Render-time value transforms.

Transforms modify attribute values at resolution/render time before they reach consumers. Declare a transform on any attribute via its data_config["transform"] key (a string name or list of string names).

Built-in transforms

• "redact" — replaces any non-nil value with "••••••••"
• "redact_secret" — replaces non-nil with "[provided]", returns "[not set]" for nil

Registering custom transforms

Implement the AttrEngine.Transform behaviour and register via application config:

config :attr_engine, transforms: %{
  "currency" => MyApp.Transforms.Currency
}

Or supply a function (arity 1 or 2):

config :attr_engine, transforms: %{
  "upcase" => &String.upcase/1,
  "prefix" => fn value, opts -> opts["prefix"] <> value end
}

External transforms take precedence over built-ins with the same name.

Behaviour

Modules implementing this behaviour must define apply/2:

defmodule MyApp.Transforms.Currency do
  @behaviour AttrEngine.Transform

  @impl true
  def apply(value, _opts) when is_number(value) do
    :erlang.float_to_binary(value / 1.0, decimals: 2)
  end
  def apply(value, _opts), do: value
end

The second argument (opts) is the full data_config map for the attribute, allowing transforms to read additional configuration keys.