Implement Fn Trait (call operator) for different t

Question

For example I have a simple classifier

struct Clf {
    x: f64
}

The classifier returns 0 if the observed value is smaller than x and 1 if bigger than x.

I now want to implement the call operator for this classifier. However, the function should be able to take either a float or a vector as arguments. In case of a vector, the output is a vector of 0 or 1 which has the same size as the input vector. It should work like this

let c = Clf { x : 0 };
let v = vec![-1, 0.5, 1];
println!("{}", c(0.5));     // prints 1
println!("{}", c(v));       // prints [0, 1, 1]

How can I write the

impl Fn for Clf{
    extern "rust-call" fn call ...
    ...
}

in this case?

Answer 1

This is indeed possible, but you need a new trait and a ton of mess.

If you start with the abstraction

enum VecOrScalar<T> {
    Scalar(T),
    Vector(Vec<T>),
}

use VecOrScalar::*;

You want a way to use the type transformations

T      (hidden) -> VecOrScalar<T> -> T      (known)
Vec<T> (hidden) -> VecOrScalar<T> -> Vec<T> (known)

because then you can take a "hidden" type T, wrap it in a VecOrScalar and extract the real type T with a match.

You also want

T      (known) -> bool      = T::Output
Vec<T> (known) -> Vec<bool> = Vec<T>::Output

but without HKT this is a bit tricky. Instead, you can do

T      (known) -> VecOrScalar<T> -> T::Output
Vec<T> (known) -> VecOrScalar<T> -> Vec<T>::Output

if you allow for a branch that can panic.

The trait will thus be

trait FromVecOrScalar<T> {
    fn put(self) -> VecOrScalar<T>;

    type Output;
    fn get(out: VecOrScalar<bool>) -> Self::Output;
}

with implementations

impl<T> FromVecOrScalar<T> for T {
    fn put(self) -> VecOrScalar<T> {
        Scalar(self)
    }

    type Output = bool;
    fn get(out: VecOrScalar<bool>) -> Self::Output {
        match out {
            Scalar(val) => val,
            Vector(_) => panic!("Wrong output type!"),
        }
    }
}

impl<T> FromVecOrScalar<T> for Vec<T> {
    fn put(self) -> VecOrScalar<T> {
        Vector(self)
    }

    type Output = Vec<bool>;
    fn get(out: VecOrScalar<bool>) -> Self::Output {
        match out {
            Vector(val) => val,
            Scalar(_) => panic!("Wrong output type!"),
        }
    }
}

Your class

#[derive(Copy, Clone)]
struct Clf {
    x: f64,
}

will first implement the two branches:

impl Clf {
    fn calc_scalar(self, f: f64) -> bool {
        f > self.x
    }

    fn calc_vector(self, v: Vec<f64>) -> Vec<bool> {
        v.into_iter().map(|x| self.calc_scalar(x)).collect()
    }
}

Then it will dispatch by implementing FnOnce for T: FromVecOrScalar<f64>

impl<T> FnOnce<(T,)> for Clf
    where T: FromVecOrScalar<f64>
{

with types

    type Output = T::Output;
    extern "rust-call" fn call_once(self, (arg,): (T,)) -> T::Output {

The dispatch first boxes the private type up, so you can extract it with the enum, and then T::gets the result, to hide it again.

        match arg.put() {
            Scalar(scalar) =>
                T::get(Scalar(self.calc_scalar(scalar))),
            Vector(vector) =>
                T::get(Vector(self.calc_vector(vector))),
        }
    }
}

Then, success:

fn main() {
    let c = Clf { x : 0.0 };
    let v = vec![-1.0, 0.5, 1.0];
    println!("{}", c(0.5f64));
    println!("{:?}", c(v));
}

Since the compiler can see through all of this malarky, it actually compiles away completely to the basically the same assembly as a direct call to the calc_ methods.

But that's not to say it's nice to write. Overloading like this is a pain, fragile and most certainly A Bad Idea™. Don't do it, though it's fine to know that you can.

Answer 2

The short answer is: You can't. At least it won't work the way you want. I think the best way to show that is to walk through and see what happens, but the general idea is that Rust doesn't support function overloading.

For this example, we will be implementing FnOnce, because Fn requires FnMut which requires FnOnce. So, if we were to get this all sorted, we could do it for the other function traits.

