Rust Typed Builder

Creates a compile-time verified builder:

use typed_builder::TypedBuilder;

#[derive(TypedBuilder)]
struct Foo {
    // Mandatory Field:
    x: i32,

    // #[builder(default)] without parameter - use the type's default
    // #[builder(setter(strip_option))] - wrap the setter argument with `Some(...)`
    #[builder(default, setter(strip_option))]
    y: Option<i32>,

    // Or you can set the default
    #[builder(default=20)]
    z: i32,
}

Build in any order:

Foo::builder().x(1).y(2).z(3).build();
Foo::builder().z(1).x(2).y(3).build();

Omit optional fields(the one marked with #[default]):

Foo::builder().x(1).build()

But you can't omit non-optional arguments - or it won't compile:

Foo::builder().build(); // missing x
Foo::builder().x(1).y(2).y(3); // y is specified twice

Features

• Custom derive for generating the builder pattern.
• Ability to annotate fields with #[builder(setter(into))] to make their setters accept Into values.
• Compile time verification that all fields are set before calling .build().
• Compile time verification that no field is set more than once.
• Ability to annotate fields with #[builder(default)] to make them optional and specify a default value when the user does not set them.
• Generates simple documentation for the .builder() method.
• Customizable method name and visibility of the .build() method.

Limitations

• The build errors when you neglect to set a field or set a field describe the actual problem as a deprecation warning, not as the main error.
• The generated builder type has ugly internal name and many generic parameters. It is not meant for passing around and doing fancy builder tricks - only for nicer object creation syntax(constructor with named arguments and optional arguments).
  • For the that reason, all builder methods are call-by-move and the builder is not cloneable. Saves the trouble of determining if the fields are cloneable...
  • If you want a builder you can pass around, check out derive-builder. It's API does not conflict with typed-builder's so you can be able to implement them both on the same type.

Conflicts

• TypedBuilder accepts arbitrary Rust code for #[builder(default = ...)], but other custom derive proc-macro crates may try to parse them using the older restrictions that allow only literals. To solve this, use #[builder(default_code = "...")] instead.

Alternatives - and why typed-builder is better

• derive-builder - does all the checks in runtime, returning a Result you need to unwrap.
• safe-builder-derive - this one does compile-time checks - by generating a type for each possible state of the builder. Rust can remove the dead code, but your build time will still be exponential. typed-builder is encoding the builder's state in the generics arguments - so Rust will only generate the path you actually use.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.