如何在Rust中为FlatMap trait实现基于flat_map的默认flatten方法？

我正在开发一个对标Scala Cats库的Rust函数式编程库，采用HKT1 trait模拟高阶类型。尝试为FlatMap trait添加签名为flatten(ffa: F<F<A>>) -> F<A>的方法，希望通过flat_map提供默认实现，但代码编译报错。

核心代码 #

trait HKT1 {
    type Unwrapped;
    type Wrapped<T>;
}

trait FlatMap: HKT1 + Sized {
    fn flat_map<B, F>(self, f: F) -> Self::Wrapped<B>
    where
        F: FnOnce(Self::Unwrapped) -> Self::Wrapped<B>;

    // 无法编译
    fn flatten<A>(ffa: Self::Wrapped<Self::Wrapped<A>>) -> Self::Wrapped<A>
    // 如何正确设置泛型约束？
    where
        Self::Wrapped<Self::Wrapped<A>>: FlatMap
    {
        ffa.flat_map(|x| x)
    }
}

编译错误信息 #

error[E0308]: mismatched types
  --> src/main.rs:15:26
   |
15 |         ffa.flat_map(|x| x)
   |                          ^ expected HKT1::Wrapped, found HKT1::Unwrapped
   |
   = note: expected associated type `<<Self as HKT1>::Wrapped<<Self as HKT1>::Wrapped<A>> as HKT1>::Wrapped<_>`
              found associated type `<<Self as HKT1>::Wrapped<<Self as HKT1>::Wrapped<A>> as HKT1>::Unwrapped`

error[E0308]: mismatched types
  --> src/main.rs:15:9
   |
6  | trait FlatMap: HKT1 + Sized {
   | --------------------------- this type parameter
...
11 |     fn flatten<A>(ffa: Self::Wrapped<Self::Wrapped<A>>) -> Self::Wrapped<A>
   |                                                            ---------------- expected `<Self as HKT1>::Wrapped<A>` because of return type
...
15 |         ffa.flat_map(|x| x)
   |         ^^^^^^^^^^^^^^^^^^^ expected type parameter `Self`, found associated type
   |
   = note: expected associated type `<Self as HKT1>::Wrapped<A>`
              found associated type `<<Self as HKT1>::Wrapped<<Self as HKT1>::Wrapped<A>> as HKT1>::Wrapped<_>`
   = note: you might be missing a type parameter or trait bound

解决方案 #

问题出在缺少对嵌套Wrapped类型的关联类型约束：编译器无法推断Self::Wrapped<Self::Wrapped<A>>作为FlatMap实现时，其HKT1::Unwrapped和HKT1::Wrapped<T>与外层类型的对应关系。需要在flatten的where子句中明确这些等价性：

trait HKT1 {
    type Unwrapped;
    type Wrapped<T>;
}

trait FlatMap: HKT1 + Sized {
    fn flat_map<B, F>(self, f: F) -> Self::Wrapped<B>
    where
        F: FnOnce(Self::Unwrapped) -> Self::Wrapped<B>;

    fn flatten<A>(ffa: Self::Wrapped<Self::Wrapped<A>>) -> Self::Wrapped<A>
    where
        // 确保嵌套的Wrapped类型实现FlatMap
        Self::Wrapped<Self::Wrapped<A>>: FlatMap,
        // 约束嵌套Wrapped的Unwrapped就是外层的Wrapped<A>
        <Self::Wrapped<Self::Wrapped<A>> as HKT1>::Unwrapped == Self::Wrapped<A>,
        // 约束嵌套Wrapped的Wrapped<T>与外层的Wrapped<T>完全一致
        for<'a> <Self::Wrapped<Self::Wrapped<A>> as HKT1>::Wrapped<A> == Self::Wrapped<A>,
    {
        ffa.flat_map(|x| x)
    }
}

约束说明 #

1. <Self::Wrapped<Self::Wrapped<A>> as HKT1>::Unwrapped == Self::Wrapped<A>：保证调用flat_map时，闭包接收的参数x类型是Self::Wrapped<A>，与闭包返回的类型一致。
2. for<'a> <Self::Wrapped<Self::Wrapped<A>> as HKT1>::Wrapped<A> == Self::Wrapped<A>：确保flat_map返回的类型就是flatten需要的Self::Wrapped<A>，解决返回类型不匹配的问题。

这样就能通过flat_map为flatten提供通用的默认实现，无需为每个类型单独编写flatten方法。

内容的提问来源于stack exchange，提问作者Campbell He