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# Dynamize
In order to turn a trait into a [trait object] the trait must be [object-safe]
and the values of all [associated types] must be specified. Sometimes you
however want a trait object to be able to encompass trait implementations with
different associated type values. This crate provides a procedural macro to
achieve that.
[trait object]: https://doc.rust-lang.org/book/ch17-02-trait-objects.html
[object-safe]: https://doc.rust-lang.org/reference/items/traits.html#object-safety
[associated types]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
The following code illustrates a scenario where dynamize can help you:
```rust ignore
trait Client {
type Error;
fn get(&self, url: String) -> Result<Vec<u8>, Self::Error>;
}
impl Client for HttpClient { type Error = HttpError; ...}
impl Client for FtpClient { type Error = FtpError; ...}
let client: HttpClient = ...;
let object = &client as &dyn Client;
```
The last line of the above code fails to compile with:
> error\[E0191\]: the value of the associated type `Error` (from trait `Client`)
> must be specified
To use dynamize you only have to make some small changes:
```rust ignore
#[dynamize::dynamize]
trait Client {
type Error: Into<SuperError>;
fn get(&self, url: String) -> Result<Vec<u8>, Self::Error>;
}
```
1. You add the `#[dynamize::dynamize]` attribute to your trait.
2. You specify a trait bound for each associated type.
Dynamize defines a new trait for you, named after your trait but
with the `Dyn` prefix, so e.g. `Client` becomes `DynClient`:
```rust ignore
let client: HttpClient = ...;
let object = &client as &dyn DynClient;
```
The new "dynamized" trait can then be used without having to specify the
associated type value.
## How does this work?
For the above example dynamize generates the following code:
```rust ignore
trait DynClient {
fn get(&self, url: String) -> Result<Vec<u8>, SuperError>;
}
impl<__to_be_dynamized: Client> DynClient for __to_be_dynamized {
fn get(&self, url: String) -> Result<Vec<u8>, SuperError> {
Client::get(self, url).map_err(|x| x.into())
}
}
```
As you can see in the dynamized trait the associated type was replaced with the
destination type of the `Into` bound. The magic however happens afterwards:
dynamize generates a blanket implementation: each type implementing `Client`
automatically also implements `DynClient`!
## How does this actually work?
The destination type of an associated type is
determined by looking at its trait bounds:
* if the first trait bound is `Into<T>` the destination type is `T`
* otherwise the destination type is the boxed trait object of all trait bounds
e.g. `Error + Send` becomes `Box<dyn Error + Send>`
(for this the first trait bound needs to be object-safe)
Dynamize can convert associated types in:
* return types, e.g. `fn example(&self) -> Self::A`
* callback parameters, e.g. `fn example<F: Fn(Self::A)>(&self, f: F)`
Dynamize also understands if you wrap associated types in the following types:
* tuples
* `Option<_>`
* `Result<_, _>`
* `some::module::Result<_>` (type alias with fixed error type)
* `&mut dyn Iterator<Item = _>`
* `Vec<_>`, `VecDeque<_>`, `LinkedList<_>`, `HashSet<K>`, `BinaryHeap<K>`,
`BTreeSet<K>`, `HashMap<K, _>`, `BTreeMap<K, _>`
(for `K` only `Into`-bounded associated types work because they require `Eq`)
Note that since these are resolved recursively you can actually nest these
arbitrarily so e.g. the following also just works:
```rust ignore
fn example(&self) -> Result<Vec<Self::Item>, Self::Error>;
```
## How does dynamize deal with method generics?
In order to be object-safe methods must not have generics, so dynamize simply
moves them to the trait definition:
```rust
trait Gen {
fn foobar<A>(&self, a: A) -> A;
}
```
becomes
```rust
trait DynGen<A> {
fn foobar(&self, a: A) -> A;
}
```
If two method type parameters have the same name, dynamize enforces that they
also have the same bounds and only adds the parameter once to the trait.
## Dynamize supports async
Dynamize supports async out of the box. Since Rust however does not yet support
async functions in traits, you'll have to additionally use another library like
[async-trait](https://crates.io/crates/async-trait), for example:
```rust ignore
#[dynamize::dynamize]
#[dyn_trait_attr(async_trait)]
#[blanket_impl_attr(async_trait)]
#[async_trait]
trait Client: Sync {
type Error: std::error::Error + Send;
async fn get(&self, url: String) -> Result<Vec<u8>, Self::Error>;
}
```
* `#[dyn_trait_attr(foo)]` attaches `#[foo]` to the dynamized trait
* `#[blanket_impl_attr(foo)]` attaches `#[foo]` to the blanket implementation
Note that it is important that the `#[dynamize]` attribute comes before the
`#[async_trait]` attribute, since dynamize must run before async_trait.
## Using dynamize with other collections
Dynamize automatically recognizes collections from the standard library like
`Vec<_>` and `HashMap<_, _>`. Dynamize can also work with other collection
types as long as they implement `IntoIterator` and `FromIterator`, for example
dynamize can be used with [indexmap](https://crates.io/crates/indexmap) as
follows:
```rust ignore
#[dynamize::dynamize]
#[collection(IndexMap, 2)]
trait Trait {
type A: Into<String>;
type B: Into<i32>;
fn example(&self) -> IndexMap<Self::A, Self::B>;
}
```
The passed number tells dynamize how many generic type parameters to expect.
* for 1 dynamize expects: `Type<A>: IntoIterator<Item=A> + FromIterator<A>`
* for 2 dynamize expects: `Type<A,B>: IntoIterator<Item=(A,B)> + FromIterator<(A,B)>`
* for 3 dynamize expects: `Type<A,B,C>: IntoIterator<Item=(A,B,C)> + FromIterator<(A,B,C)>`
* etc ...
|
