defmodule Funx.Validator.Required do
  @moduledoc """
  Validates that a value is present (not nil, not empty string, not Nothing).

  ## Special Semantics

  **Required is the ONLY validator that runs on `Maybe.Nothing`.**

  All other validators skip `Nothing` values (from Prism projections).
  This makes fields optional-by-default with explicit presence checks.

  ## Failure Conditions

  - `nil`
  - `""` (empty string)
  - `%Maybe.Nothing{}` (from Prism projections)

  ## Success Conditions

  All other values, including:
  - `0`, `false`, `[]` (falsy but present values)

  ## Options

  - `:message` - Custom error message callback `(value -> String.t())`

  ## Examples

      iex> Funx.Validator.Required.validate("hello")
      %Funx.Monad.Either.Right{right: "hello"}

      iex> Funx.Validator.Required.validate(nil)
      %Funx.Monad.Either.Left{left: %Funx.Errors.ValidationError{errors: ["is required"]}}

      iex> Funx.Validator.Required.validate(0)
      %Funx.Monad.Either.Right{right: 0}
  """

  @behaviour Funx.Validate.Behaviour

  alias Funx.Monad.Either
  alias Funx.Monad.Maybe.Nothing
  alias Funx.Predicate
  alias Funx.Validator

  def validate(value) do
    validate(value, [], %{})
  end

  def validate(value, opts) when is_list(opts) do
    validate(value, opts, %{})
  end

  @impl true
  def validate(value, opts, env)

  def validate(%Nothing{}, opts, _env) do
    error = Validator.validation_error(opts, nil, "is required")
    Either.left(error)
  end

  def validate(value, opts, _env) do
    predicate = Predicate.Required.pred(opts)

    Either.lift_predicate(
      value,
      predicate,
      fn v -> Validator.validation_error(opts, v, "is required") end
    )
  end
end