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Overview

You can create custom rules by implementing the Rule trait. This allows you to define rules that are not provided by the library.

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Get Started

Let’s create a custom rule that only accepts strings that start with Hello.

At first, define a struct for the custom rule.

struct StartsWithHelloRule;

Next, implement the Rule trait for the struct.

use refined_type::Rule;
use refined_type::result::Error;

impl Rule for StartsWithHelloRule {
    type Item = String;

    fn validate(target: Self::Item) -> refined_type::Result<Self::Item> {
        if target.starts_with("Hello") {
            Ok(target)
        } else {
            Err(Error::new(target, "String does not start with 'Hello'"))
        }
    }
}

Creating a type alias improves readability.

type StartsWithHello = Refined<StartsWithHelloRule>;

Now you can use the custom rule.

fn starts_with_hello_example() -> Result<(), Error<String>> {
    let starts_with_hello = StartsWithHello::new("Hello, world".to_string())?;
    assert_eq!(starts_with_hello.into_value(), "Hello, world");

    let starts_with_hello = StartsWithHello::new("Hi, world".to_string());
    assert!(starts_with_hello.is_err());

    Ok(())
}

That’s all for creating custom rules! It’s very simple!

Now, if the standard provided rules aren’t sufficient, you can define your own rules!

CollectionDefine a complex rule