What does an empty set of parentheses mean when used in a generic type declaration?

() is an empty tuple, a simple zero-sized type (it uses no memory) with only one value possible, (). It’s also known as the unit type. Its use in a return type of Result<(), E> means “if nothing goes wrong, there’s no further value produced”. The semantics are what’s important—the call was OK.

Result<(), ()> would also make sense as a return type—either something succeeded, or it failed, with nothing more to report in either case.

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