Overview

bon is a Rust crate for generating compile-time-checked builders for structs and functions. It also provides idiomatic partial application with optional and named parameters for functions and methods.

If you wonder "Why would I use builders?", see the motivational blog post.

Function Builder

You can turn a function with positional parameters into a function with named parameters with #[builder].

use bon::builder;

#[builder]
fn greet(name: &str, level: Option<u32>) -> String {
    let level = level.unwrap_or(0);

    format!("Hello {name}! Your level is {level}")
}

let greeting = greet()
    .name("Bon")
    .level(24) // <- setting `level` is optional, we could omit it
    .call();

assert_eq!(greeting, "Hello Bon! Your level is 24");

Any syntax for functions is supported including async, fallible, generic functions, impl Trait, etc.

Many things are customizable with additional attributes described in the API reference, but let's see what else bon has to offer.

Struct Builder

Use #[derive(Builder)] to generate a builder for a struct.

use bon::Builder;

#[derive(Builder)]
struct User {
    name: String,
    is_admin: bool,
    level: Option<u32>,
}

let user = User::builder()
    .name("Bon".to_owned())
    // `level` is optional, we could omit it here
    .level(24)
    // call setters in any order
    .is_admin(true)
    .build();

assert_eq!(user.name, "Bon");
assert_eq!(user.level, Some(24));
assert!(user.is_admin);

Method Builder

Associated methods require #[bon] on top of the impl block additionally.

Method new

The method named new generates builder()/build() methods.

use bon::bon;

struct User {
    id: u32,
    name: String,
}

#[bon]
impl User {
    #[builder]
    fn new(id: u32, name: String) -> Self {
        Self { id, name }
    }
}

let user = User::builder()
    .id(1)
    .name("Bon".to_owned())
    .build();

assert_eq!(user.id, 1);
assert_eq!(user.name, "Bon");

#[derive(Builder)] on a struct generates builder API that is fully compatible with placing #[builder] on the new() method with a signature similar to the struct's fields (more details on the Compatibility page).

Other Methods

All other methods generate {method_name}()/call() methods.

use bon::bon;

struct Greeter {
    name: String,
}

#[bon]
impl Greeter {
    #[builder]
    fn greet(&self, target: &str, prefix: Option<&str>) -> String {
        let prefix = prefix.unwrap_or("INFO");
        let name = &self.name;

        format!("[{prefix}] {name} says hello to {target}")
    }
}

let greeter = Greeter { name: "Bon".to_owned() };

let greeting = greeter
    .greet()
    .target("the world")
    // `prefix` is optional, omitting it is fine
    .call();

assert_eq!(greeting, "[INFO] Bon says hello to the world");

Methods with or without self are both supported.

No Panics Possible

Builders generated by bon's macros use the typestate pattern to ensure all required parameters are filled, and the same setters aren't called repeatedly to prevent unintentional overwrites. If something is wrong, a compile error will be created.

What's Next?

What you've seen above is the first page of the 📖 Guide Book. If you want to learn more, jump to the Basics section. And remember: knowledge is power 🐱!

Feel free to jump to code and use the #[builder] and #[derive(Builder)] once you've seen enough docs to get started.

The 🔍 API Reference will help you navigate the attributes once you feel comfortable with the basics of bon. Both #[derive(Builder)] on structs and #[builder] on functions/methods have almost identical attributes API, so the documentation for them is common.

Installation

Add bon to your Cargo.toml.

[dependencies]
bon = "3.3"

You can opt out of std and alloc cargo features with default-features = false for no_std environments.

Acknowledgments

This project was heavily inspired by such awesome crates as buildstructor, typed-builder and derive_builder. This crate was designed with many lessons learned from them.

See alternatives for comparison.

Who's Using bon?

Some notable users:

• crates.io backend
• ractor
• comrak
• soldeer (package manager endorsed by foundry)
• tachyonfx

Getting Help

If you can't figure something out, consult the docs and maybe use the 🔍 Search bar on our docs website. You may also create an issue or a discussion on the Github repository for help or write us a message on Discord.

Socials

License

lib.rs:

Not a real crate! It's just a showcase of examples used by bon's documentation to demonstrate the rustdoc output for code generated by builder macros.

Don't use this crate, it doesn't follow semver at all and serves no other purpose other than linking to its docs as an example!