<h2>ch9. Error Handling</h2> - HackMD

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width: 34px !important; height: 21px !important; border: none !important; float: right; margin-top: -7px; margin-right: -10px; } .my-icon-css { width: 100%; height: 200px; background-position:center; background-repeat: no-repeat; background-image:url('https://mir-s3-cdn-cf.behance.net/project_modules/disp/fe36cc42774743.57ee5f329fae6.gif'); } </style>  # <h2>ch9. Error Handling</h2> <div class="my-icon-css"></div> @Kais(VagrantPi) ###### tags: `Rust`, `簡報`, `Rust 讀書會` ---  ## Agenda - Unrecoverable Errors with panic! - Recoverable Errors with Result - To panic! or Not To panic! ---  ## Unrecoverable Errors with panic! ----  ### Rust 中的錯誤 - 可恢復錯誤（recoverable）  - 不可恢復錯誤（unrecoverable）  ----  ### Panic 又可分為兩種模式： - 展開（unwinding）預設值  - 終止（abort）  *Cargo.toml 的 [profile] 部分增加 panic = ‘abort’* ----  ```javascript= fn main() { panic!("crash and burn"); } ``` ``` $ cargo run Compiling rust v0.1.0 (file:///Users/kais.lin/rust) Finished dev [unoptimized + debuginfo] target(s) in 3.42 secs Running `target/debug/rust` thread 'main' panicked at 'crash and burn', main.rs:2:5 note: Run with `RUST_BACKTRACE=1` for a backtrace. ```  ----  ### 使用 panic! 的 backtrace ```javascript= fn main() { let v = vec![1, 2, 3]; v[99]; } ``` ``` $ cargo run Compiling rust v0.1.0 (file:///Users/kais.lin/rust) Finished dev [unoptimized + debuginfo] target(s) in 0.34 secs Running `target/debug/rust` thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', /Users/travis/build/rust-lang/rust/src/libcore/slice/mod.rs:865:10 note: Run with `RUST_BACKTRACE=1` for a backtrace. ``` ----  ``` $ RUST_BACKTRACE=1 cargo run Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs Running `target/debug/rust` thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', /Users/travis/build/rust-lang/rust/src/libcore/slice/mod.rs:865:10 stack backtrace: 0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49 1: std::sys_common::backtrace::print at libstd/sys_common/backtrace.rs:71 at libstd/sys_common/backtrace.rs:59 2: std::panicking::default_hook::{{closure}} at libstd/panicking.rs:207 3: std::panicking::default_hook at libstd/panicking.rs:223 4: std::panicking::begin_panic at libstd/panicking.rs:402 5: std::panicking::try::do_call at libstd/panicking.rs:349 6: std::panicking::try::do_call at libstd/panicking.rs:325 7: core::ptr::drop_in_place at libcore/panicking.rs:72 8: core::ptr::drop_in_place at libcore/panicking.rs:58 9: <usize as core::slice::SliceIndex<[T]>>::index at /Users/travis/build/rust-lang/rust/src/libcore/slice/mod.rs:865 10: core::slice::<impl core::ops::index::Index<I> for [T]>::index at /Users/travis/build/rust-lang/rust/src/libcore/slice/mod.rs:767 11: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index at /Users/travis/build/rust-lang/rust/src/liballoc/vec.rs:1648 12: rust::main at ./main.rs:4 13: std::rt::lang_start::{{closure}} at /Users/travis/build/rust-lang/rust/src/libstd/rt.rs:74 14: std::panicking::try::do_call at libstd/rt.rs:59 at libstd/panicking.rs:306 15: panic_unwind::dwarf::eh::read_encoded_pointer at libpanic_unwind/lib.rs:102 16: <std::io::Write::write_fmt::Adaptor<'a, T> as core::fmt::Write>::write_str at libstd/panicking.rs:285 at libstd/panic.rs:361 at libstd/rt.rs:58 17: std::rt::lang_start at /Users/travis/build/rust-lang/rust/src/libstd/rt.rs:74 18: rust::main ``` ``` 12: rust::main at ./main.rs:4 ``` ---  ## Recoverable Errors with Result ----  ### 面對它使用 `Result` 類型來處理潛在錯誤 ```javascript enum Result<T, E> { Ok(T), Err(E), } ``` 如何知道某某函數會回傳 `Rusult` 呢？