139 lines
2.5 KiB
Markdown
139 lines
2.5 KiB
Markdown
1.
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```rust
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fn main() {
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// impl From<bool> for i32
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let i1:i32 = false.into();
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let i2:i32 = i32::from(false);
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assert_eq!(i1, i2);
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assert_eq!(i1, 0);
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let i3: u32 = 'a'.into();
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let s: String = 'a'.into();
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}
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```
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```rust
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fn main() {
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// impl From<bool> for i32
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let i1:i32 = false.into();
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let i2:i32 = i32::from(false);
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assert_eq!(i1, i2);
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assert_eq!(i1, 0);
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let i3: u32 = 'a' as u32 ;
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let s: String = String::from('a');
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}
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```
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2.
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```rust
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// From is now included in `std::prelude`, so there is no need to introduce it into the current scope
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// use std::convert::From;
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#[derive(Debug)]
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struct Number {
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value: i32,
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}
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impl From<i32> for Number {
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// IMPLEMENT `from` method
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fn from(item: i32) -> Self {
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Number { value: item }
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}
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}
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fn main() {
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let num = Number::from(30);
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assert_eq!(num.value, 30);
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let num: Number = 30.into();
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assert_eq!(num.value, 30);
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println!("Success!")
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}
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```
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3.
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```rust
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use std::fs;
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use std::io;
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use std::num;
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enum CliError {
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IoError(io::Error),
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ParseError(num::ParseIntError),
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}
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impl From<io::Error> for CliError {
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fn from(error: io::Error) -> Self {
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CliError::IoError(error)
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}
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}
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impl From<num::ParseIntError> for CliError {
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fn from(error: num::ParseIntError) -> Self {
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CliError::ParseError(error)
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}
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}
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fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
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// ? automatically converts io::Error to CliError
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let contents = fs::read_to_string(&file_name)?;
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// num::ParseIntError -> CliError
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let num: i32 = contents.trim().parse()?;
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Ok(num)
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}
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fn main() {
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println!("Success!")
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}
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```
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4.
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```rust
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fn main() {
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let n: i16 = 256;
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let n: u8 = match n.try_into() {
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Ok(n) => n,
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Err(e) => {
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println!("there is an error when converting: {:?}, but we catch it", e.to_string());
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0
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}
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};
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assert_eq!(n, 0);
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println!("Success!")
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}
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```
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5.
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```rust,editable
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#[derive(Debug, PartialEq)]
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struct EvenNum(i32);
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impl TryFrom<i32> for EvenNum {
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type Error = ();
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fn try_from(value: i32) -> Result<Self, Self::Error> {
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if value % 2 == 0 {
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Ok(EvenNum(value))
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} else {
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Err(())
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}
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}
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}
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fn main() {
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assert_eq!(EvenNum::try_from(8), Ok(EvenNum(8)));
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assert_eq!(EvenNum::try_from(5), Err(()));
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let result: Result<EvenNum, ()> = 8i32.try_into();
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assert_eq!(result, Ok(EvenNum(8)));
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let result: Result<EvenNum, ()> = 5i32.try_into();
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assert_eq!(result, Err(()));
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}
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``` |