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#21 in Rust patterns

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Used in 742 crates (217 directly)

MIT/Apache

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Build Status Latest Version Rust Documentation

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

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.

Dependencies

~340–800KB
~19K SLoC