$ cargo run
+ Compiling panic v0.1.0 (file:///projects/panic)
+ Finished debug [unoptimized + debuginfo] target(s) in 0.27 secs
+ Running `target/debug/panic`
+thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
+100', /stable-dist-rustc/build/src/libcollections/vec.rs:1362
+note: Run with `RUST_BACKTRACE=1` for a backtrace.
+error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
+
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)
+ },
+ };
+}
+
+
+
Listing 9-3: Using a match expression to handle the Result variants we
+might have
thread 'main' panicked at 'There was a problem opening the file: Error { repr:
+Os { code: 2, message: "No such file or directory" } }', src/main.rs:8
+
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
+ )
+ },
+ };
+}
+
+
+
Listing 9-4: Handling different kinds of errors in different ways
if error.kind() == ErrorKind::NotFound条件被称作 match guard：它是一个进一步完善match分支模式的额外的条件。这个条件必须为真才能使分支的代码被执行；否则，模式匹配会继续并考虑match中的下一个分支。模式中的ref是必须的，这样error就不会被移动到 guard 条件中而只是仅仅引用它。第十八章会详细解释为什么在模式中使用ref而不是&来获取一个引用。简而言之，在模式的上下文中，&匹配一个引用并返回它的值，而ref匹配一个值并返回一个引用。
+
在 match guard 中我们想要检查的条件是error.kind()是否是ErrorKind枚举的NotFound成员。如果是，尝试用File::create创建文件。然而File::create也可能会失败，我们还需要增加一个内部match语句。当文件不能被打开，会打印出一个不同的错误信息。外部match的最后一个分支保持不变这样对任何除了文件不存在的错误会使程序 panic。
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error {
+repr: Os { code: 2, message: "No such file or directory" } }',
+/stable-dist-rustc/build/src/libcore/result.rs:868
+
thread 'main' panicked at 'Failed to open hello.txt: Error { repr: Os { code:
+2, message: "No such file or directory" } }',
+/stable-dist-rustc/build/src/libcore/result.rs:868
+
use std::net::IpAddr;
+
+let home = "127.0.0.1".parse::<IpAddr>().unwrap();
+
+
我们通过解析一个硬编码的字符来创建一个IpAddr实例。可以看出127.0.0.1是一个有效的 IP 地址，所以这里使用unwrap是没有问题的。然而，拥有一个硬编码的有效的字符串也不能改变parse方法的返回值类型：它仍然是一个Result值，而编译器仍然就好像还是有可能出现Err成员那样要求我们处理Result，因为编译器还没有智能到可以识别出这个字符串总是一个有效的 IP 地址。如果 IP 地址字符串来源于用户而不是硬编码进程序中的话，那么就确实有失败的可能性，这时就绝对需要我们以一种更健壮的方式处理Result了。
fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let mut largest = numbers[0];
+
+ for number in numbers {
+ if number > largest {
+ largest = number;
+ }
+ }
+
+ println!("The largest number is {}", largest);
+# assert_eq!(largest, 100);
+}
+
+
+
Listing 10-1: Code to find the largest number in a list of numbers
fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let mut largest = numbers[0];
+
+ for number in numbers {
+ if number > largest {
+ largest = number;
+ }
+ }
+
+ println!("The largest number is {}", largest);
+
+ let numbers = vec![102, 34, 6000, 89, 54, 2, 43, 8];
+
+ let mut largest = numbers[0];
+
+ for number in numbers {
+ if number > largest {
+ largest = number;
+ }
+ }
+
+ println!("The largest number is {}", largest);
+}
+
+
+
Listing 10-2: Code to find the largest number in two lists of numbers
fn largest(list: &[i32]) -> i32 {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let result = largest(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 100);
+
+ let numbers = vec![102, 34, 6000, 89, 54, 2, 43, 8];
+
+ let result = largest(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 6000);
+}
+
+
+
Listing 10-3: Abstracted code to find the largest number in two lists
fn largest_i32(list: &[i32]) -> i32 {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn largest_char(list: &[char]) -> char {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let result = largest_i32(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 100);
+
+ let chars = vec!['y', 'm', 'a', 'q'];
+
+ let result = largest_char(&chars);
+ println!("The largest char is {}", result);
+# assert_eq!(result, 'y');
+}
+
+
+
Listing 10-4: Two functions that differ only in their names and the types in
+their signatures
$ cargo run
+ Compiling panic v0.1.0 (file:///projects/panic)
+ Finished debug [unoptimized + debuginfo] target(s) in 0.27 secs
+ Running `target/debug/panic`
+thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
+100', /stable-dist-rustc/build/src/libcollections/vec.rs:1362
+note: Run with `RUST_BACKTRACE=1` for a backtrace.
+error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
+
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)
+ },
+ };
+}
+
+
+
Listing 9-3: Using a match expression to handle the Result variants we
+might have
thread 'main' panicked at 'There was a problem opening the file: Error { repr:
+Os { code: 2, message: "No such file or directory" } }', src/main.rs:8
+
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
+ )
+ },
+ };
+}
+
+
+
Listing 9-4: Handling different kinds of errors in different ways
if error.kind() == ErrorKind::NotFound条件被称作 match guard：它是一个进一步完善match分支模式的额外的条件。这个条件必须为真才能使分支的代码被执行；否则，模式匹配会继续并考虑match中的下一个分支。模式中的ref是必须的，这样error就不会被移动到 guard 条件中而只是仅仅引用它。第十八章会详细解释为什么在模式中使用ref而不是&来获取一个引用。简而言之，在模式的上下文中，&匹配一个引用并返回它的值，而ref匹配一个值并返回一个引用。
+
在 match guard 中我们想要检查的条件是error.kind()是否是ErrorKind枚举的NotFound成员。如果是，尝试用File::create创建文件。然而File::create也可能会失败，我们还需要增加一个内部match语句。当文件不能被打开，会打印出一个不同的错误信息。外部match的最后一个分支保持不变这样对任何除了文件不存在的错误会使程序 panic。
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error {
+repr: Os { code: 2, message: "No such file or directory" } }',
+/stable-dist-rustc/build/src/libcore/result.rs:868
+
thread 'main' panicked at 'Failed to open hello.txt: Error { repr: Os { code:
+2, message: "No such file or directory" } }',
+/stable-dist-rustc/build/src/libcore/result.rs:868
+
fn largest(list: &[i32]) -> i32 {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let result = largest(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 100);
+
+ let numbers = vec![102, 34, 6000, 89, 54, 2, 43, 8];
+
+ let result = largest(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 6000);
+}
+
+
+
Listing 10-3: Abstracted code to find the largest number in two lists
fn largest_i32(list: &[i32]) -> i32 {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn largest_char(list: &[char]) -> char {
+ let mut largest = list[0];
+
+ for &item in list.iter() {
+ if item > largest {
+ largest = item;
+ }
+ }
+
+ largest
+}
+
+fn main() {
+ let numbers = vec![34, 50, 25, 100, 65];
+
+ let result = largest_i32(&numbers);
+ println!("The largest number is {}", result);
+# assert_eq!(result, 100);
+
+ let chars = vec!['y', 'm', 'a', 'q'];
+
+ let result = largest_char(&chars);
+ println!("The largest char is {}", result);
+# assert_eq!(result, 'y');
+}
+
+
+
Listing 10-4: Two functions that differ only in their names and the types in
+their signatures