rust - Expected closure, found a different closure

Question

A is a structure which contains a vector of B. A implements the add_b method which adds a B instance to the list of B. B contains a closure property f.

If I add one B to the vector with add_b, it's OK. If I add two vectors with add_b, I got an error saying the two closures are different. Here is a minimal example:

// A struct...
struct A<F> {
    b_vec: Vec<B<F>>, // A vector of B
}

// ...and its implementation
impl<F> A<F>
where
    F: Fn(),
{
    fn new() -> A<F> {
        A { b_vec: Vec::new() }
    }

    fn add_b(&mut self, b: B<F>) {
        self.b_vec.push(b);
    }
}

// B struct...
struct B<F> {
    f: F,
}

// ...and its implementation
impl<F> B<F>
where
    F: Fn(),
{
    fn new(f: F) -> B<F> {
        B { f: f }
    }
}

// I add two B (with their closures arguments) in A
fn main() {
    let mut a = A::new();
    a.add_b(B::new(|| println!("test")));
    a.add_b(B::new(|| println!("test2")));
}

This code results in:

error[E0308]: mismatched types
  --> src/main.rs:39:20
   |
39 |     a.add_b(B::new(|| println!("test2")));
   |                    ^^^^^^^^^^^^^^^^^^^^ expected closure, found a different closure
   |

How can I add multiple B with their different closures to A's b_vec?

Answer 1

It's always worth taking a look at the full compiler output:

error[E0308]: mismatched types
  --> src/main.rs:39:20
   |
39 |     a.add_b(B::new(|| println!("test2")));
   |                    ^^^^^^^^^^^^^^^^^^^^ expected closure, found a different closure
   |
   = note: expected type `[closure@src/main.rs:38:20: 38:39]`
              found type `[closure@src/main.rs:39:20: 39:40]`
   = note: no two closures, even if identical, have the same type
   = help: consider boxing your closure and/or using it as a trait object

Especially helpful:

• no two closures, even if identical, have the same type

• consider boxing your closure and/or using it as a trait object

We can simplify your example further by removing the type B altogether. Then the only task is to save a vector of closures. As the compiler tells us, no two closures have the same type, but Vec is a homogeneous data structure, meaning that every item in it has the same type.

We can work around that restriction by introducing one level of indirection. As the compiler suggests, this can either be done by trait objects or boxing (the latter kind of includes the first one). The corresponding types would look like this:

• Vec<&Fn()> (reference to trait objects)
• Vec<Box<Fn()>> (trait object in a box)

In your example you want to own all closures, thus the correct choice is to box all closures, as Box<T> is an owning wrapper while references only borrow stuff.

A fully working example:

struct A {
    b_vec: Vec<B>,
}

impl A {
    fn new() -> A {
        A { b_vec: Vec::new() }
    }

    fn add_b(&mut self, b: B) {
        self.b_vec.push(b);
    }
}

struct B {
    f: Box<Fn()>,
}

impl B {
    fn new<F>(f: F) -> B
    where
        F: Fn() + 'static,
    {
        B { f: Box::new(f) }
    }
}

fn main() {
    let mut a = A::new();
    a.add_b(B::new(|| println!("test")));
    a.add_b(B::new(|| println!("test2")));
}