CODEWITHSUNDEEP
error-handlingrust
Galaxy
Galaxy

Reputation: 1279

How to use a Result when there is no success value to return from a function?

To define a function with void return, my code goes like the following:

trait Handler {
    fn on_message(&mut self, msg: String) -> Result<()> {
        println!("on_message: {}", msg);
        Ok(())
    }
}

The compiler tell me this is wrong because a Result should always have 2 parameters:

error[E0243]: wrong number of type arguments: expected 2, found 1
 --> src/lib.rs:2:46
  |
2 |     fn on_message(&mut self, msg: String) -> Result<()> {
  |                                              ^^^^^^^^^^ expected 2 type arguments

This confuses me. How should I define a function when I don't care about its return value?

Upvotes: 5

Views: 2382

Answers (2)

Scott Maddox
Scott Maddox

Reputation: 139

The error message is trying to tell you that Result takes two type parameters, i.e. Result<(), ()>. Here's a working version:

trait Handler {
    fn on_message(&mut self, msg: String) -> Result<(), ()> {
        println!("on_message: {}", msg);
        Ok(())
    }
}

Upvotes: 1

Peter Hall
Peter Hall

Reputation: 58735

If you don't need error handling, then there is no need to use a Result: 

fn on_message(&mut self, msg: String) { 
    // ... 
}

If you need to handle errors, but don't have a meaningful "success" value, then you can say that the success type is (), but you still need to say what the error type is:

fn on_message(&mut self, msg: String) -> Result<(), MyError> {
    // ...
}

Where I put MyError, you could use a built-in error like io::Error, a custom error struct or enum, or any other type such as String. It could even be (), making your return type Result<(), ()>, which would let users of your function know if it was successful or not, without providing any information about either the successful outcome or the possible error. At that point, you might consider using an Option<()> or even just a bool, which carry the same amount of information.

Upvotes: 7

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