async-graphql 2.0.0-alpha.9

A GraphQL server library implemented in Rust
Documentation 
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# Union

The definition of a `Union` is similar to an `Interface`, **but with no fields allowed.**.
The implementation is quite similar for `Async-graphql`; from `Async-graphql`'s perspective, `Union` is a subset of `Interface`.

The following example modified the definition of `Interface` a little bit and removed fields.

```rust
use async_graphql::*;

struct Circle {
    radius: f32,
}

#[GQLObject]
impl Circle {
    async fn area(&self) -> f32 {
        std::f32::consts::PI * self.radius * self.radius
    }

    async fn scale(&self, s: f32) -> Shape {
        Circle { radius: self.radius * s }.into()
    }
}

struct Square {
    width: f32,
}

#[GQLObject]
impl Square {
    async fn area(&self) -> f32 {
        self.width * self.width
    }

    async fn scale(&self, s: f32) -> Shape {
        Square { width: self.width * s }.into()
    }
}

#[derive(GQLUnion)]
enum Shape {
    Circle(Circle),
    Square(Square),
}
```