Since TYPO was introduced and then upgraded to TYPO v2, typing has become a big thing for YASS. The standard library uses types on all built-in functions and all of my own programs are moving to be typed. One of the new features that is coming to ZPE in the near future is forcing typing and disallowing the execution of programs that don't use strong typing. But before that, ZPE is looking to bring union types.
Union types are crucially important for return types on functions which may need to return two separate data types. Rather than using the very abstract mixed type, these functions would use the union type. Assume the following function which returns an integer (the index of the found item) or a Boolean false if the item is not found. Using types with this function currently means specifying a mixed type to allow both integer or Boolean return types:
function linearSearch(string searchTerm, list items) : mixed
This same program could be written with union types as shown below:
function linearSearch(string searchTerm, list items) : integer | boolean
This is a much less abstract and more concrete solution to the problem, forcing the return type to be one or the other. This is the first step in union types, as union types will also come to variable declaration, but this isn't planned to come any time soon.