使用 Rayon into_par_iter().sum() 和自定义结构

Question

I'm trying to get use Rayon::prelude::into_par_iter to sum up a bunch of instances of a struct. I've implemented std::iter::Sum for the struct, but am still running into an error. Here is my example code;

use std::iter::Sum;

use rayon::prelude::*;

pub struct TestStruct {
    val: f32,
}

impl TestStruct {
    pub fn new(v: f32) -> Self {
        Self { val: v }
    }
}

impl<'a> Sum<&'a Self> for TestStruct {
    fn sum<I>(iter: I) -> Self
    where
        I: Iterator<Item = &'a Self>,
    {
        iter.fold(Self { val: 0. }, |a, b| Self {
            val: a.val + b.val,
        })
    }
}

fn main() {
    let default_val: f32 = 1.1;
    let sum: TestStruct = (0..5).into_par_iter().map(|&default_val| {
        TestStruct::new(default_val)
    })
    .sum();

    println!("{}", sum.val);
}

and I am told the trait 'std::iter::Sum' is not implemented for 'TestStruct' , but I think I am implementing it. Am I missing something here?

Answer 1

Two small compounded issues. One is that the std::iter::Sum is defined as:

pub trait Sum<A = Self> {
    fn sum<I>(iter: I) -> Self
    where
        I: Iterator<Item = A>;
}

The Self here refers to whatever the struct gets the implementation and the default generic parameter has already been filled in. You don't need to specify it any more if implementing Sum for struct itself.

The other is that the map closure parameter default_val , which is an integer, shadows the previous float one with the same name. Since TestStruct::new expects a f32 , it leads to a type error.

This works:

impl Sum for TestStruct {
    fn sum<I>(iter: I) -> Self
    where
        I: Iterator<Item = Self>,
    {
        ...
    }
}

fn main() {
    let default_val: f32 = 1.1;
    let sum: TestStruct = (0..5)
        .into_par_iter()
        .map(|_| TestStruct::new(default_val))
        .sum();

    println!("{}", sum.val);
}