rust-by-practice/en/src/type-conversions/from-into.md

From/Into

The From trait allows for a type to define how to create itself from another type, hence providing a very simple mechanism for converting between several types.

The From and Into traits are inherently linked, and this is actually part of its implementation. It means if we write something like this: impl From<T> for U, then we can use let u: U = U::from(T) or let u:U = T.into().

The Into trait is simply the reciprocal of the From trait. That is, if you have implemented the From trait for your type, then the Into trait will be automatically implemented for the same type.

Using the Into trait will typically require the type annotations as the compiler is unable to determine this most of the time.

For example we can easily convert &str into String :

fn main() {
    let my_str = "hello";

    // three conversions below all depends on the fact: String implements From<&str>:
    let string1 = String::from(my_str);
    let string2 = my_str.to_string();
    // explict type annotation is required here
    let string3: String = my_str.into();
}

because the standard library has already implemented this for us : impl From<&'_ str> for String .

Some implementations of From trait can be found here.

1. 🌟🌟🌟

fn main() {
     // impl From<bool> for i32
    let i1:i32 = false.into();
    let i2:i32 = i32::from(false);  
    assert_eq!(i1, i2);
    assert_eq!(i1, 0);

    // FIX the error in two ways
    // 1. impl From<char> for ? , maybe you should check the docs mentiond above to find the answer
    // 2. a keyword from the last chapter
    let i3: i32 = 'a'.into();

    // FIX the error in two ways
    let s: String = 'a' as String;

    println!("Success!")
}

Implement From for custom types

2. 🌟🌟

// From is now included in `std::prelude`, so there is no need to introduce it into the current scope
// use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl From<i32> for Number {
    // IMPLEMENT `from` method
}

// FILL in the blanks
fn main() {
    let num = __(30);
    assert_eq!(num.value, 30);

    let num: Number = __;
    assert_eq!(num.value, 30);

    println!("Success!")
}

3. 🌟🌟🌟 When performing error handling it is often useful to implement From trait for our own error type. Then we can use ? to automatically convert the underlying error type to our own error type.

use std::fs;
use std::io;
use std::num;

enum CliError {
    IoError(io::Error),
    ParseError(num::ParseIntError),
}

impl From<io::Error> for CliError {
    // IMPLEMENT from method
}

impl From<num::ParseIntError> for CliError {
    // IMPLEMENT from method
}

fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
    // ? automatically converts io::Error to CliError
    let contents = fs::read_to_string(&file_name)?;
    // num::ParseIntError -> CliError
    let num: i32 = contents.trim().parse()?;
    Ok(num)
}

fn main() {
    println!("Success!")
}

TryFrom/TryInto

Similar to From and Into, TryFrom and TryInto are generic traits for converting between types.

Unlike From/Into, TryFrom and TryInto are used for fallible conversions and return a Result instead of a plain value.

4. 🌟🌟

// TryFrom and TryInto are included in `std::prelude`, so there is no need to introduce it into the current scope
// use std::convert::TryInto;

fn main() {
    let n: i16 = 256;

    // Into trait has a method `into`,
    // hence TryInto has a method ?
    let n: u8 = match n.__() {
        Ok(n) => n,
        Err(e) => {
            println!("there is an error when converting: {:?}, but we catch it", e.to_string());
            0
        }
    };

    assert_eq!(n, __);

    println!("Success!")
}

5. 🌟🌟🌟

#[derive(Debug, PartialEq)]
struct EvenNum(i32);

impl TryFrom<i32> for EvenNum {
    type Error = ();

    // IMPLEMENT `try_from`
    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value % 2 == 0 {
            Ok(EvenNum(value))
        } else {
            Err(())
        }
    }
}

fn main() {
    assert_eq!(EvenNum::try_from(8), Ok(EvenNum(8)));
    assert_eq!(EvenNum::try_from(5), Err(()));

    // FILL in the blanks
    let result: Result<EvenNum, ()> = 8i32.try_into();
    assert_eq!(result, __);
    let result: Result<EvenNum, ()> = 5i32.try_into();
    assert_eq!(result, __);

    println!("Success!")
}