While I was building the position heuristic function for Connect Four I ran into an interesting gotcha with F# pattern matching. Lets see if you see it before I tell you what it is: let rec heuristic (positions: (int * int) list) (pos: int*int) = match positions with | [] -> 0 | position::_ -> 1 + (heuristic (List.tl positions) pos) | _ -> heuristic (List.tl positions) pos Looking at it, it seems the code should tell me how often value is in position, right? Well it doesn’t! And there is...