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42 lines
1.2 KiB
Markdown
42 lines
1.2 KiB
Markdown
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# Dynamic Error Types
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Sometimes we want to allow any type of error to be returned without writing our own enum covering
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all the different possibilities. `std::error::Error` makes this easy.
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```rust,editable,compile_fail
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use std::fs::{self, File};
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use std::io::Read;
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use thiserror::Error;
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use std::error::Error;
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#[derive(Clone, Debug, Eq, Error, PartialEq)]
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#[error("Found no username in {0}")]
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struct EmptyUsernameError(String);
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fn read_username(path: &str) -> Result<String, Box<dyn Error>> {
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let mut username = String::with_capacity(100);
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File::open(path)?.read_to_string(&mut username)?;
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if username.is_empty() {
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return Err(EmptyUsernameError(String::from(path)).into());
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}
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Ok(username)
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}
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fn main() {
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//fs::write("config.dat", "").unwrap();
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match read_username("config.dat") {
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Ok(username) => println!("Username: {username}"),
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Err(err) => println!("Error: {err}"),
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}
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}
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```
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<details>
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This saves on code, but gives up the ability to cleanly handle different error cases differently in
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the program. As such it's generally not a good idea to use `Box<dyn Error>` in the public API of a
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library, but it can be a good option in a program where you just want to display the error message
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somewhere.
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</details>
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