# frozen_string_literal: true

require_relative 'immutable_struct'

module Upgrow
  # Results are special Structs that are generated dynamically to accommodate a
  # set of pre-defined members. Since different Actions might want to return
  # zero to multiple values, they are always returned as members of a Result
  # instance.
  #
  # Regardless of the values the Action might want to return, a Result has one
  # default member called errors, which holds any errors that might occur when
  # the Action is performed. If Result errors are empty, the Result is a
  # success; if there are errors present, however, the Result is a failure. This
  # empowers Actions with a predictable public interface, so callers can expect
  # how to evaluate if an operation was successful or not by simply checking the
  # success or failure of a Result.
  #
  # Additionally, Result instances behave like monadic values by offering
  # bindings to be called only in case of success or failure, which further
  # simplifies the caller's code by not having to use conditional to check for
  # errors.
  class Result < ImmutableStruct
    class << self
      # Creates a new Result class that can handle the given members.
      #
      # @param members [Array<Symbol>] the list of members the new Result should
      #   be able to hold.
      #
      # @return [Result] the new Result class with the given members.
      def new(*members)
        members << :errors unless members.include?(:errors)
        super(*members)
      end
    end

    # Returns a new Result instance populated with the given values.
    #
    # @param values [Hash<Symbol, Object>] the list of values for each member
    #   provided as keyword arguments.
    #
    # @return [Result] the Result instance populated with the given values.
    def initialize(**values)
      errors = values.fetch(:errors, [])
      super(**values.merge(errors: errors))
    end

    # Check if the Result is successful or not. A successful Result means there
    # are no errors present.
    #
    # @return [true] if the Result is successful.
    # @return [false] if the Result is a failure.
    def success?
      errors.none?
    end
  end
end