Behaviour for casting a single value to and from its wire representation.

A Dowser.Client.Field knows how to cast one backend field type — a date, a geo point, an IP, whatever a backend's mapping describes — in both directions:

• load/2 — turns a decoded-but-still-raw value (e.g. the string "2024-01-01") into a richer Elixir term (e.g. ~D[2024-01-01]).
• dump/2 — the reverse, turning a richer term back into whatever the backend expects on the wire.

Both callbacks receive the value and a second argument — typically the field's own metadata (e.g. an Elasticsearch mapping entry) — and return the cast term directly. A value the field doesn't recognize should pass through unchanged rather than raise, so a bad cast degrades to identity instead of failing the whole document.

dowser_client ships only this behaviour, not any implementation — it has no knowledge of any particular backend's field types. A backend package (e.g. dowser_elasticsearch) implements one module per field type and wires them together with Dowser.Client.Codec.Builder, which assembles a load/2/dump/2 dispatcher from them — see its moduledoc for how that dispatcher is then wired into a Dowser.Client.Codec as :codec_adapter.

defmodule MyApp.Fields.Date do
  @behaviour Dowser.Client.Field

  @impl true
  def load(value, %{"format" => "strict_date"}) when is_binary(value) do
    case Date.from_iso8601(value) do
      {:ok, date} -> date
      {:error, _reason} -> value
    end
  end

  def load(value, _field), do: value

  @impl true
  def dump(%Date{} = date, %{"format" => "strict_date"}) do
    Date.to_iso8601(date)
  end

  def dump(value, _field), do: value
end