mirror of
https://github.com/rust-lang-cn/book-cn.git
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Update codes
This commit is contained in:
Aaran Xu
parent
f8f8119884
commit
0b2c9ff951
@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,5 +1,5 @@
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// ANCHOR: here
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#[derive(Debug)] // so we can inspect the state in a minute
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#[derive(Debug)] // 这样可以立刻看到州的名称
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enum UsState {
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Alabama,
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Alaska,
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,8 +1,8 @@
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fn main() {
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// ANCHOR: here
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let some_u8_value = Some(0u8);
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match some_u8_value {
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Some(3) => println!("three"),
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let config_max = Some(3u8);
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match config_max {
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Some(max) => println!("The maximum is configured to be {}", max),
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_ => (),
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}
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// ANCHOR_END: here
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -8,9 +8,5 @@ error[E0277]: cannot add `Option<i8>` to `i8`
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= help: the trait `Add<Option<i8>>` is not implemented for `i8`
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error: aborting due to previous error
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For more information about this error, try `rustc --explain E0277`.
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error: could not compile `enums`
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To learn more, run the command again with --verbose.
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error: could not compile `enums` due to previous error
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -9,9 +9,5 @@ error[E0004]: non-exhaustive patterns: `None` not covered
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= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
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= note: the matched value is of type `Option<i32>`
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error: aborting due to previous error
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For more information about this error, try `rustc --explain E0004`.
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error: could not compile `enums`
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To learn more, run the command again with --verbose.
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error: could not compile `enums` due to previous error
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@ -1,12 +0,0 @@
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fn main() {
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// ANCHOR: here
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let some_u8_value = 0u8;
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match some_u8_value {
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1 => println!("one"),
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3 => println!("three"),
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5 => println!("five"),
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7 => println!("seven"),
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_ => (),
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}
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// ANCHOR_END: here
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}
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,8 +1,8 @@
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fn main() {
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// ANCHOR: here
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let some_u8_value = Some(0u8);
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if let Some(3) = some_u8_value {
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println!("three");
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let config_max = Some(3u8);
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if let Some(max) = config_max {
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println!("The maximum is configured to be {}", max);
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}
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// ANCHOR_END: here
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}
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -1,7 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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authors = ["Your Name <you@example.com>"]
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edition = "2018"
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edition = "2021"
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[dependencies]
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@ -0,0 +1,14 @@
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fn main() {
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// ANCHOR: here
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let dice_roll = 9;
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match dice_roll {
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3 => add_fancy_hat(),
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7 => remove_fancy_hat(),
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other => move_player(other),
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}
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fn add_fancy_hat() {}
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fn remove_fancy_hat() {}
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fn move_player(num_spaces: u8) {}
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// ANCHOR_END: here
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}
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6
listings/ch06-enums-and-pattern-matching/no-listing-16-underscore-catchall/Cargo.lock
generated
Normal file
6
listings/ch06-enums-and-pattern-matching/no-listing-16-underscore-catchall/Cargo.lock
generated
Normal file
@ -0,0 +1,6 @@
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# This file is automatically @generated by Cargo.
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# It is not intended for manual editing.
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[[package]]
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name = "enums"
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version = "0.1.0"
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@ -0,0 +1,6 @@
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[package]
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name = "enums"
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version = "0.1.0"
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edition = "2021"
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[dependencies]
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@ -0,0 +1,14 @@
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fn main() {
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// ANCHOR: here
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let dice_roll = 9;
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match dice_roll {
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3 => add_fancy_hat(),
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7 => remove_fancy_hat(),
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_ => reroll(),
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}
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fn add_fancy_hat() {}
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fn remove_fancy_hat() {}
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fn reroll() {}
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// ANCHOR_END: here
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}
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6
listings/ch06-enums-and-pattern-matching/no-listing-17-underscore-unit/Cargo.lock
generated
Normal file
6
listings/ch06-enums-and-pattern-matching/no-listing-17-underscore-unit/Cargo.lock
