将闭包传递给期望 std::ops::Fn 的函数

Question

我正在尝试使用闭包来调用一组特征向量上的函数。这里最重要的代码行是第一行，我们说 query 必须是什么类型，最后一行我们尝试给出一个闭包。

fn stat_query(dataset: &Vec<Vec<i32>>, query: Fn(&Vec<i32>) -> f64) -> f64 {
    // takes in a dataset of feature vectors and a query,
    // returns the average of the query on the dataset
    if dataset.len() == 0 {
        0.
    } else {
        dataset.iter().map(|ref fv| query(&fv)).sum() / (dataset.len() as f64)
    }
}

fn main() {
    // build dataset
    let fv1: Vec<i32> = vec![1, 1, 1, 1, 1];
    let fv2: Vec<i32> = vec![1, 0, 1, 0, 1];
    let my_dataset = vec![fv1, fv2];

    // query checks whether sum of features is greater than a threshold
    fn my_query(ref fv: &Vec<i32>, threshold: i32) -> f64 {
        if fv.iter().sum() > threshold {
            1.
        } else {
            0.
        }
    }

    // run query on dataset with threshold 3
    println!("{}", stat_query(&my_dataset, |ref fv| my_query(fv, 3)));
}

当我运行这个时，我得到一个错误：

error[E0277]: the trait bound `for<'r> std::ops::Fn(&'r std::vec::Vec<i32>) -> f64 + 'static: std::marker::Sized` is not satisfied
 --> src/main.rs:1:40
  |
1 | fn stat_query(dataset: &Vec<Vec<i32>>, query: Fn(&Vec<i32>) -> f64) -> f64 {
  |                                        ^^^^^ `for<'r> std::ops::Fn(&'r std::vec::Vec<i32>) -> f64 + 'static` does not have a constant size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `for<'r> std::ops::Fn(&'r std::vec::Vec<i32>) -> f64 + 'static`
  = note: all local variables must have a statically known size

error[E0308]: mismatched types
  --> src/main.rs:27:44
   |
27 |     println!("{}", stat_query(&my_dataset, |ref fv| my_query(fv, 3)));
   |                                            ^^^^^^^^^^^^^^^^^^^^^^^^ expected trait std::ops::Fn, found closure
   |
   = note: expected type `for<'r> std::ops::Fn(&'r std::vec::Vec<i32>) -> f64 + 'static`
              found type `[closure@src/main.rs:27:44: 27:68]`

我以为闭包implemented Fn（或FnOnce或FnMut）？当您期望 Fn 时提供闭包是否无效？我假设我需要在第一行指定关于 Fn 的其他内容？但是什么？

我假设我错误地指定了第一个函数的 query: Fn(&Vec<i32>) -> f64 参数的类型，或者写错了闭包 |ref fv| my_query(fv, 3).

我读过，但这似乎更多地是关于传递一个不是函数接受的唯一类型的闭包。

Answer 1

第二个错误比较重要：

error[E0277]: the trait bound `for<'r> std::ops::Fn(&'r std::vec::Vec<i32>) -> f64 + 'static: std::marker::Sized` is not satisfied

简而言之：Fn(&Vec<i32>) -> f64不是大小类型，不能按值传递。您只能传递 trait objects（引用或框）或实现特征的（大小）类型的值。在这种情况下，您可能需要后者：

Playground

fn stat_query<F>(dataset: &Vec<Vec<i32>>, query: F) -> f64
where
    F: Fn(&Vec<i32>) -> f64,
{
    // takes in a dataset of feature vectors and a query,
    // returns the average of the query on the dataset
    if dataset.len() == 0 {
        0.
    } else {
        dataset.iter().map(|ref fv| query(&fv)).sum::<f64>() / (dataset.len() as f64)
    }
}

fn main() {
    // build dataset
    let fv1: Vec<i32> = vec![1, 1, 1, 1, 1];
    let fv2: Vec<i32> = vec![1, 0, 1, 0, 1];
    let my_dataset = vec![fv1, fv2];

    // query checks whether sum of features is greater than a threshold
    fn my_query(ref fv: &Vec<i32>, threshold: i32) -> f64 {
        if fv.iter().sum::<i32>() > threshold {
            1.
        } else {
            0.
        }
    }

    // run query on dataset with threshold 3
    println!("{}", stat_query(&my_dataset, |ref fv| my_query(fv, 3)));
}

将闭包传递给期望 std::ops::Fn 的函数

Passing a closure to a function expecting a std::ops::Fn

closures

traits

rust