Rust Conversion - From, Into, TryFrom, TryInto

Primitive types can be converted to each other through casting.

Rust addresses conversion between custom types (i.e., struct and enum) by the use of traits. The generic conversions will use the From and Into traits. However there are more specific ones for the more common cases, in particular when converting to and from Strings.

From and Into

The From and Into traits are inherently linked, and this is actually part of its implementation. If you are able to convert type A from type B, then it should be easy to believe that we should be able to convert type B to type A.

From

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. There are numerous implementations of this trait within the standard library for conversion of primitive and common types.

For example we can easily convert a str into a String

let my_str = "hello";
let my_string = String::from(my_str);

We can do something similar for defining a conversion for our own type.

use std::convert::From;

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

impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let num = Number::from(30);
    println!("My number is {:?}", num);
}

Into

The Into trait is simply the reciprocal of the From trait. It defines how to convert a type into another type.

Calling into() typically requires us to specify the result type as the compiler is unable to determine this most of the time.

use std::convert::Into;

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

impl Into<Number> for i32 {
    fn into(self) -> Number {
        Number { value: self }
    }
}

fn main() {
    let int = 5;
    // Try removing the type annotation
    let num: Number = int.into();
    println!("My number is {:?}", num);
}

From and Into are interchangable

From and Into are designed to be complementary. We do not need to provide an implementation for both traits. If you have implemented the From trait for your type, Into will call it when necessary. Note, however, that the converse is not true: implementing Into for your type will not automatically provide it with an implementation of From.

use std::convert::From;

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

// Define `From`
impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let int = 5;
    // use `Into`
    let num: Number = int.into();
    println!("My number is {:?}", num);
}

TryFrom and TryInto

Similar to From and Into, TryFrom and TryInto are generic traits for converting between types. Unlike From/Into, the TryFrom/TryInto traits are used for fallible conversions, and as such, return Results.

use std::convert::TryFrom;
use std::convert::TryInto;

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

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

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

fn main() {
    // TryFrom

    assert_eq!(EvenNumber::try_from(8), Ok(EvenNumber(8)));
    assert_eq!(EvenNumber::try_from(5), Err(()));

    // TryInto

    let result: Result<EvenNumber, ()> = 8i32.try_into();
    assert_eq!(result, Ok(EvenNumber(8)));
    let result: Result<EvenNumber, ()> = 5i32.try_into();
    assert_eq!(result, Err(()));
}

To and from Strings

Converting to String

To convert any type to a String is as simple as implementing the ToString trait for the type. Rather than doing so directly, you should implement the fmt::Display trait which automagically provides ToString and also allows printing the type as discussed in the section on print!.

use std::fmt;

struct Circle {
    radius: i32
}

impl fmt::Display for Circle {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Circle of radius {}", self.radius)
    }
}

fn main() {
    let circle = Circle { radius: 6 };
    println!("{}", circle.to_string());
}

Parsing a String

It's useful to convert strings into many types, but one of the more common string operations is to convert them from string to number. The idiomatic approach to this is to use the parse function and either to arrange for type inference or to specify the type to parse using the 'turbofish' syntax. Both alternatives are shown in the following example.

This will convert the string into the type specified as long as the FromStr trait is implemented for that type. This is implemented for numerous types within the standard library.

fn main() {
    let parsed: i32 = "5".parse().unwrap();
    let turbo_parsed = "10".parse::<i32>().unwrap();

    let sum = parsed + turbo_parsed;
    println!("Sum: {:?}", sum);
}

To obtain this functionality on a user defined type simply implement the FromStr trait for that type.

use std::num::ParseIntError;
use std::str::FromStr;

#[derive(Debug)]
struct Circle {
    radius: i32,
}

impl FromStr for Circle {
    type Err = ParseIntError;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s.trim().parse() {
            Ok(num) => Ok(Circle{ radius: num }),
            Err(e) => Err(e),
        }
    }
}

fn main() {
    let radius = "    3 ";
    let circle: Circle = radius.parse().unwrap();
    println!("{:?}", circle);
}