degeneric-macros

Quick Start

use degeneric_macros::{Degeneric};
use std::marker::PhantomData;

use trait_set::trait_set;
use typed_builder::TypedBuilder;

trait_set!(trait FactoryFn<T> = 'static + Send + Sync + Fn() -> T);

#[derive(Degeneric, TypedBuilder)]
#[degeneric(trait = "pub trait ContainerTrait")]
/// This is doc for ContainerTrait!
struct Container<T: Default, A: FactoryFn<T>, B> {
    a: A,
    b: B,
    c: u32,
    #[builder(default)]
    _t: PhantomData<T>,
}

fn my_fact() -> String {
    format!("hello world!")
}

let c = Container::builder().a(my_fact).b(true).c(20).build();
do_something(&c);
access_inner_types(&c);

fn do_something(c: &impl ContainerTrait) {}
fn access_inner_types<C: ContainerTrait>(c: &C) {
    let same_as_a: C::A;
}

Elevator pitch

The problem

Degeneric is a utility library that solves the common problem of having too many generics. Let's say we want to construct a dependency container like this:

struct Container<Logger, HttpClient> {
    logger: Logger,
    client: HttpClient,
    // ...and so on...
}

let container = Container {
    logger: String::from("logger"),
    client: String::from("http"),
};

accepts_container(container);
// now to consume such a container, one needs to write the function like this:
fn accepts_container<Logger, HttpClient>(c: Container<Logger, HttpClient>) {}

This creates a problem of ever growing list of generics in all functions that touch the container and pollutes APIs with unnecessary generics.

Degeneric solution

Degeneric proposes solution to this problem by creating a trait and stuffing all of the generic types into the trait as associated types. Instead of the pattern above, you'll end up with this:

use degeneric_macros::Degeneric;

#[derive(Degeneric)]
#[degeneric(trait = "pub trait ContainerTrait")]
struct Container<Logger, HttpClient> {
    logger: Logger,
    client: HttpClient,
}

let c = Container {
    logger: String::from("logger"),
    client: String::from("http"),
};

accepts_container(c);
fn accepts_container(c: impl ContainerTrait) {}

How is this different, you ask? Instead of accepting a whole lot of generic arguments, I can now write the function without even using angular brackets and I think that's beautiful. What is even more beautiful is that you can add more generics without having to modify the signature of accepts_container.

Degeneric understands lifetimes

use std::borrow::Cow;
use std::fmt::Debug;

use degeneric_macros::{Degeneric};
use typed_builder::TypedBuilder;

#[derive(Degeneric, TypedBuilder)]
#[degeneric(trait = "trait ContainerTrait")]
struct Container<'a, T: 'a + PartialEq<i32> + Debug> {
    cow: &'a Cow<'a, str>,
    reference: &'a T,
}

let cow = Cow::Owned(String::from("hello lifetimes"));
{
    let reference = 42;
    let c = Container::builder().cow(&cow).reference(&reference).build();

    fn accept_container<'a>(cont: &impl ContainerTrait<'a>) {
        assert_eq!(cont.cow().as_ref(), "hello lifetimes");
        assert_eq!(cont.reference(), &42_i32);
    }

    accept_container(&c);
}

Degeneric can be used with galemu!

If you're into hiding generics, you'll be surprised that the galemu crate makes it possible to hide even lifetimes!

The way this example works is, that your Container contains an impl GCon. This object is able to produce galemu::Bound<GCon::Transaction>.

The particular implementation of GTran is provided by galemu::create_gal_wrapper_type. One must manually implement GTran on it.

In principle, galemu lifts the lifetime of Transaction<'a> into the galemu::BoundExt trait. The lifetime inference happens in Connection::transaction. At that point, it's apparent that the connection's lifetime is passed to Transaction.

use std::fmt::Debug;
use std::borrow::Cow;
use std::ops::Deref;

use degeneric_macros::Degeneric;

use galemu::{Bound, BoundExt, create_gal_wrapper_type};

// begin galemu

struct Connection {
    count: usize
}

struct Transaction<'conn> {
    conn: &'conn mut Connection
}

impl Connection {
    fn transaction(&mut self) -> Transaction {
        Transaction { conn: self }
    }
}

trait GCon {
    type Transaction: GTran;

    fn create_transaction(&mut self) -> Bound<Self::Transaction>;
}

trait GTran: for<'s> BoundExt<'s> {
    fn commit<'s>(me: Bound<'s, Self>);
    fn abort<'s>(me: Bound<'s, Self>);
}

create_gal_wrapper_type!{ struct TransWrap(Transaction<'a>); }

impl GCon for Connection {
    type Transaction = TransWrap;

    fn create_transaction(&mut self) -> Bound<Self::Transaction> {
        let transaction = self.transaction();
        TransWrap::new(transaction)
    }
}

impl GTran for TransWrap {
    fn commit<'s>(me: Bound<'s, Self>) {
        let trans = TransWrap::into_inner(me);
        trans.conn.count += 10;
    }

    fn abort<'s>(me: Bound<'s, Self>) {
        let trans = TransWrap::into_inner(me);
        trans.conn.count += 3;
    }
}

// end galemu

#[derive(Degeneric)]
#[degeneric(trait = "pub trait ContainerTrait")]
struct Container<T: GCon> {
    conn: T,
}

let conn = Connection { count : 0 };

let cont = Container {
    conn,
};

fn commit_transaction(mut c: impl ContainerTrait) {
    let conn = c.conn_mut();
    let tran = conn.create_transaction();
    GTran::commit(tran);
}

commit_transaction(cont);

Degeneric + dynamize

Degeneric supports dynamizing the generated trait. How does that work?

