<分区>
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，我们在这里说什么类型 query必须是最后一行，我们尝试在此结束。
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)));
}
当我运行它时，出现错误:
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？ ？我假设我需要指定关于 Fn 的其他内容在第一行？但是什么？
我假设我错误地指定了 query: Fn(&Vec<i32>) -> f64 的类型我的第一个函数的参数，或者闭包写错了|ref fv| my_query(fv, 3) .
我读了Passing closure to trait method: expected type parameter, found closure ，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。