The syntax proposed has problems. Many languages allow you to declare nested data structures and a natural way to express an array of arrays in swift would be the following:
let a = [[1,2,3],[6,7,8]] let a = [[1,2,3],[6,7,8]] let a = [: [: 1, 2, 3 :], [: 6, 7, 8 :] :]
let a = [ [: 1, 2, 3 :], [: 6, 7, 8 :] ]