implement inherent associated items (see RFC 195).

[pnkfelix]

This issue is tracking RFC 195's inherent associated items

Original description follows.

When developing a type-parametric impl, I found myself writing code somewhat like this:

    struct Grammar<T, NT> { ... }

    impl<T:Clone,NT:Eq+Hash+Clone+Primable> Grammar<T,NT> {
        fn to_nt_map(&self) -> HashMap<NT, ~[Prod<T,NT>]>
        {
            type Rules = ~[Prod<T,NT>];
            type NTMap = HashMap<NT, Rules>;        
          ...
        }

        fn from_nt_map(start: &NT, rules: &HashMap<NT, ~[Prod<T,NT>]>)
            -> Grammar<T,NT>
        {
            type Rules = ~[Prod<T,NT>];
            type NTMap = HashMap<NT, Rules>;
          ...
        }

        pub fn eliminate_left_recursion(&self) -> Grammar<T,NT>
        {
          ...
        }

        ...

    }

I cannot make a type definition for HashMap<NT, ~[Prod<T,NT>]> outside of the impl because it then those free references to NT and T would be unbound.

And I cannot put a type definition within the impl, even an impl for a struct such as this (it simply does not parse with the current grammar).

(Being able to put type definitions within impls will probably arise naturally from #5033, when we get around to that. But that is nonetheless a separate issue from this: Associated Types (and Associated Items) are about enabling certain patterns for the user of a trait, while this ticket describes a convenience for the implementor of a struct.)

[cmr]

Nominating; is backwards compatible. I can try and take this on if we decide we want it.

[cmr]

(type aliases would need to preceed all methods)

[pnkfelix]

@cmr said "(type aliases would need to preceed all methods)"

why? To my knowledge there is no such ordering constraint for type aliases within a mod item, so why would an impl item require this?

[cmr]

@pnkfelix yikes, you're right. I think they should be. Why? Because:

#[crate_type="lib"];

mod bar {
    fn foo() -> T {
        32i
    }
    type T = int;
}

is crazy.

[huonw]

Why is that less crazy than

fn foo() { bar() }

fn bar() {}

?

[cmr]

I cede.

On Tue, Dec 31, 2013 at 1:05 AM, Huon Wilson notifications@github.comwrote:

Why is that less crazy than

fn foo() { bar() }
fn bar() {}

?

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/8995#issuecomment-31385141
.

[pnkfelix]

assigning P-low priority (is backwards compatible going foward, as cmr said).

[reem]

Triage: How does this interact with Associated Types? At least a new syntax would probably have to be chosen.

[pnkfelix]

I think that this feature request is actually satisfied by the Associated Items RFC.

In particular, the section "inherent associated items" of RFC 59 implies (to me) that you can write the exact example that I had asked for.

(I did write in the issue description that this feature request is distinct from associated types, but nonetheless, it seems that the author of RFC 59 decided that it was a natural feature to include.)

[pnkfelix]

So, this now seems to be a subissue of #17307.

I will change the title to "implement inherent associated items."

[rohel01]

Hello, I hope this the right place to ask for my request.

Associated types currently only work with traits.
I think they are also useful for structs. Is there a reason for the current limitation ?

pub struct GenericStruct<A, B> {
    fa : A,
    fb : B,
}

struct GenericResult<A, B> {
    fa : A,
    fb : B,
}

impl <A, B> GenericStruct<A, B> {


    // I d like to write
    // type Result = Option<GenericResult<A, B>>;
    // and use it in the code

    fn new(a: A, b : B) -> Option<GenericResult<A, B>> {
        Some(GenericResult {fa: a, fb : b})

    }
}

[steveklabnik]

I'm going to tag this with the tags from #8995 since it was closed without this being implemented.

[yodaldevoid]

@steveklabnik I think you meant to reference #17307 rather than #8995, which is this issue.

[tinaun]

note that inherent associated consts work perfectly fine today

#[derive(Debug)]
struct Vector {
    x: f32,
    y: f32,
    z: f32,
}

impl Vector {
    const ZERO: Self = Vector { x: 0.0, y: 0.0, z: 0.0 };
    
    fn new(x: f32, y: f32, z: f32) -> Self {
        Vector {
            x, y, z
        }
    }
}

[eddyb]

@nikomatsakis Is it right to assume that this can't be implemented without lazy normalization?

[varkor]

It would be good to link to this issue in the error message for https://doc.rust-lang.org/nightly/error-index.html#E0202, as it's not immediately clear that this is supposed to be supported.

[alexreg]

Any developments on this lately?