The PostTransforms extension adds the :map option to Flint schemas.

This works similarly to the PreTransforms extension, but uses the :map option rather than the :derive option used by PreTransforms, and by default, applies to the field after all validations.

The same caveats apply to the :map expression as all other expressions, with the exception that the :map function only accepts arity-1 anonymous functions or non-anonymous function expressions (eg. using variable replacement).

In the following example, :derived is used to normalize incoming strings to downcase to prepare for the validation, then the output is mapped to the uppercase string using the :map option.

defmodule Character do
  use Flint.Schema

  embedded_schema do
    field! :type, :string, derive: &String.downcase/1, map: String.upcase(type) do
      type not in ~w[elf human] -> "Expected elf or human, got: #{type}"
    end

    field! :age, :integer do
      age < 0 ->
        "Nobody can have a negative age"

      type == "elf" and age > max_elf_age ->
        "Attention! The elf has become a bug! Should be dead already!"

      type == "human" and age > max_human_age ->
        "Expected human to have up to #{max_human_age}, got: #{age}"
    end
  end
end

max_elf_age = 400
max_human_age = 120
Character.new!(%{type: "Elf", age: 10}, binding())

%Character{type: "ELF", age: 10}