除文件之外，書中也提到了另一個方法 ----  ### 執行它 ```javascript= use std::fs::File; fn main() { let f = File::open("hello.txt"); } ``` ``` error[E0308]: mismatched types --> src/main.rs:4:18 | 4 | let f: u32 = File::open("hello.txt"); | ^^^^^^^^^^^^^^^^^^^^^^^ expected u32, found enum `std::result::Result` | = note: expected type `u32` found type `std::result::Result<std::fs::File, std::io::Error>` ``` ----  ### 處理它 ```javascript= use std::fs::File; fn main() { let f = File::open("hello.txt"); let f = match f { Ok(file) => file, Err(error) => { panic!("There was a problem opening the file: {:?}", error) }, }; } ``` ----  ```javascript= use std::fs::File; use std::io::ErrorKind; fn main() { let f = File::open("hello.txt"); let f = match f { Ok(file) => file, Err(ref error) if error.kind() == ErrorKind::NotFound => { match File::create("hello.txt") { Ok(fc) => fc, Err(e) => { panic!( "Tried to create file but there was a problem: {:?}", e ) }, } }, Err(error) => { panic!( "There was a problem opening the file: {:?}", error ) }, }; } ``` if error.kind() == ErrorKind::NotFound 被稱為 `match guard`，用來更完善 match 的而外條件判斷 ----  ### 失敗時 panic 的簡寫：unwrap ```javascript= use std::fs::File; fn main() { let f = File::open("hello.txt").unwrap(); } ``` ``` thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "No such file or directory" } }', src/libcore/result.rs:906:4 ``` ----  ### 失敗時 panic 的簡寫：expect ```javascript= use std::fs::File; fn main() { let f = File::open("hello.txt").expect("Failed to open hello.txt"); } ``` ``` thread 'main' panicked at 'Failed to open hello.txt: Error { repr: Os { code: 2, message: "No such file or directory" } }', src/libcore/result.rs:906:4 ``` ----  ### 傳播(propagating)錯誤簡單說就是 function 會回傳 error，這樣可以讓呼叫他的地方能夠判斷要不要做錯誤處裡 ```javascript= use std::io; use std::io::Read; use std::fs::File; fn read_username_from_file() -> Result<String, io::Error> { let f = File::open("hello.txt"); let mut f = match f { Ok(file) => file, Err(e) => return Err(e), }; let mut s = String::new(); match f.read_to_string(&mut s) { Ok(_) => Ok(s), Err(e) => Err(e), } } ``` ----  ### 傳播錯誤的簡寫: ? ```javascript= use std::io; use std::io::Read; use std::fs::File; fn read_username_from_file() -> Result<String, io::Error> { let mut f = File::open("hello.txt")?; let mut s = String::new(); f.read_to_string(&mut s)?; Ok(s) } ``` ----  ### 與 match 的差別原文理解不能，以下是我個人理解，請大大們勘誤 `?` 會將錯誤值丟給 from 函數(定義於 From trait 中)，可以將 Result 回傳的錯誤型別，轉換成該 fn 回傳的錯誤型別 ----  ```javascript= use std::io::{self, Read}; use std::num; enum CliError { IoError(io::Error), ParseError(num::ParseIntError), } impl From<io::Error> for CliError { fn from(error: io::Error) -> Self { CliError::IoError(error) } } impl From<num::ParseIntError> for CliError { fn from(error: num::ParseIntError) -> Self { CliError::ParseError(error) } } fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> { let mut file = std::fs::File::open("test")?; let mut contents = String::new(); file.read_to_string(&mut contents)?; let num: i32 = contents.trim().parse()?; Ok(num) } ``` > [trait.From](https://doc.rust-lang.org/std/convert/trait.From.html) ----  ### ? 