What is an empty parens?
I was reading some Rust documentation on futures and I fell over something I found pretty unclear.
A timeout. When time is up, the future is completed, and its value is ().
The page was an overview, so I found it confusing the all of a sudden a reference code was included, since all of the other examples where on a higher abstraction level.
- A database query, when the query finishes, the future is completed, and its value is the result of the query.
- An RPC invocation to a server. When the server replies, the future is completed, and its value is the server’s response.
- A timeout. When time is up, the future is completed, and its value is ().
- A long-running CPU-intensive task, running on a thread pool. When the task finishes, the future is completed, and its value is the return value of the task.
- Reading bytes from a socket. When the bytes are ready, the future is completed – and depending on the buffering strategy, the bytes might be returned directly, or written as a side-effect into some existing buffer.
Luckily I am not the only one who have wondered about this and I found a good response to a question on StackOverflow.
() 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.
So the empty parethesis is a empty tuple, called Unit, in Rust with the following characteristics:
- It has zero cost memory-wise
- It is referred to as: “unit”, “the unit type” or “nil”
- It resembles
void,null,undefin other languages
A tuple is a primitive type: Unit in Rust