Why do try!() and ? not compile when used in a function that doesn't return Option or Result?

Question

Why does this code not compile?

use std::{fs, path::Path};

fn main() {
    let dir = Path::new("../FileSystem");

    if !dir.is_dir() {
        println!("Is not a directory");
        return;
    }

    for item in try!(fs::read_dir(dir)) {
        let file = match item {
            Err(e) => {
                println!("Error: {}", e);
                return;
            }
            Ok(f) => f,
        };

        println!("");
    }

    println!("Done");
}

This is the error I get

error[E0308]: mismatched types
  --> src/main.rs:11:17
   |
11 |     for item in try!(fs::read_dir(dir)) {
   |                 ^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
   |
   = note: expected type `()`
              found type `std::result::Result<_, _>`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

I also tried the question mark operator:

for item in fs::read_dir(dir)? {

Which had a different error:

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:11:17
   |
11 |     for item in fs::read_dir(dir)? {
   |                 ^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

Previous versions of Rust had a similar error about std::ops::Carrier

Should I avoid try!() and ?? What is the best way to handle errors? Mostly I do it like this:

match error_prone {
    Err(e) => {
        println!("Error: {}", e);
        return;
    },
    Ok(f) => f,
};

But if I have to use that in a for loop, it's a complete mess

for i in match error_prone {
    // match code
} {
    // loop code
}

try! is a macro that returns Errs automatically. You can't use it in main because main doesn't return anything. — Veedrac
Aaaahn that explains a lot.. Thank you for that! And how would I handle the errors in this specific case? Without having to resort to the weird for match syntax mentioned above — Mathieu David
See also. In general the lazy route is to stick your code in some wrapper function that you call from main that returns Result<T, Box<Error>> and then unwrap or handle that in main. — Veedrac
println!("{}", path.unwrap().path().file_name().unwrap().to_str().unwrap()); really? :p — Mathieu David

Veedrac Veedrac · Accepted Answer · 2015-05-31T10:04:29

try! is a macro that returns Errs automatically; ? is syntax that does mostly the same thing, but it works with any type that implements the Try trait.

As of Rust 1.22.0, Option implements Try, so it can be used with ?. Before that, ? could only be used in functions that return a Result. try! continues to only work with Results.

As of Rust 1.26.0, main is allowed to return a value that implements Termination. Before that, it doesn't return any value.

As of Rust 1.26.0

Your original code works if you mark main as returning a Result and then return Ok(()) in all the "success" cases:

use std::{fs, io, path::Path};

fn main() -> Result<(), io::Error> {
    let dir = Path::new("../FileSystem");

    if !dir.is_dir() {
        println!("Is not a directory");
        return Ok(());
    }

    for item in fs::read_dir(dir)? {
        let file = match item {
            Err(e) => {
                println!("Error: {}", e);
                return Ok(());
            }
            Ok(f) => f,
        };

        println!("");
    }

    println!("Done");
    Ok(())
}

Before that

This is how you might transform your code to use ?:

use std::{error::Error, fs, path::Path};

fn print_dir_contents() -> Result<String, Box<Error>> {
    let dir = Path::new("../FileSystem");

    if !dir.is_dir() {
        return Err(Box::from("Is not a directory!"));
    }

    for entry in fs::read_dir(dir)? {
        let path = entry?.path();
        let file_name = path.file_name().unwrap();
        println!("{}", file_name.to_string_lossy());
    }

    Ok("Done".into())
}

fn main() {
    match print_dir_contents() {
        Ok(s) => println!("{}", s),
        Err(e) => println!("Error: {}", e.to_string()),
    }
}

There's a lot of error handling here that you might not expect - other languages don't tend to require it! But they exist in other languages - Rust just makes you know it. Here are the errors:

entry?

IO errors can happen during iteration.

path.file_name().unwrap()

Not all paths have file names. We can unwrap this because read_dir won't give us a path without a file name.

file_name.to_string_lossy()

You can also to_str and throw an error, but it's nicer to do this. This error exists because not all file names are valid Unicode.

try! and ? throw errors into the return value, converting them to Box::Error. It's actually more reasonable to return an amalgamated error of all the things that can go wrong. Luckily io::Error is just the right type:

use std::io;

// ...

fn print_dir_contents() -> Result<String, io::Error> {
    // ...

    if !dir.is_dir() {
        return Err(io::Error::new(io::ErrorKind::Other, "Is not a directory!"));
    }

    // ...
}

Frankly, though, this check is already in fs::read_dir, so you can actually just remove the if !dis.is_dir altogether:

use std::{fs, io, path::Path};

fn print_dir_contents() -> Result<String, io::Error> {
    let dir = Path::new("../FileSystem");

    for entry in fs::read_dir(dir)? {
        let path = entry?.path();
        let file_name = path.file_name().unwrap();
        println!("{}", file_name.to_string_lossy());
    }

    Ok("Done".into())
}

fn main() {
    match print_dir_contents() {
        Ok(s) => println!("{}", s),
        Err(e) => println!("Error: {}", e.to_string()),
    }
}

Why do try!() and ? not compile when used in a function that doesn't return Option or Result?

4 Answers

As of Rust 1.26.0

Before that