Soccer Game AI

I am currently working on the A.I. for my soccer game. My task right now is to try to create various positions that the A.I. players can be. I'm thinking of having an attacker that is always on the attacking side of the field being ready to score goals, a defender who will stay on the defensive side of the field but will always try to get in between the ball and the goal and a midfielder who will chase the ball anywhere. The trickiest part at this stage is that I believe I will have to use polymorphism as I am going to want all three different position classes to be kept in one AI dynamic array.