generated
Normal file
@ -0,0 +1,6 @@
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||||
# This file is automatically @generated by Cargo.
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||||
# It is not intended for manual editing.
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||||
[[package]]
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||||
name = "enums"
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||||
version = "0.1.0"
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||||
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@ -0,0 +1,6 @@
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||||
[package]
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name = "enums"
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version = "0.1.0"
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edition = "2021"
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||||
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||||
[dependencies]
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||||
@ -0,0 +1,13 @@
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fn main() {
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// ANCHOR: here
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let dice_roll = 9;
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match dice_roll {
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3 => add_fancy_hat(),
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7 => remove_fancy_hat(),
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_ => (),
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}
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fn add_fancy_hat() {}
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fn remove_fancy_hat() {}
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// ANCHOR_END: here
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}
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@ -7,21 +7,7 @@ Rust 有一个叫做 `match` 的极为强大的控制流运算符，它允许我
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因为刚刚提到了硬币，让我们用它们来作为一个使用 `match` 的例子！我们可以编写一个函数来获取一个未知的硬币，并以一种类似验钞机的方式，确定它是何种硬币并返回它的美分值，如示例 6-3 中所示。
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```rust
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enum Coin {
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Penny,
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Nickel,
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Dime,
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Quarter,
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}
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fn value_in_cents(coin: Coin) -> u8 {
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match coin {
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Coin::Penny => 1,
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Coin::Nickel => 5,
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Coin::Dime => 10,
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Coin::Quarter => 25,
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}
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}
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{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-03/src/main.rs:here}}
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```
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<span class="caption">示例 6-3：一个枚举和一个以枚举成员作为模式的 `match` 表达式</span>
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@ -37,24 +23,7 @@ fn value_in_cents(coin: Coin) -> u8 {
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如果分支代码较短的话通常不使用大括号，正如示例 6-3 中的每个分支都只是返回一个值。如果想要在分支中运行多行代码，可以使用大括号。例如，如下代码在每次使用`Coin::Penny` 调用时都会打印出 “Lucky penny!”，同时仍然返回代码块最后的值，`1`：
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```rust
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# enum Coin {
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# Penny,
