It is possible to embed type alias in a struct in Rust?

help
#1

Is that possible to embed type alias FooResult into Foo and use it with Foo::Result, so that I don't need to prefix it with Foo?

struct Foo {}
type FooResult = String

// Is it possible to make the type alias FooResult be a static part of Foo, like Foo::Result?

impl Foo {
    // A function for parsing a random string
    // fn parse(input: &str) -> FooResult{
        
    // }
}

#![allow(unused)]
fn main() {
    let result = Foo.parse("hello word");
}

(Playground)

#2

I'm unsure as to what you mean by "embedding" or "static part of". Are you looking for an associated type?

#3

The most straightforward way would be an inherent associated type, used like this: playground. Unfortunately, this isn't possible currently and from a brief glance at the tracking issue, I don't think that it will land soon.

You could achieve it via associated types as mentioned. Alternatively, you could namespace it via modules this way:


pub mod foo {

    pub struct Foo;
    pub type Result = String;
}

// Outside of Foo


pub mod another_module {
    use super::foo::{self, Foo};
    
    fn use_foo(foo: Foo) -> foo::Result {
        todo!()
    }
    
}

(playground)
However this way, foo::Result is not statically linked to the type Foo.

Edit: This is how you could do it with a trait and an associated type:

struct Foo;

trait FooExt {
    type Result;
}

impl FooExt for Foo {
    type Result = String;
}

impl Foo {
    fn parse(input: &str) -> <Self as FooExt>::Result {
        todo!()
    }
}

(playground)
Unfortunately, the <Self as FooExt>::Result is part is necessary. As explained by rustc --explain E0223:

This syntax specifies that we want the X type from MyTrait, as made concrete in
MyStruct. The reason that we cannot simply use MyStruct::X is that MyStruct
might implement two different traits with identically-named associated types.
This syntax allows disambiguation between the two.

#4

To expand on this: given how you want your impl to be "strictly-shaped", it does look like:

which is currently an inherent impl, ought to be a trait impl:

impl Parse for Foo {
    fn parse(input: &str) -> Foo::Result {
        …
    }
}

And, in that case, we can see that Foo::Result is an associated type relative to the Parse trait:

  trait Parse {
+     type Result;

      fn parse(input: &str) -> Self::Result {
  }

And then you can write:

struct Foo {}

impl Parse for Foo {
    type Result = String;

    fn parse(input: &str) -> Self::Result {
        "todo!".into()
    }
}

  • Note that Self::Result, or, more generally, when T : Parse, T::Result is an unambiguous associated type. But if using Foo or some other concrete type, you'll need to use the fully qualified associated type syntax:

    <Foo as Parse>::Result

    Which, to be honest, is even better: a Result may be anything, whereas with this syntax it is clear that we are talking of the Result of something related to Parse-ing :slight_smile:

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