Type Safe Builder Pattern

Infallible compile-time checked builders for your structs.

No more worrying whether the build call on your builder will return Ok or not. Maybe you forgot to set a field? typesafe-builders solves this by using the Rust type-system to ensure correct usage.

Example

use typesafe_builders::prelude::*;

fn main() {
	#[derive(Builder)]
	struct Point {
		#[builder(constructor)]
		x: u8,
		y: u8,
		#[builder(optional)]
		z: Option<u8>,
	}

	// `builder` requires `x` since it is marked as `constructor`.
	let builder = Point::builder(1);
	// These do not compile:
	// partial.x(6); 		// `x` is already set
	// partial.build();		// `y` is not set

	// `build` is only available once all required fields are set:
	let result = builder.y(2).build();

	assert_eq!(result.x, 1);
	assert_eq!(result.y, 2);
	assert_eq!(result.z, None);
}

Known Downside

I can recommend this only for internal use. It is best to not expose these builder types as an API of your crate, since they look extremely ugly and verbose. For example:

use typesafe_builders::prelude::*;

#[derive(Builder)]
struct Point {
	x: u8,
	y: u8,
	z: u8,
}

// Ugly type name here... and it only gets worse for const-generics etc.
fn preset() -> GenericPointBuilder<false, false, true> {
	Point::builder().z(0)
}

fn main() {
	let partial = preset();
	let point = partial.x(1).y(2).build();
}

Please open an MR/Issue if you know how to improve this.

Field Attributes

Attributes can be combined. Ones that do not work together will throw an explicit error at compile time. Duplicates always error.

Optional

A field can be set, but does not have to be. Requires the field type to be Default.

use typesafe_builders::prelude::*;

#[derive(Builder)]
pub struct Struct {
	#[builder(optional)]
	x: u8,
}

fn main() {
	// without x
	Struct::builder().build();
	 // with x
	Struct::builder().x(4).build();
}

Constructor

Require a field to be set upon builder construction.

use typesafe_builders::prelude::*;

#[derive(Builder)]
pub struct Struct {
	#[builder(constructor)]
	x: u8,
}

fn main() {
	Struct::builder(4).build();
	// does not work:
	// Struct::builder(4).x(5).build();
}

Decay

Decay the type to its first generic. Eases use for Option, Box etc. Requires that the decayed type can be intoed its original. Works on all types with one generic arg.

use typesafe_builders::prelude::*;

#[derive(Builder)]
pub struct Struct {
	#[builder(decay)]
	x: Option<u8>,
}

fn main() {
	// Use `4` of `Some(4)`
	Struct::builder().x(4).build();
}

How does it work?

Const generic one-hot bitfields. What you get is similar to this:

pub struct Builder<const x_set: bool, const y_set: bool> {
	x: Option<u8>,
	y: Option<u8>,
}

impl<const y_set: bool> Builder<false, y_set> {
    fn set_x(self, x: u8) -> Builder<true, y_set,> {
        unimplemented!()
    }
}

impl<const x_set: bool> Builder<x_set, false> {
    fn set_y(self, y: u8) -> Builder<x_set, true> {
        unimplemented!()
    }
}

// The build function is only available once all fields are set:
impl Builder<true, true> {
    fn build() {

    }
}

More Examples

Lifetimes

They work as expected

use typesafe_builders::prelude::*;

#[derive(Builder)]
pub struct Struct<'a, 'b, 'c> {
	x: &'a Box<&'b Option<&'c str>>, // yikes
}

fn main() {
	Struct::builder().x(&Box::new(&Some("hi"))).build();
}

Generics

Works as expected, but does not yet support defaults.

mod other {
	use typesafe_builders::prelude::*;

	#[derive(Builder)]
	pub struct Struct<T: Clone> {
		y: Option<T>,
	}
}

fn main() {
	other::Struct::<u8>::builder().y(Some(4)).build();
}

Const Generics

Works as expected, but does not yet support defaults.

mod other {
	use typesafe_builders::prelude::*;

	#[derive(Builder)]
	pub struct Struct<const LEN: usize> {
		x: [u8; LEN],
	}
}

fn main() {
	other::Struct::<1>::builder().x([1]).build();
}

TODOs

• Lifetimes
• Generics
  • Bounds
  • With default
• Const generics
  • With default
• Add optional fields.
• Add rename field attribute.
• Add constructor or something like this to have mandatory args directly in the builder function.
• Add Into or whatever to cast types.
• Add way to pass options as Some automatically.
• Cleanup