如何在 Rust 中组合函数? [英] How to compose functions in Rust?

问题描述

我正在尝试编写一个由两个函数组成的函数.最初的设计非常简单:一个函数接受两个函数并返回一个组合函数，然后我可以与其他函数组合，因为 Rust 没有 rest 参数.我遇到了一堵墙，里面堆满了令人沮丧的无用编译器错误.

I'm trying to write a function that composes two functions. The initial design is pretty simple: a function that takes two functions and returns a composed function which I can then compose with other functions, since Rust doesn't have rest parameters. I've run into a wall built with frustrating non-helpful compiler errors.

我的撰写功能:

fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
where
    F: 'a + Fn(A) -> B + Sized,
    G: 'a + Fn(B) -> C + Sized,
{
    Box::new(move |x| g(f(x)))
}

我想如何使用它:

fn main() {
    let addAndMultiply = compose(|x| x * 2, |x| x + 2);
    let divideAndSubtract = compose(|x| x / 2, |x| x - 2);

    let finally = compose(*addAndMultiply, *divideAndSubtract);
    println!("Result is {}", finally(10));
}

编译器不喜欢那样，无论我尝试什么，特征边界都不会得到满足.错误是:

The compiler doesn't like that, no matter what I try, the trait bounds are never satisfied. The error is:

error[E0277]: the size for values of type `dyn std::ops::Fn(_) -> _` cannot be known at compilation time
  --> src/main.rs:13:19
   |
13 |     let finally = compose(*addAndMultiply, *divideAndSubtract);
   |                   ^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn std::ops::Fn(_) -> _`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
note: required by `compose`
  --> src/main.rs:1:1
   |
1  | / fn compose<'a, A, B, C, G, F>(f: F, g: G) -> Box<Fn(A) -> C + 'a>
2  | | where
3  | |     F: 'a + Fn(A) -> B + Sized,
4  | |     G: 'a + Fn(B) -> C + Sized,
5  | | {
6  | |     Box::new(move |x| g(f(x)))
7  | | }
   | |_^

推荐答案

正如 @ljedrz 指出的，要做到工作你只需要再次引用组合函数:

As @ljedrz points out, to make it work you only need to reference the composed functions again:

let finally = compose(&*multiply_and_add, &*divide_and_subtract);

(请注意，在 Rust 中，约定规定变量名应采用蛇形大小写)

(Note that in Rust, convention dictates that variable names should be in snake_case)

但是，我们可以做得更好！

However, we can make this better!

从 Rust 1.26 开始，我们可以使用 抽象返回类型(以前的特色门控为#![feature(conservative_impl_trait)]).这可以帮助您极大地简化示例，因为它允许您跳过生命周期、引用、Sized 约束和 Boxes:

Since Rust 1.26, we can use abstract return types (previously featured gated as #![feature(conservative_impl_trait)]). This can help you simplify your example greatly, as it allows you to skip the lifetimes, references, Sized constraints and Boxes:

fn compose<A, B, C, G, F>(f: F, g: G) -> impl Fn(A) -> C
where
    F: Fn(A) -> B,
    G: Fn(B) -> C,
{
    move |x| g(f(x))
}

fn main() {
    let multiply_and_add = compose(|x| x * 2, |x| x + 2);
    let divide_and_subtract = compose(|x| x / 2, |x| x - 2);

    let finally = compose(multiply_and_add, divide_and_subtract);
    println!("Result is {}", finally(10));
}

<小时>

最后，既然你提到了 rest 参数，我怀疑你真正想要的是有一种方法以灵活的方式链式组合尽可能多的函数.我为此目的编写了这个宏:

---

Finally, since you mention rest parameters, I suspect that what you actually want is to have a way to chain-compose as many functions as you want in a flexible manner. I wrote this macro for this purpose:

macro_rules! compose {
    ( $last:expr ) => { $last };
    ( $head:expr, $($tail:expr), +) => {
        compose_two($head, compose!($($tail),+))
    };
}

fn compose_two<A, B, C, G, F>(f: F, g: G) -> impl Fn(A) -> C
where
    F: Fn(A) -> B,
    G: Fn(B) -> C,
{
    move |x| g(f(x))
}

fn main() {
    let add = |x| x + 2;
    let multiply = |x| x * 2;
    let divide = |x| x / 2;
    let intermediate = compose!(add, multiply, divide);

    let subtract = |x| x - 2;
    let finally = compose!(intermediate, subtract);

    println!("Result is {}", finally(10));
}

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