Simple: call that function with that variable name as its output argument:
var_name = A(X,Y);
will define var_name in the workspace where the function is being called. Passing arguments is the fastest, simplest and neatest way of passing variables between workspaces, and this is what MATLAB themselves recommend (see link below). Whatever you do, do not learn sloppy buggy programing by passing arguments using assignin or globals. Although beginners love using them, actually both of these methods make code slower and buggier.
The entire concept of functions is very similar to encapsulation, which is broken when you start passing variables willy-nilly in and out of their workspaces. It is a bad idea.
