list.rb

module Pathfinder
  class Feat
    class List
      class NoMatchingSlotsAvailable < StandardError; end

      include Enumerable

      def initialize
        @feats = []
      end

      def add_slot(&rule)
        # general feats go at the end, so restricted feats get filled in first
        slot = Slot.new(rule)
        rule ? @feats.unshift(slot) : @feats.push(slot)
      end

      def <<(feat)
        slot = @feats.detect {|slot| slot.matches?(feat) }
        slot ? slot.fill(feat) : raise(NoMatchingSlotsAvailable, "#{feat} can't be added")
      end

      def to_a
        @feats.map {|slot| slot.feat }
      end

      def each(&block)
        to_a.each(&block)
      end

      class Slot
        attr_reader :feat

        def initialize(rule)
          @rule = rule
        end

        def matches?(feat)
          empty? && (@rule.nil? || @rule[feat])
        end

        def fill(feat)
          @feat = feat
        end

        def empty?
          @feat.nil?
        end
      end
    end
  end
end

use_example.rb

character.feats.add_slot # any feat
character.feats.add_slot {|feat| feat.metamagic? }
character.feats.add_slot {|feat| feat.combat? }

character.feats << some_metamagic_feat # fills the slot that matches feat.metamagic?
character.feats << some_metamagic_feat # fills the general slot, because the metamagic one is filled
character.feats << some_non_combat_feat # raises because the only available slot allows combat feats only