TryFrom for checked conversions in Rust

TryFrom is From for conversions that may fail, and it gives you try_into.

use std::convert::TryFrom;

struct Stars(u8);

impl TryFrom<i64> for Stars {
    type Error = String;

    fn try_from(value: i64) -> Result<Self, Self::Error> {
        match value {
            0..=3 => Ok(Stars(value as u8)),
            other => Err(format!("{other} is not 0 to 3")),
        }
    }
}

fn main() {
    let ok = Stars::try_from(3).map(|s| s.0);
    println!("{:?}", ok);

    let bad: Result<Stars, String> = 9_i64.try_into();
    println!("{}", bad.err().unwrap());
}

How it works

1. Implementing TryFrom provides try_into for free.
2. The associated Error type names the failure.
3. Validation lives in one place instead of every call site.

Keywords and builtins used here

• Err
• Error
• Ok
• Result
• Self
• Stars
• String
• as
• fn
• for
• i64
• impl
• let
• main
• match
• struct
• try_from
• type
• u8
• use

The run, in numbers

Lines

22

Characters to type

421

Tokens

146

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 83 seconds.

Type this snippet

Step 5 of 5 in Your own error types, step 12 of 15 in Error handling.

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