First, this is unstable, so we need some feature flags

#![feature(unboxed_closures, fn_traits)]

Then, let's do the impl for taking an f64:

impl FnOnce<(f64,)> for Clf {
    type Output = i32;
    extern "rust-call" fn call_once(self, args: (f64,)) -> i32 {
        if args.0 > self.x {
            1
        } else {
            0
        }
    }
}

The arguments to the Fn family of traits are supplied via a tuple, so that's the (f64,) syntax; it's a tuple with just one element.

This is all well and good, and we can now do c(0.5), although it will consume c until we implement the other traits.

Now let's do the same thing for Vecs:

impl FnOnce<(Vec<f64>,)> for Clf {
    type Output = Vec<i32>;
    extern "rust-call" fn call_once(self, args: (Vec<f64>,)) -> Vec<i32> {
        args.0.iter().map(|&f| if f > self.x { 1 } else { 0 }).collect()
    }
}

Now, we have a problem. If we try c(v) or even c(0.5) (which worked before), we get an error about the type of the function not being known. Basically, Rust doesn't support function overloading. But we can still call the functions using ufcs, where c(0.5) becomes FnOnce::call_once(c, (0.5,)).

---

Not knowing your bigger picture, I would want to solve this simply by giving Clf two functions like so:

impl Clf {
    fn classify(&self, val: f64) -> u32 {
        if val > self.x { 1 } else { 0 }
    }

    fn classify_vec(&self, vals: Vec<f64>) -> Vec<u32> {
        vals.map(|v| self.classify(v)).collect()
    }
}

Then your use example becomes

let c = Clf { x : 0 };
let v = vec![-1, 0.5, 1];
println!("{}", c.classify(0.5));     // prints 1
println!("{}", c.classify_vec(v));       // prints [0, 1, 1]

I would actually want to make the second function classify_slice and take &[f64] to be a bit more general, then you could still use it with vecs by referencing them: c.classify_slice(&v).

Answer 3

You can't.

First of all, implementing the Fn* family of traits explicitly is unstable and subject to change at any time, so it'd be a bad idea to depend on that.

Secondly, and more importantly, the Rust compiler just will not let you call a value that has Fn* implementations for different argument types. It just can't work out what you want it to do, since there's normally no way for it to happen. The only way around that is fully specifying the trait you wanted to call, but at that point, you've lost any possible ergonomic benefit of this approach.

Just define and implement your own trait instead of trying to use the Fn* traits. I took some liberties with the question to avoid/fix questionable aspects.

struct Clf {
    x: f64,
}

trait ClfExt<T: ?Sized> {
    type Result;
    fn classify(&self, arg: &T) -> Self::Result;
}

impl ClfExt<f64> for Clf {
    type Result = bool;
    fn classify(&self, arg: &f64) -> Self::Result {
        *arg > self.x
    }
}

impl ClfExt<[f64]> for Clf {
    type Result = Vec<bool>;
    fn classify(&self, arg: &[f64]) -> Self::Result {
        arg.iter()
            .map(|v| self.classify(v))
            .collect()
    }
}

fn main() {
    let c = Clf { x : 0.0 };
    let v = vec![-1.0, 0.5, 1.0];
    println!("{}", c.classify(&0.5f64));
    println!("{:?}", c.classify(&v[..]));
}

---

Note: included for the sake of completeness; do not actually do this. Not only is it unsupported, it's damn ugly.

#![feature(fn_traits, unboxed_closures)]

#[derive(Copy, Clone)]
struct Clf {
    x: f64,
}

impl FnOnce<(f64,)> for Clf {
    type Output = bool;
    extern "rust-call" fn call_once(self, args: (f64,)) -> Self::Output {
        args.0 > self.x
    }
}

impl<'a> FnOnce<(&'a [f64],)> for Clf {
    type Output = Vec<bool>;
    extern "rust-call" fn call_once(self, args: (&'a [f64],)) -> Self::Output {
        args.0.iter().cloned()
            .map(|v| { FnOnce::call_once(self, (v,)) })
            .collect()
    }
}

fn main() {
    let c = Clf { x : 0.0 };
    let v = vec![-1.0, 0.5, 1.0];
    println!("{}", FnOnce::call_once(c, (0.5f64,)));
    println!("{:?}", FnOnce::call_once(c, (&v[..],)));
}