unwrap extracts the successful value from a Result. For example, it turns Ok(File) into File; when the Result contains Err(String), it crashes the program instead of handling the error. section 3.4.2
Unwrapping a numeric conversion
A conversion returns a Result, so unwrap makes its successful value available for comparison: let b_ = b.try_into().unwrap();. If conversion fails, the program panics. section 2.3.3
Use unwrap with care:
- Use it in small programs or main functions when an immediate crash is acceptable. section 8.5.3
- Avoid relying on it in library code when errors may result from conditions outside the caller’s control. section 8.5.3
- Prefer more sophisticated error handling when program stability matters. section 3.4.2 section 2.3.3
Listing 3.8 Using Result to mark functions liable to filesystem errors
use rand::prelude::*; #1
fn one_in(denominator: u32) -> bool { #2
thread_rng().gen_ratio(1, denominator) #3
}
#[derive(Debug)]
struct File {
name: String,
data: Vec<u8>,
}
impl File {
fn new(name: &str) -> File {
File {
name: String::from(name),
data: Vec::new()
} #4
}
fn new_with_data(name: &str, data: &Vec<u8>) -> File {
let mut f = File::new(name);
f.data = data.clone();
f
}
fn read(
self: &File,
save_to: &mut Vec<u8>,
) -> Result<usize, String> { #5
let mut tmp = self.data.clone();
let read_length = tmp.len();
save_to.reserve(read_length);
save_to.append(&mut tmp);
Ok(read_length) #6
}
}
fn open(f: File) -> Result<File, String> {
if one_in(10_000) { #7
let err_msg = String::from("Permission denied");
return Err(err_msg);
}
Ok(f)
}
fn close(f: File) -> Result<File, String> {
if one_in(100_000) { #8
let err_msg = String::from("Interrupted by signal!");
return Err(err_msg);
}
Ok(f)
}
fn main() {
let f4_data: Vec<u8> = vec![114, 117, 115, 116, 33];
let mut f4 = File::new_with_data("4.txt", &f4_data);
let mut buffer: Vec<u8> = vec![];
f4 = open(f4).unwrap(); #9
let f4_length = f4.read(&mut buffer).unwrap(); #9
f4 = close(f4).unwrap(); #9
let text = String::from_utf8_lossy(&buffer);
println!("{:?}", f4);
println!("{} is {} bytes long", &f4.name, f4_length);
println!("{}", text);
}
See unwrap applied to filesystem operations whose functions return Result values. section 3.4.2
