Implementing the fitness function.

however they can still be in the same row

The genotype function generates an Elixir list of the columns that each queen is in (Enum.shuffle(0…7). A list 8 elements long consisting of all the digits 0-7 in some random order.
Each element of the list represents the row for the queens column. As each element index must be unique (0, 1, 2, …,7) otherwise it would be a very strange list, each row can only contain one column.

Therefore there is no need to check that the rows only contain one queen.