Implement trait for all function pointers

[kylem]

I am trying to implement a trait for all function pointers. I've tried to do this by both implementing the trait for fn() as well as F: Fn() but am running into limitations with both options.

A simplified example is:

trait A { }
trait C { }

impl A for fn() { }
impl A for &fn() { }

impl<P> A for fn(&P) { }
impl<P> A for &fn(&P) { }

impl<F> C for F where F: Fn() { }
// impl<F, P> C for F where F: Fn(&P) { } -- unconstrained type parameter

fn my_func() { }
fn my_func_with_p(_: &usize) { }

fn accept_a<F: A>(_: &F) { }
fn accept_dyn_a(_: &dyn A) { }

fn accept_c<F: C>(_: &F) { }
fn accept_dyn_c(_: &dyn C) { }

fn main() {
    accept_a(&(my_func as fn()));
    accept_a(&(my_func_with_p as fn(&usize)));
    // accept_dyn_a(&my_func); -- trait bound not satisfied
    
    accept_c(&my_func);
    accept_dyn_c(&my_func);
}

And the playground is available at: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=889ed167fbd2dad717b6e6a63a178764

1. If I implement the trait for fn() I have to cast the value, and can't use it as an argument for a parameter of &dyn A.
2. If I implement the trait for Fn(), I don't have to worry about casting the value and can use it as &dyn C, but can't implement it for functions with an argument since the type parameter is unconstrained.

Am I doing something wrong and/or missing some alternative option?

[cole-miller]

What behavior/interface is available on all function pointers and closures with arbitrary number and type of arguments? That is, if you succeed in implementing this trait MyFn, and I give you a Box<dyn MyFn>, what do you expect to be able to do with it?

(I'm not aware of any way to be generic over functions and closures with different type signatures.)

[kylem]

Good question, so this is a better example of what I am wanting to do:

use std::any::Any;

struct Context {
    arg: Box<dyn Any>,
}

trait Runnable {
    fn caller(&self) -> &str {
        std::any::type_name::<Self>()
    }

    fn exec(&self, ctx: &Context);
}

trait Callable<P>: Runnable {
    fn call(&self, arg: &P);
}

impl<P: Any> Runnable for fn(&P) {
    fn exec(&self, ctx: &Context) {
        println!("caller :: {}", self.caller());

        let arg = &ctx.arg.downcast_ref::<P>().unwrap();
        (self)(arg)
    }
}

impl<P: Any> Callable<P> for fn(&P) {
    fn call(&self, arg: &P) {
        (self)(arg)
    }
}

impl<F> Runnable for F
where
    F: Fn(),
{
    fn exec(&self, _: &Context) {
        println!("caller :: {}", self.caller());
        (self)()
    }
}

fn my_runnable() {}
fn my_callable(_: &usize) {}

fn to_runnable<P: Any, F: Callable<P> + 'static>(f: F) -> Box<dyn Runnable> {
    Box::new(f)
}

fn run_runnable(f: impl Runnable, ctx: &Context) {
    f.exec(ctx);
}

fn main() {
    let runner = to_runnable(my_callable as fn(&usize));

    let ctx = Context {
        arg: Box::new(0usize),
    };

    runner.exec(&ctx);
    // run_runnable(my_callable, &ctx);

    run_runnable(my_runnable, &ctx);   
}

playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6a4a99b1462215976f9a27a56c7d759b

So the primary benefits that I see for implementing Runnable for F: Fn() is that it makes calling it easier since there isn't any casting involved as well as giving a more accurate type_name. However, I can't implement the Runnable for F with generic P (F: Fn(P)).

[Yandros]

@kylem the issue is that you seem to assume that the callable contract of Fn{,Mut,Once} seems to be unique (at most one) per type / implementor, which is not true (future-true, to be exact).

That is, in the future / in nightly, it is possible for a given type F, to be both Fn() and Fn(&P), as well as Fn(&Q), Fn(&R), etc.

See:

overloadable

for an example.

---

So, to solve:

impl<F> C for F where F: Fn() { }
// impl<F, P> C for F where F: Fn(&P) { } -- unconstrained type parameter

you will need the trait C to be in some way, generic over (something that depends on) P.

That being said, then you'd have issues with "no arity" and "arity over a parameter with a higher-order lifetime" to be hard-to-impossible to abstract over.

So let's abandon the idea of a generic trait (some sub usage could be possible (&P or &Dummy), but then that wouldn't be a trait that could be used as a dyn, because of its generic parm).

---

The last option, then, is to use wrapper types, that allow to get back "unique implementors":

• Playground