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* Update error-contexts.md Adding more speaker notes about `anyhow`. * Add markup Co-authored-by: Martin Geisler <martin@geisler.net>
48 lines
1.4 KiB
Markdown
48 lines
1.4 KiB
Markdown
# Adding Context to Errors
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The widely used [anyhow](https://docs.rs/anyhow/) crate can help you add
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contextual information to your errors and allows you to have fewer
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custom error types:
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```rust,editable,compile_fail
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use std::{fs, io};
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use std::io::Read;
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use thiserror::Error;
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use anyhow::{Context, Result};
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#[derive(Error, Debug)]
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enum ReadUsernameError {
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#[error("Found no username in {0}")]
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EmptyUsername(String),
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}
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fn read_username(path: &str) -> Result<String> {
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let mut username = String::with_capacity(100);
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fs::File::open(path)
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.context(format!("Failed to open {path}"))?
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.read_to_string(&mut username)
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.context("Failed to read")?;
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if username.is_empty() {
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return Err(ReadUsernameError::EmptyUsername(String::from(path)))?;
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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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* `anyhow::Result<T>` is generic and it can hold any `Error` implementation without changing the type signature.
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* Actual error type inside of it can be extracted for examination if necessary.
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* Functionality provided by `anyhow::Result<T>` may be familiar to Go developers, as it provides similar usage patterns and ergonomics
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of `(T, error)` from Go.
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</details>
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