Result Type For Error Handling

The Result type is used to return values. It can contain either and Ok or an Err. The basic signature is:

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

The T stands for type, and E stands for error. Those are generic types which means that we can put any type in the slot. The first T and E in Result<T, E> setup the result of enum to be able to use them (TODO: write up generic types before this)

The Ok(T) turns into whatever type the process that made it needs to send.

For example, the code below is used to get the string value from an Environmental Variable. But, the string doesn't come in directly. Instead the env::var returns a Result that contains either Ok(String) or Err(VarError).

So, to get to the string we have to examine the Result with match to see if it contains Ok or Err

Here's an example

#![allow(unused)]
fn main() {
use std::env;

let key = "HOMEx";
let returnValueAsResult = env::var(key);

match returnValueAsResult {
    Ok(value) => {
      println!("{} is {}", key, value);
    }
    Err(error) => {
      println!("{} - {}", key, error);
    }
}
}

#![allow(unused)]
fn main() {
use std::env;

let key = "NON_EXISTANT_KEY";
match env::var(key) {
    Ok(val) => println!("{key}: {val:?}"),
    Err(e) => println!("couldn't interpret {key}: {e}"),
}
}

Typing Rust

Result Type For Error Handling