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# Nickel,
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# Dime,
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# Quarter,
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# }
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#
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fn value_in_cents(coin: Coin) -> u8 {
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match coin {
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Coin::Penny => {
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println!("Lucky penny!");
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1
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},
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Coin::Nickel => 5,
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Coin::Dime => 10,
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Coin::Quarter => 25,
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}
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}
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{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-08-match-arm-multiple-lines/src/main.rs:here}}
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```
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### 绑定值的模式
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@ -64,19 +33,7 @@ fn value_in_cents(coin: Coin) -> u8 {
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作为一个例子，让我们修改枚举的一个成员来存放数据。1999 年到 2008 年间，美国在 25 美分的硬币的一侧为 50 个州的每一个都印刷了不同的设计。其他的硬币都没有这种区分州的设计，所以只有这些 25 美分硬币有特殊的价值。可以将这些信息加入我们的 `enum`，通过改变 `Quarter` 成员来包含一个 `State` 值，示例 6-4 中完成了这些修改：
|
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|
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```rust
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#[derive(Debug)] // 这样可以立刻看到州的名称
|
||||
enum UsState {
|
||||
Alabama,
|
||||
Alaska,
|
||||
// --snip--
|
||||
}
|
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|
||||
enum Coin {
|
||||
Penny,
|
||||
Nickel,
|
||||
Dime,
|
||||
Quarter(UsState),
|
||||
}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-04/src/main.rs:here}}
|
||||
```
|
||||
|
||||
<span class="caption">示例 6-4：`Quarter` 成员也存放了一个 `UsState` 值的 `Coin` 枚举</span>
|
||||
@ -86,30 +43,7 @@ enum Coin {
|
||||
在这些代码的匹配表达式中，我们在匹配 `Coin::Quarter` 成员的分支的模式中增加了一个叫做 `state` 的变量。当匹配到 `Coin::Quarter` 时，变量 `state` 将会绑定 25 美分硬币所对应州的值。接着在那个分支的代码中使用 `state`，如下：
|
||||
|
||||
```rust
|
||||
# #[derive(Debug)]
|
||||
# enum UsState {
|
||||
# Alabama,
|
||||
# Alaska,
|
||||
# }
|
||||
#
|
||||
# enum Coin {
|
||||
# Penny,
|
||||
# Nickel,
|
||||
# Dime,
|
||||
# Quarter(UsState),
|
||||
# }
|
||||
#
|
||||
fn value_in_cents(coin: Coin) -> u8 {
|
||||
match coin {
|
||||
Coin::Penny => 1,
|
||||
Coin::Nickel => 5,
|
||||
Coin::Dime => 10,
|
||||
Coin::Quarter(state) => {
|
||||
println!("State quarter from {:?}!", state);
|
||||
25
|
||||
},
|
||||
}
|
||||
}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-09-variable-in-pattern/src/main.rs:here}}
|
||||
```
|
||||
|
||||
如果调用 `value_in_cents(Coin::Quarter(UsState::Alaska))`，`coin` 将是 `Coin::Quarter(UsState::Alaska)`。当将值与每个分支相比较时，没有分支会匹配，直到遇到 `Coin::Quarter(state)`。这时，`state` 绑定的将会是值 `UsState::Alaska`。接着就可以在 `println!` 表达式中使用这个绑定了，像这样就可以获取 `Coin` 枚举的 `Quarter` 成员中内部的州的值。
|
||||
@ -123,16 +57,7 @@ fn value_in_cents(coin: Coin) -> u8 {
|
||||
得益于 `match`，编写这个函数非常简单，它将看起来像示例 6-5 中这样：
|
||||
|
||||
```rust
|
||||
fn plus_one(x: Option<i32>) -> Option<i32> {
|
||||
match x {
|
||||
None => None,
|
||||
Some(i) => Some(i + 1),
|
||||
}
|
||||
}
|
||||
|
||||
let five = Some(5);
|
||||
let six = plus_one(five);
|
||||
let none = plus_one(None);
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:here}}
|
||||
```
|
||||
|
||||
<span class="caption">示例 6-5：一个在 `Option<i32>` 上使用 `match` 表达式的函数</span>
|
||||
@ -142,13 +67,13 @@ let none = plus_one(None);
