optionable

Library to derive structs/enums with all fields recursively replaced with Option-variants.

One common problem when expressing patches e.g. for Kubernetes apply configurations. is that one would need for a given rust struct T a corresponding struct TOpt where all fields are optional. While trivial to write for plain structures this quickly becomes tedious for nested structs/enums.

Deriving optional structs/enums

The core utility of this library is to provide an Optionable-derive macro that derives such an optioned type. It supports nested structures as well as various container and pointer wrapper.

The general logic is the same as for other rust derives, If you want to use the derive Optionable for a struct/enum every field of it needs to also have implemented the corresponding Optionable trait (see below):

#[derive(Optionable)]
#[optionable(derive(Serialize,Deserialize))]
struct DeriveExample {
    name: String,
    addresses: Vec<Address>,
}
#[derive(Optionable)]
#[optionable(derive(Serialize,Deserialize))]
struct Address {
    street_name: String,
    number: u8,
}

The generated optioned struct is (with resolved associated types):

#[derive(Serialize,Deserialize)]
struct DeriveExampleOpt {
    name: Option<String>,
    addresses: Option<Vec<AddressOpt>>,
}
#[derive(Serialize,Deserialize)]
struct AddressOpt {
    street_name: Option<String>,
    number: Option<u8>,
}

Also works for enums

Enums are also supported for the derive macro, e.g.

#[derive(Optionable)]
enum DeriveExample {
    Unit,
    Plain(String),
    Address { street: String, number: u32 },
    Address2(String, u32),
}

generates the following enum (with resolved associated types):

enum DeriveExampleOpt {
    Unit,
    Plain(Option<String>),
    Address { street: Option<String>, number: Option<u32> },
    AddressTuple(Option<String>, Option<u32>),
}

How it works

The main Optionable trait is quite simple

pub trait Optionable {
    type Optioned;
}

It is a marker trait that allows to express for a given type T which type should be considered its Optioned type such that Option<Optioned> would represent all variants of partial completeness. For types without inner structure this means that the Optioned type will just resolve to the type itself, e.g.

impl Optionable for String {
    type Optioned = String;
}

For many primitive types as well as common wrapper or collection types the Optionable-trait is already implemented.

Crate features

• chrono: Derive Optionable for types from chrono
• serde_json: Derive Optionable for serde_json::Value

Limitations

External types

Due to the orphan rule the usage of the library becomes cumbersome if one has a use case which heavily relies on crate-external types. For well-established libraries adding corresponding impl to this crate (feature-gated) would be a worthwhile approach.

IDE: Resolving associated types

Due to the use of associated types some IDE-hints do not fully resolve the associated types leaving you with <i32 as Optionable>::Optioned instead of i32. Luckily, for checking type correctness and also for error messages when using wrong types the associated types are resolved.

Similar crates

Another crate with similar scope is optional_struct. It focuses specifically on structs (not enums) and offers a more manual approach, especially in respect to nested sub-struct, providing many fine-grained configuration options.