handling errors in rust

content/posts/2022.04.13.handling-errors-in-rust.md

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+---
+title: "Handling Errors in Rust"
+date: 2022-04-13T15:31:11-04:00
+toc: false
+images:
+tags:
+  - dev
+  - rust
+---
+
+Rust uses a pretty interesting way to deal with errors. Languages like Python
+and JavaScript allow you to throw errors completely unchecked. C does error
+handling through the `errno` which is also completely unchecked and not enforced
+in any way by the programming language. This means any function you call may
+throw an error, and other than documentation there's nothing to let you know
+that it might do so.
+
+Worse, most of these languages don't tell you what kind of error a function
+might throw. That's obvious in JavaScript, and TypeScript doesn't help with it
+either (errors caught have `any` or `unknown` type). C++ and Python let you
+catch specific types of errors, but you have to rely on documentation to know
+which types those are.
+
+```typescript
+try {
+  // ...
+} catch (err: any) {
+  // Is it a file error? Did I access something undefined? Who knows.
+```
+
+Java, on the other hand, requires you to explicitly mark what errors a function
+can throw and enforces that you handle all the errors or explicitly mark that
+you are propagating it.
+
+```java
+public class Main {
+  static void example() throws ArithmeticException;
+}
+```
+
+Rust is a lot closer to this as it enforces that you handle errors. The main
+difference is that instead of using a special syntax for error handling, it's
+built into the return type of the function directly. Which is why you'll see
+functions like this:
+
+```rust
+fn example() -> Result<String, io::Error>;
+```
+
+This sometimes makes error handling a little harder, but luckily we have many
+tools that can help us handle errors.
+
+## When you don't care about the error
+
+Sometimes you just don't care about the error. Perhaps the error is impossible
+to recover from and the best you can do is print an error message and exit, or
+how you handle it is the same regardless of what the error is.
+
+### To just exit
+
+Two great options in this case is [die](https://crates.io/crates/die) and
+[tracing-unwrap](https://crates.io/crates/tracing-unwrap). Both of these options
+allow you to unwrap a `Result` type, and print a message if it's an `Error` and
+exit. `die` allows you to pick the error code to exit with, while
+`tracing-unwrap` uses the [tracing](https://crates.io/crates/tracing) logging
+framework. You can also always use the built-in [unwrap or expect](https://learning-rust.github.io/docs/e4.unwrap_and_expect.html) functions.
+
+```rust
+// die
+let output = example().die_code("some error happened", 12);
+// tracing-unwrap
+let output = example().unwrap_or_log()
+```
+
+If you are writing a function that might return any type of error, then
+[anyhow](https://crates.io/crates/anyhow) is your best option.
+
+```rust
+fn main() -> anyhow::Result<()> {
+  let output = example()?;
+}
+```
+
+## When you do care about the error
+
+If you do care about what type of error you return, then you need
+[thiserror](https://crates.io/crates/thiserror). `thiserror` allows you to write
+your own error types and propagate errors.
+
+```rust
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ExampleError {
+    #[error("The error message for this type")]
+    Simple(String),
+    #[error("An error that you are propagating")]
+    FileError(#[from] io::Error),
+}
+```