[FIXED] How to specify associated type <Item=...> when calling associated function of a trait?

Issue

I have this rust code playground:

use itertools::{repeat_n,RepeatN};

pub trait MyIterator: Iterator {
    fn fill1(elem: Self::Item, n1: usize) -> RepeatN<Self::Item>
    where
        Self::Item: Clone,
    {
        repeat_n(elem, n1)
    }
}

My Problem is that I can't call this method because rustc can't infer the type.

// error[E0284]: type annotations needed
// let r: RepeatN<char> = MyIterator::fill1('a', 5);
//                        ^^^^^^^^^^^^^^^^^ cannot infer type
// note: cannot satisfy `<_ as Iterator>::Item == _`
let r: RepeatN<char> = MyIterator::fill1('a', 5);

I tried this but it doesn't compile:

// error[E0229]: associated type bindings are not allowed here
let r: RepeatN<char> = MyIterator::<Item=char>::fill1('a', 5);

How can I specify the type of Item in this call? Or is a function outside of the trait (like itertools::repeat_n) the best way here?

Solution

Well, you haven't implemented the trait for any types, so no fill1 function actually exists.

If you implement MyIterator for some type that implements Iterator<Item = char>, for example std::iter::Empty<char>, then you'll be able to call fill1 through that type.

use std::iter::Empty;
impl MyIterator for Empty<char> {}

fn main() {
    let r: RepeatN<char> = Empty::fill1('a', 5);
}

This is, however, pointless. You will note that Empty plays no role in the actual function definition - it could have been any iterator. Furthermore there is no sensible generalization of this behavior to other Self types. This is why itertools::repeat_n is not a member of the Itertools trait: it is nonsense as a trait function because it is not a generalization of behavior shared by multiple iterator types.

In Rust, unlike some other languages, not everything has to be a member of a class. If you merely want to put fill1 and related things in a common namespace, simply use a module, which is the most basic unit of code organization.

mod my_iterator {
    use itertools::{repeat_n, RepeatN};

    fn fill1<T>(elem: T, n1: usize) -> RepeatN<T>
    where
        T: Clone,
    {
        repeat_n(elem, n1)
    }
}

Answered By - trent

Answer Checked By - Clifford M. (PHPFixing Volunteer)