Builder patterns are also super helpful for libraries. Allows you to create APIs with lots of optional fields without having call sites that pass in a dozen Non

Builder patterns are also super helpful for libraries. Allows you to create APIs with lots of optional fields without having call sites that pass in a dozen None values. There's also the type state pattern where you have a struct take a generic so you can allow certain methods depending on the generic type. Like say you have a struct with a field that you want to be optional, but you know statically that at a certain point in the code it'll be set. You can do: struct User<IdType> { id: IdType } impl User<()> { fn create() -> User<u64> { ... } } impl User<u64> { fn id() -> u64 { user.id } } That way you can have some methods where you can be certain that the ID is set and some methods where it's not set.