Strange error: cannot use the `?` operator in a function that returns `()`

Question

I have tried to reproduce the problem in a very tiny program (you can find it here Rust REPL)

#[macro_use]
extern crate quick_error;

quick_error! {
    #[derive(Debug)]
    pub enum Error {
        SomeError{
            description("SomeError")
        }
    }
}


pub struct Version {
    foo: u8,
}

pub struct Bar();

impl Bar {
    pub fn version() -> Result<Version, Error> {
        Ok(Version{foo: 1})
    }
}

fn main() {
    let tmp = Bar::version()?;
}

when trying to compile, I get the following:

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:27:15
   |
27 |     let tmp = Bar::version()?;
   |               ^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`

HOWEVER, version is returning Result<Version, Error>. What is going on?

`version` isn't the function you're trying to use `?` in. – trent Sep 07 '18 at 15:21 — trent, Sep 07 '18 at 15:21
`main` returns `()` which is where the error is. – Lee Sep 07 '18 at 15:22 — Lee, Sep 07 '18 at 15:22
OMG!! :) silly me. Thanks! – Juan Leni Sep 07 '18 at 15:22 — Juan Leni, Sep 07 '18 at 15:22

score 25 · Accepted Answer · edited Feb 06 '22 at 20:13

You misunderstand the function that the operator is talking about:

cannot use the ? operator in a function that returns ()

Is about the function in which the expression appears and not the expression on which ? is applied:

the function in which the expression appears is main.
the expression to which ? is applied is Bar::version()?.

Therefore, the compiler is complaining that you cannot use ? in main because the return type of main is ().

You can use -> Result<(), Box<dyn Error>> as a return type for main:

use std::error::Error;

fn main() -> Result<(), Box<dyn Error>> {
    let tmp = Bar::version()?;
    Ok(())
}

Thanks! yes, I have just realized I was misreading the error message :) haha! — Juan Leni, Sep 07 '18 at 15:29

Strange error: cannot use the `?` operator in a function that returns `()`

1 Answers1

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