Error Handling

Rust does not have exceptions. Instead, it uses:

Result<T, E> type for recoverable errors
panic! macro for unrecoverable errors

Panics

We can call panic! when there is no way out of the problem. It terminates the program.

fn main() {
  panic!("fatal error");
}

Call stack

By default, when panic occurs we’ll only see the line in our code that led to the panic. We can see the whole callstack by setting the RUST_BACKTRACE environment variable to anything other than 0.

Recoverable Errors

Our functins might return Result if there is a chance of failure. It’s brough in by the prelude.

enum Result<T, E> {
  Ok(T),
  Err(E),
}

An example of a built-in API that uses Result:

use std::fs::File;

fn main() {
  let f = File::open("hello.txt");

  let f = match f {
    Ok(file) => file,
    Err(error) => panic!("Problem opening the file: {:?}", error),
  };
}

In this case we’re pacic!ing when error occurs.

Methods on Result

Result has some helper methods:

unwrap - returns value inside of Ok, or panic!s if there’s error. It’s a shortcut which can be used instead of match.
expect - like unwrap, but allows to specify error message for the potential panic.
unwrap_or_else - returns value inside of Ok or executes a lambda passed to it in the case of Error.

The ? Operator

The ? placed after a Result value works as follows:

if it’s Ok(value), the value gets returned
if there’s an error, the containing function returns that error

? can convert the error to the expected Error type that a function normally would return (theFrom trait needs to be implemented).

Example:

fn read_username_from_file() -> Result<String, io::Error> {
  let mut f = File::open("hello.txt")?;
  let mut s = String::new();
  f.read_to_string(&mut s)?;
  Ok(s)
}

It could be even shorter with chaining calls:

fn read_username_from_file() -> Result<String, io::Error> {
  let mut s = String::new();
  File::open("hello.txt")?.read_to_string(&mut s)?;
  Ok(s)
}

or even:

fn read_username_from_file() -> Result<String, io::Error> {
  fs::read_to_string("hello.txt")
}

In the case above, ? does not need to convert the error since all the expected errors would be of type io:Error - the same type that the function returns.

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Generics →