# Run code for each @target.
def each_target(current_hero, other_hero, target, position)
    current_field = current_hero.field
    other_field = other_hero.field
    case target
        # your hero and creatures
        when 'self:all'
            current_field.position.each { |slot| yield slot unless slot.empty? }
            yield current_hero

        # your creatures
        when 'self:creatures'
            current_field.position.each { |slot| yield slot unless slot.empty? }

        # one of your creatures
        when 'self:one'
            yield current_field.position[position] unless current_field.position[position].empty?

        # your hero
        when 'self:hero'
            yield current_hero

        # enemy hero and creatures
        when 'foe:all'
            other_field.position.each { |slot| yield slot unless slot.empty? }
            yield other_hero

        # enemy creatures
        when 'foe:creatures'
            other_field.position.each { |slot| yield slot unless slot.empty? }

        # one of enemy creatures
        when 'foe:one'
            yield other_field.position[position] unless other_field.position[position].empty?

        # enemy hero
        when 'foe:hero'
            yield other_hero
    end
end
# …
card_abilities.each do |ability|
    case ability['name']
        # Damage to enemy hero and opposite creature equals
        # to opposite creature's attack
        when 'damage'
            # self method
            each_target(current_hero, other_hero, ability['target'], position) do |t|
                t.do_damage(ability['param'].to_i, :magic)
            end

        when 'heal'
            each_target(current_hero, other_hero, ability['target'], position) do |t|
                t.do_heal(ability['param'].to_i)
            end
# …