How to manually return a Result<(), Box<dyn Error>>?

Question

I want to return an error from a function in case a condition is true:

use std::error::Error;  pub fn run() -> Result<(), Box<dyn Error>> {     // -- snip ---      if condition {         // return error     }      // -- snip --      Ok(()) }  fn main() {} 

I probably don't have the basics of the typesystem down, but everywhere I looked people use the ? operator, so I can't figure out what type to return.

1. Is it possible to just return an error like this?
2. Is there a better way to handle this logic?

Answer 1

Error is a trait and you want to return a trait object (note the dyn keyword), so you need to implement this trait:

use std::error::Error; use std::fmt;  #[derive(Debug)] struct MyError(String);  impl fmt::Display for MyError {     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {         write!(f, "There is an error: {}", self.0)     } }  impl Error for MyError {}  pub fn run() -> Result<(), Box<dyn Error>> {     let condition = true;      if condition {         return Err(Box::new(MyError("Oops".into())));     }      Ok(()) }  fn main() {     if let Err(e) = run() {         println!("{}", e); // "There is an error: Oops"     } } 

• Create your own error type,
• Implement Debug, Display, then Error for it,
• If there is an error, return the Err variant of Result.

I advise you to use failure that remove all the error boilerplate:

#[derive(Fail, Debug)] #[fail(display = "There is an error: {}.", _0)] struct MyError(String); 

--

Note that if you expect an Error, you can return whatever type you want, given that it implements Error. This includes the error types in std.

Answer 2

I am new to Rust, but here is my dirty hack to return custom errors, given that the function is set to return Result<(), Box<dyn Error>>:

fn serve(config: &Config, stream: TcpStream) -> Result<(), Box<dyn Error>> {     // ...     if request_is_bad() {         // This returns immediately a custom "Bad request" error         Err("Bad request")?;     }     // ... }