|
||||
让我们更仔细地检查 `plus_one` 的第一行操作。当调用 `plus_one(five)` 时，`plus_one` 函数体中的 `x` 将会是值 `Some(5)`。接着将其与每个分支比较。
|
||||
|
||||
```rust,ignore
|
||||
None => None,
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:first_arm}}
|
||||
```
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||||
|
||||
值 `Some(5)` 并不匹配模式 `None`，所以继续进行下一个分支。
|
||||
|
||||
```rust,ignore
|
||||
Some(i) => Some(i + 1),
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:second_arm}}
|
||||
```
|
||||
|
||||
`Some(5)` 与 `Some(i)` 匹配吗？当然匹配！它们是相同的成员。`i` 绑定了 `Some` 中包含的值，所以 `i` 的值是 `5`。接着匹配分支的代码被执行，所以我们将 `i` 的值加一并返回一个含有值 `6` 的新 `Some`。
|
||||
@ -156,7 +81,7 @@ Some(i) => Some(i + 1),
|
||||
接着考虑下示例 6-5 中 `plus_one` 的第二个调用，这里 `x` 是 `None`。我们进入 `match` 并与第一个分支相比较。
|
||||
|
||||
```rust,ignore
|
||||
None => None,
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:first_arm}}
|
||||
```
|
||||
|
||||
匹配上了！这里没有值来加一，所以程序结束并返回 `=>` 右侧的值 `None`，因为第一个分支就匹配到了，其他的分支将不再比较。
|
||||
@ -168,21 +93,13 @@ None => None,
|
||||
`match` 还有另一方面需要讨论。考虑一下 `plus_one` 函数的这个版本，它有一个 bug 并不能编译：
|
||||
|
||||
```rust,ignore,does_not_compile
|
||||
fn plus_one(x: Option<i32>) -> Option<i32> {
|
||||
match x {
|
||||
Some(i) => Some(i + 1),
|
||||
}
|
||||
}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-10-non-exhaustive-match/src/main.rs:here}}
|
||||
```
|
||||
|
||||
我们没有处理 `None` 的情况，所以这些代码会造成一个 bug。幸运的是，这是一个 Rust 知道如何处理的 bug。如果尝试编译这段代码，会得到这个错误：
|
||||
|
||||
```text
|
||||
error[E0004]: non-exhaustive patterns: `None` not covered
|
||||
-->
|
||||
|
|
||||
6 | match x {
|
||||
| ^ pattern `None` not covered
|
||||
```console
|
||||
{{#include ../listings/ch06-enums-and-pattern-matching/no-listing-10-non-exhaustive-match/output.txt}}
|
||||
```
|
||||
|
||||
Rust 知道我们没有覆盖所有可能的情况甚至知道哪些模式被忘记了！Rust 中的匹配是**穷举式的**（*exhaustive*）：必须穷举到最后的可能性来使代码有效。特别的在这个 `Option<T>` 的例子中，Rust 防止我们忘记明确的处理 `None` 的情况，这让我们免于假设拥有一个实际上为空的值，从而使之前提到的价值亿万的错误不可能发生。
|
||||
@ -192,16 +109,7 @@ Rust 知道我们没有覆盖所有可能的情况甚至知道哪些模式被忘
|
||||
让我们看一个例子，我们希望对一些特定的值采取特殊操作，而对其他的值采取默认操作。想象我们正在玩一个游戏，如果你掷出骰子的值为 3，角色不会移动，而是会得到一顶新奇的帽子。如果你掷出了 7，你的角色将失去新奇的帽子。对于其他的数值，你的角色会在棋盘上移动相应的格子。这是一个实现了上述逻辑的 `match`，骰子的结果是硬编码而不是一个随机值，其他的逻辑部分使用了没有函数体的函数来表示，实现它们超出了本例的范围：
|
||||
|
||||
```rust
|
||||
let dice_roll = 9;
|
||||
match dice_roll {
|
||||
3 => add_fancy_hat(),
|
||||
7 => remove_fancy_hat(),
|
||||
other => move_player(other),
|
||||
}
|
||||
|
||||
fn add_fancy_hat() {}
|
||||
fn remove_fancy_hat() {}
|
||||
fn move_player(num_spaces: u8) {}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-15-binding-catchall/src/main.rs:here}}
|
||||
```
|
||||
|
||||
+对于前两个分支，匹配模式是字面值 3 和 7，最后一个分支则涵盖了所有其他可能的值，模式是我们命名为 `other` 的一个变量。`other` 分支的代码通过将其传递给 `move_player` 函数来使用这个变量。
|
||||
@ -213,34 +121,15 @@ Rust 还提供了一个模式，当我们不想使用通配模式获取的值时
|
||||
让我们改变游戏规则，当你掷出的值不是 3 或 7 的时候，你必须再次掷出。这种情况下我们不需要使用这个值，所以我们改动代码使用 `_` 来替代变量 `other` ：
|
||||
|
||||
```rust
|
||||
let dice_roll = 9;
|
||||
match dice_roll {
|
||||
3 => add_fancy_hat(),
|
||||
7 => remove_fancy_hat(),
|
||||
_ => reroll(),
|
||||
}
|
||||
|
||||
fn add_fancy_hat() {}
|
||||
fn remove_fancy_hat() {}
|
||||
fn reroll() {}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-16-underscore-catchall/src/main.rs:here}}
|
||||
```
|
||||
|
||||
这个例子也满足穷举性要求，因为我们在最后一个分支中明确地忽略了其他的值。我们没有忘记处理任何东西。
|
||||
|
||||
让我们再次改变游戏规则，如果你掷出 3 或 7 以外的值，你的回合将无事发生。我们可以使用单元值（在[“元组类型”][tuples]<!-- ignore -->一节中提到的空元组）作为 `_` 分支的代码：
|
||||
|
||||
[tuples]: ch03-02-data-types.html#元组类型
|
||||
|
||||
```rust
|
||||
let dice_roll = 9;
|
||||
match dice_roll {
|
||||
3 => add_fancy_hat(),
|
||||
7 => remove_fancy_hat(),
|
||||
_ => (),
|
||||
}
|
||||
|
||||
fn add_fancy_hat() {}
|
||||
fn remove_fancy_hat() {}
|
||||
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-17-underscore-unit/src/main.rs:here}}
|
||||
```
|
||||
|
||||
在这里，我们明确告诉 Rust 我们不会使用与前面模式不匹配的值，并且这种情况下我们不想运行任何代码。
|
||||
|
||||
Loading…
Reference in New Issue
Block a user