Return value from match to Err(e)

Question

I'm trying to write a simple TCP echo server in Rust, and I'm a little confused over how to return a value from a match to Err.

I know the return type is supposed to be usize, and I'd like to return a zero. In other languages, I would just return 0; but Rust doesn't let me. I've also tried usize::Zero(). I did get it to work by doing let s:usize = 0; s but that seems awfully silly and I imagine there would be a better way to do it.

let buffer_length =  match stream.read(&mut buffer) {
    Err(e) => {
         println!("Error reading from socket stream: {}", e);
         // what do i return here?
         // right now i just panic!
         // return 0;
    },
    Ok(buffer_length) => {
        stream.write(&buffer).unwrap();
        buffer_length
    },
};

I know I could also just not have the match return anything and consume buffer_length inside the match with a function call or something, but I'd prefer not to in this case.

What is the most idiomatic way to handle something like this?

It's not clear what your problem actually is. "Return value from match to Err(e)" just doesn't make any sense, and everything you've described sounds like it *should* work. Your best bet is to update the question including a minimal, compilable example of what you're trying and the output of the compiler. Also, a clear indication of what *result* you're trying to get (what should happen after this "return") would help. — DK., Oct 14 '15 at 04:28

score 16 · Accepted Answer · answered Oct 14 '15 at 07:31

The same way you "returned" buffer_length from inside the Ok arm, you can simply "return" a 0 from the Err arm by leaving the trailing expression without return or a semicolon.

return always returns from a function. If you want to get the value of an expression, there's nothing you need to do in Rust. This always happens automatically.

This works for the same reason your let s = 0; s workaround worked. All you did was insert a temporary variable, but you can also directly let the expression get forwarded to the outer expression.

let buffer_length = match stream.read(&mut buffer) {
    Err(e) => {
         println!("Error reading from socket stream: {}", e);
         0
    },
    Ok(buffer_length) => {
        stream.write(&buffer).unwrap();
        buffer_length
    },
};

Return value from match to Err(e)

1 Answers1

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