A union type in Nim can only be used in funciton arguments, and it does the obvious thing: when you actually call the function, it specializes to the type you a

A union type in Nim can only be used in funciton arguments, and it does the obvious thing: when you actually call the function, it specializes to the type you are calling with. Think about templates in C++, where the type parameter can only assume one of two (or more) values. Hence it would generate exactly what you would write by hand, but the syntax is much less messy than C++ templates