform do
  use Toml.Transform

  # Transform IP strings to Erlang address tuples
  def transform(:ip, ip) when is_binary(ip) do
    case :inet.parse_ipv4_address(String.to_charlist(ip)) do
      {:ok, result} ->
        result
      {:error, reason} ->
        {:error, {:invalid_ip, ip, reason}}
    end
  end

  # Non-binary values for IP addresses should return an error
  def transform(:ip, ip), do: {:error, {:invalid_ip, ip, :expected_string}}

  # Transform the server objects to Server structs
  def transform(:servers, servers) when is_map(servers) do
    for {name, server} <- servers do
      struct(Server, Map.put(server, :name, name))
    end
  end

  # Non-map values for servers should return an error
  def transform(:servers, s), do: {:error, {:invalid_server, s}}

  # Ignore all other values
  def transform(_key, v), do: v
end

Assuming we decode with the following options:

  Toml.decode!(content, keys: :atoms, transforms: [ServerTransform])

The result would be:

  %{
    servers: [
      %Server{name: :alpha, ip: {192,168,1,1}, ports: [8080, 8081]},
      %Server{name: :beta, ip: {192,168,1,2}, ports: [8082, 8083]}
    ]
   }