How to get error using if else method in rust? - error-handling

Question

I know the idiomatic way to handle errors in rust is using match statement but I am looking for other ways to do the same.
use std::fs;
fn main() {
if let Ok(f) = fs::read_dir("/dummy"){
println!("First: {:?}", f);
}else{
println!("Error");
}
}
This works but I need the original Result error from fs::read_dir("/dummy") to be printed in the else statement. How can I do it?

Answer 1

Generally, I'd consider such an approach a bad idea, but you do have a few options, the most obvious two being "multiple if lets" and a "functional style" approach. I've included the match version for comparison. The code is available on the playground.
fn multiple_if_lets() {
let f = std::fs::read_dir("/dummy");
if let Ok(f) = &f {
println!("First: {:?}", f);
}
if let Err(f) = &f {
println!("Error: {:?}", f);
}
}
fn functional_style() {
std::fs::read_dir("/dummy")
.map(|f| println!("First: {:?}", f))
.unwrap_or_else(|f| println!("Error: {:?}", f));
}
fn match_style() {
match std::fs::read_dir("/dummy") {
Ok(f) => println!("First: {:?}", f),
Err(f) => println!("Error: {:?}", f),
}
}

Answer 2

I'm not quite sure why you don't want to use match because it is like a better if let! But you might find it useful to use an external crate like anyhow. You can very easily propagate all errors as one type and also add context where it makes sense.
In your Cargo.toml
[dependencies]
anyhow = "1"
In your code
use std::fs;
use anyhow::Context;
fn main() -> anyhow::Result<()> {
let dir = fs::read_dir("/dummy").context("failed to read dir")?;
// another fallible function which can return a `Result` with a
// different error type.
do_something(dir).context("failed to do something")?;
Ok(())
}
If read_dir had to fail here your program would exit and it would output the following
Error: failed to read dir
Caused by:
No such file or directory (os error 2)
If you wanted to throw away the error but still print it out you could still use match.
let dir = match fs::read_dir("/dummy").context("failed to read dir") {
Ok(dir) => Some(dir),
Err(err) => {
eprintln!("Error: {:?}", err);
None
}
};
This would output something like the following but still continue:
Error: failed to read dir
Caused by:
No such file or directory (os error 2)

Answer 3

Since Rust 1.65.0 (Nov. 2022), as mentioned by Mara Bos, you can do:
let Ok(f) = fs::read_dir("/dummy") else{ println!("Error"); return };
println!("First: {:?}", f);
let-else statements.
You can now write things like:
let Ok(a) = i32::from_str("123") else { return };
without needing an if or match statement.
This can be useful to avoid deeply nested if statements.