Good to hear!
Not yet. It’s something we want, but it’s not in the language yet.
The way to do this is if you have arrays of different length, concatenate them all into one big array, and then have another array of start indices. For instance:
real big_array[N]; // N total values
int starts[S + 1]; // Start index of S arrays (and the last index is N + 1)
big_array[starts[1]:(starts[2] - 1)] // first array
big_array[starts[s]:(starts[s + 1] - 1)] // sth array
More doc is here: 8.2 Ragged Data Structures | Stan User’s Guide