closures - 如何编写可以组成 `FnMut` 闭包的函数？

Question

这是一个可以组合Fn 闭包的compose 函数:

fn compose<'a, T1, T2, T3, F1, F2>(f: F1, g: F2) -> Box<Fn(T1) -> T3 + 'a>
    where F1: Fn(T1) -> T2 + 'a,
          F2: Fn(T2) -> T3 + 'a
{
    box move |x| g(f(x))
}

如何才能让这个 compose 函数接受 FnMut 闭包？我试过:

fn compose<'a, T1, T2, T3, F1, F2>(f: F1, g: F2) -> Box<FnMut(T1) -> T3 + 'a>
    where F1: FnMut(T1) -> T2 + 'a,
          F2: FnMut(T2) -> T3 + 'a
{
    box move |x| g(f(x))
}

但它提示:

error: cannot borrow captured outer variable in an `FnMut` closure as mutable
         box move |x| g(f(x))
                      ^
error: cannot borrow captured outer variable in an `FnMut` closure as mutable
         box move |x| g(f(x))
                        ^

扩展它，是否可以让它与 FnOnce 闭包一起工作？

Answer 1

局部变量 f 和 g 必须是可变的:

fn compose<'a, T1, T2, T3, F1, F2>(mut f: F1, mut g: F2) -> Box<FnMut(T1) -> T3 + 'a>
    where F1: FnMut(T1) -> T2 + 'a,
          F2: FnMut(T2) -> T3 + 'a
{
    Box::new(move |x| g(f(x)))
}