degeneric-macros

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degeneric-macros

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Rust macro that hides struct generics into trait associated types

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# degeneric-macros

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## Quick Start

```rust
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_decl = "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:
```rust
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:
```rust
use degeneric_macros::Degeneric;

#[derive(Degeneric)]
#[degeneric(trait_decl = "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

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

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

#[derive(Degeneric, TypedBuilder)]
#[degeneric(trait_decl = "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](https://crates.io/crates/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.

```rust
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_decl = "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:

```rust
use degeneric_macros::Degeneric;

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

By convention, dynamize generates a `DynGeneratedContainerTrait` where the types are boxed.
Please refer to [dynamize documentation](https://docs.rs/dynamize/latest/dynamize/#dynamize)
for more information.

### Degeneric + haz

Degeneric is able to serve as a derive macro for the excellent
[`haz`](https://crates.io/crates/haz) crate.

```rust
use degeneric_macros::Degeneric;
use haz::Has;

#[derive(Degeneric, Default)]
 #[degeneric(haz)]
 struct Config {
   host: Host,
   port: Port,
   verbosity: Verbosity,
   restriction: Restriction,
 }

fn assert_has_all_the_things<T: Has<Host> + Has<Port> + Has<Verbosity> + Has<Restriction>>(_: T) {}
 assert_has_all_the_things(Config::default());
```

## Degeneric understands where clause

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

#[derive(Degeneric)]
#[degeneric(trait_decl = "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.

```compile_fail
use degeneric_macros::{Degeneric};

#[derive(Degeneric)]
#[degeneric(trait_decl = "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.

```rust
use degeneric_macros::{Degeneric};
#[derive(Degeneric)]
#[degeneric(trait_decl = "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);
```

```compile_fail
use degeneric_macros::{Degeneric};

#[derive(Degeneric)]
#[degeneric(trait_decl = "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\"])]`

```compile_fail
use degeneric_macros::Degeneric;

#[derive(Degeneric)]
#[degeneric(trait_decl = "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](https://lib.rs/galemu) - hide lifetimes!
+ [trait-set](https://lib.rs/trait-set) - shorten and DRY up trait bounds
+ [typed-builder](https://lib.rs/typed-builder) - generate a builder for your trait
+ [easy-ext](https://lib.rs/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:

```compile_fail

#[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:

```rust
#[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

degeneric-macros

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