Here's a minimal example on how to dynamize the generated trait:

#[derive(Degeneric)]
#[degeneric(dynamize, trait = "pub trait GeneratedContainerTrait")]
struct Container<T: Any> {
    item: T,
}

By convention, dynamize generates a DynGeneratedContainerTrait where the types are boxed. Please refer to dynamize documentation for more information.

Degeneric understands where clause

use degeneric_macros::{Degeneric};
use std::fmt::Debug;

#[derive(Degeneric)]
#[degeneric(trait = "pub trait ContainerTrait")]
struct Container<T> where T: Default + Debug + PartialEq {
    item: T,
}

let c = Container {
    item: vec![""],
};

fn construct_default_value<C: ContainerTrait>(c: C) {
    let v: C::T = Default::default();
    assert_eq!(v, Default::default());
}

construct_default_value(c);

Generate getters only for some fields

The no_getter attribute can be used to skip generating a getter.

use degeneric_macros::{Degeneric};

#[derive(Degeneric)]
#[degeneric(trait = "pub(crate) trait Something")]
struct Container<'a, T: 'a, S: 'a> {
    item: &'a T,
    item2: S,
    #[degeneric(no_getter)]
    dt: PhantomData<S>,
}

let c = Container {
    item: "hello",
    item2: format!("this won't have getter!"),
    dt: PhantomData<S>,
};

fn accept_container<C: Something>(c: C) {
    /// ERROR: dt doesn't have a getter!
    assert_eq!(c.dt(), format!("this won't have getter!"));
}

accept_container(c);

Degeneric figures out mutability

Some fields may have mutable getters, some not. Degeneric recognizes immutable pointers and references and skips generating mutable getter for them.

use degeneric_macros::{Degeneric};
#[derive(Degeneric)]
#[degeneric(trait = "pub(crate) trait Something")]
struct Container<'a, T: 'a> {
    x: &'a T,
    y: T,
}

let mut c = Container {
    x: &(),
    y: (),
};

fn accept_container<'a>(mut c: impl Something<'a>) {
    // OK
    c.x();
    c.y();
    c.y_mut();
}

accept_container(c);

use degeneric_macros::{Degeneric};

#[derive(Degeneric)]
#[degeneric(trait = "pub(crate) trait Something")]
struct Container<'a, T> {
    x: &'a T,
}

let c = Container {
    x: &(),
};

fn accept_container<'a>(c: impl Something<'a>) {
    // ERROR: x is a reference which can't be made mut
    c.x_mut();
}

Add attributes everywhere!

For some attributes, you can just add them on the field and they'll be forwarded to all getters automatically. Here's a list of such attributes:

• #[allow]
• #[doc]
• #[cfg(...)]
• #[cfg_attr(...)]

If you need more granularity, you can add attributes only on:

• Trait declaration: #[degeneric(trait_decl_attr = "#[doc = \"Trait declaration\"]")]
• Trait impl block: #[degeneric(trait_impl_attr = "#[doc = \"Trait implementation\"]")]
• Field immutable getter implementation: #[degeneric(getter_impl_attr = "#[doc = \"Getter implementation\"])]
• Field mutable getter declaration: #[degeneric(mut_getter_decl_attr = "#[doc = \"Mutable Getter declaration\"])]

use degeneric_macros::Degeneric;

#[derive(Degeneric)]
#[degeneric(trait = "pub(crate) trait Something")]
#[degeneric(trait_decl_impl_attr = "#[cfg(foo)]")]
/// This is documentation for the `Something` trait
struct Container<T> {
    x: T,
}

// this will error because the Something trait exists only in the foo configuration
#[cfg(not(foo))]
fn accept_container(c: impl Something) {}

Crates degeneric plays nice with

• galemu - hide lifetimes!
• trait-set - shorten and DRY up trait bounds
• typed-builder - generate a builder for your trait
• easy-ext - extend your trait with more methods

CloneExt

Apart from solving the dependency injection problem, degeneric also helps with cloning. There might be a situation where you're holding a non-cloneable type inside another type. In these situations, it might be possible to clone the value by different means.

Failing example:

#[derive(Default)]
struct NonClone;

#[derive(Clone)]
struct Container {
    nc: PhantomData<NonClone>,
}

In such situations, one can resort to degeneric's CloneExt derive macro. Currently, it offers a single attribute to adjust the way fields are cloned:

#[derive(Default)]
struct NonClone;

#[derive(Default, degeneric_macros::CloneExt)]
struct Container {
    #[clone_ext(clone_behavior(call_function="Default::default"))]
    nc: NonClone,
}

Container::default().clone();

License: MIT