只能被用于返回 Result 的函数 ```javascript= use std::fs::File; fn main() { let f = File::open("hello.txt")?; } ``` ``` error[E0277]: the trait bound `(): std::ops::Try` is not satisfied --> src/main.rs:4:13 | 4 | let f = File::open("hello.txt")?; | ------------------------ | | | the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`) | in this macro invocation | = help: the trait `std::ops::Try` is not implemented for `()` = note: required by `std::ops::Try::from_error` ``` ---  ## To panic! or Not To panic! ----  ### Examples, Prototype Code, and Tests 有時候只是要快速的驗證或者是 demo，可以先 unwrap 之類的方法，未來再來實現這部廢的錯誤處理 ----  ### 當你比編譯器知道更多情況 ```javascript= use std::net::IpAddr; let home: IpAddr = "127.0.0.1".parse().unwrap(); ``` 即便是寫死的，而且確定是可分析的 ip，編譯器仍會覺得還是可能會出現 Err 因此要我們處理 Result，這邊種情況下使用 unwrap 是合適的 ----  ### 當輸入值錯誤時預期外的傳入值，可能導致存取非當前資料的記憶體空間，而出現安全性問題不過透過 Rust 的 type system 就會為你做很多檢查了(EX: u32 接到的值不會有負的)  ----  ### 建立自訂義的型別驗證 v0.0.1 ```javascript= loop { // --snip-- let guess: i32 = match guess.trim().parse() { Ok(num) => num, Err(_) => continue, }; if guess < 1 || guess > 100 { println!("The secret number will be between 1 and 100."); continue; } match guess.cmp(&secret_number) { // --snip-- } ``` ----  不過這樣但這樣每次都要進一次判斷式，如果很多的函式都需要這樣的檢查，這樣寫就恨很冗餘(也許也會小小的引響到效能) ----  ```javascript= pub struct Guess { value: u32, } impl Guess { pub fn new(value: u32) -> Guess { if value < 1 || value > 100 { panic!("Guess value must be between 1 and 100, got {}.", value); } Guess { value } } pub fn value(&self) -> u32 { self.value } } ``` ----  v0.0.2 ```diff= extern crate rand; use std::io; use std::cmp::Ordering; use rand::Rng; fn main() { println!("Guess the number!"); let secret_number = rand::thread_rng().gen_range(1, 101); loop { println!("Please input your guess."); let mut guess = String::new(); io::stdin().read_line(&mut guess) .expect("Failed to read line"); let guess: u32 = match guess.trim().parse() { - Ok(num) => num, + Ok(num) => Guess::new(num).value(), Err(_) => continue, }; println!("You guessed: {}", guess); match guess.cmp(&secret_number) { Ordering::Less => println!("Too small!"), Ordering::Greater => println!("Too big!"), Ordering::Equal => { println!("You win!"); break; } } } } + pub struct Guess { + value: u32, + } + + impl Guess { + pub fn new(value: u32) -> Guess { + if value < 1 || value > 100 { + panic!("Guess value must be between 1 and +100, got {}.", value); + } + + Guess { + value + } + } + + pub fn value(&self) -> u32 { + self.value + } + } ``` ---  ## Thanks for listening ~

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- [ ] Todo List	Todo List
> Blockquote	Blockquote
Bold font	Bold font
Italics font	Italics font
~~Strikethrough~~	~~Strikethrough~~
19^th^	19^th
H~2~O	H₂O
++Inserted text++	Inserted text
==Marked text==	Marked text
[link text](https:// "title")	Link
![image alt](https:// "title")	Image
`Code`	`Code`	在筆記中貼入程式碼
```javascript var i = 0; ```	`var i = 0;`	在筆記中貼入程式碼
:smile:		Emoji list
{%youtube youtube_id %}	Externals
$L^aT_eX$	L^aT_eX
:::info This is a alert area. :::	This is a alert area.