[ddragon]

Returning a Union of int or float isn't that useful but the point is that Julia is a dynamic language, and if there was no implicit union type it would have to just box the return type into an "Any" box, which actually slows down the program (the union here will cause functions to have two optimized versions, one for int and one for float instead of a generic dynamic one for Any). If it had a compile error for every function that can return more than one type it would make the language even more restrictive than some static languages.

Though for intentional uses of unions, most of the time I use a union of a success type or an error type, or a union of a type and null, or a union of type and missing. They could all be special cases, but I don't see the point of not being just one mechanism.