Complex trait requirements on struct

问题

I have a fairly complex trait set up and I'm having trouble lining the pieces up. Right now it looks roughly like this:

/// Trait for models which can be gradient-optimized.
pub trait Optimizable {
    type Data;
    type Target;

    // The contract //
}

/// Trait for optimization algorithms.
pub trait OptimAlgorithm<M : Optimizable> {

    // The contract //
}

Now I want to be able to allow a struct implementing OptimAlgorithm to be a field in a struct implementing Optimizable. This would look something like this:

/// Model struct
pub struct Model<A: OptimAlgorithm<Self>> {
    alg: A,
}

impl Optimizable for Model<A> {
...
}

This doesn't work as the Self reference on the struct is nonsense. I tried using associated types for OptimAlgorithm but I need the algorithms to be generic over the models so this doesn't work. Is there a magic syntax I'm missing or does this need an overhaul?

Edit --

Here's a minimal example which shows error E0275 as described in Steven's answer. It's a little closer to my source code but less messy.

回答1:

Just use Model<A> instead of Self. Self is only really useful in traits where one needs to be able to refer to the concrete type implementing the trait. Here, the concrete type is always Model<A>.

pub trait Optimizable {
    type Data;
    type Target;

    // The contract //
}

/// Trait for optimization algorithms.
pub trait OptimAlgorithm<M: Optimizable> {

    // The contract //
}
pub struct Model<A> where A: OptimAlgorithm<Model<A>> {
    alg: A,
}

impl<A> Optimizable for Model<A>
    where A: OptimAlgorithm<Model<A>>
{
    type Data = ();
    type Target = ();
}

In response to your updated code, the lifetime appears to be giving rust trouble. It appears you can make this work by using a higher-ranked lifetime but I don't know why.

pub trait Optimizable {
    type Data;
    type Target;

    // The contract //
}

/// Trait for optimization algorithms.
pub trait OptimAlgorithm<M: Optimizable> {

    // The contract //
}

pub struct Algorithm;

impl Default for Algorithm {
    fn default() -> Algorithm { Algorithm }
}

impl<M: Optimizable> OptimAlgorithm<M> for Algorithm {

}


pub struct Model<'a, A> where for<'b> A: OptimAlgorithm<Model<'b, A>> {
    layer_sizes: &'a [usize],
    alg: A,
}

impl<'a, A> Model<'a, A>
    where A: for<'b> OptimAlgorithm<Model<'b, A>>
{
    pub fn new(layers: &'a [usize]) -> Model<Algorithm> {
        Model {
            layer_sizes: layers,
            alg: Algorithm::default(),
        }
    }
}

impl<'a, A> Optimizable for Model<'a, A>
    where A: for<'b> OptimAlgorithm<Model<'b, A>>
{
    type Data = ();
    type Target = ();
}

pub fn main() {
    let layers = &[1usize,2,3];
    let a = Model::<Algorithm>::new(layers as &[usize]);
}

回答2:

I think it's a bug. Or at least surprising behaviour. If you take off the where bound on the Model struct (and just leave it on the impl), your edited code compiles. I'll try to reduce a bit more and file a bug.

pub trait Optimizable {
    type Data;
    type Target;

    // The contract //
}

/// Trait for optimization algorithms.
pub trait OptimAlgorithm<M: Optimizable> {

    // The contract //
}

pub struct Algorithm;

impl Default for Algorithm {
    fn default() -> Algorithm { Algorithm }
}

impl<M: Optimizable> OptimAlgorithm<M> for Algorithm {

}


pub struct Model<'a, A> { // no bounds here
    layer_sizes: &'a [usize],
    alg: A,
}

impl<'a, A> Model<'a, A>
    where A: OptimAlgorithm<Model<'a, A>>
{
    pub fn new(layers: &[usize]) -> Model<Algorithm> {
        Model {
            layer_sizes: layers,
            alg: Algorithm::default(),
        }
    }
}

impl<'a, A> Optimizable for Model<'a, A>
    where A: OptimAlgorithm<Model<'a, A>>
{
    type Data = ();
    type Target = ();
}

pub fn main() {

}

playground

来源：https://stackoverflow.com/questions/34552927/complex-trait-requirements-on-struct

标签

struct

rust

traits