Ugly but short.
From bottom to top: a vector of all the possible win combinations for a flattened board:
012 is the top row
036 is the first column, etc
ending with
048 and 246 as the two diagonals.
Make the input a vector so I can use map and the vector to find the values at the specific locations.
Then split those values into groups of three and look for all :o's or all :x's and return a nil if none of those combinations exist.
#(some {[:o :o :o] :o [:x :x :x] :x} (partition 3 (map (vec (flatten %)) [0 1 2 3 4 5 6 7 8 0 3 6 1 4 7 2 5 8 0 4 8 2 4 6])))