Passing functions and unknown overloads

[ConnorBaker]

Hello all! I'm using Mojo to follow along with the Ray Tracing in One Weekend series: https://github.com/ConnorBaker/mojo_rt.

Background

Fairly early on, I ran into a problem due to lack of type classes. So far, I've only needed the equivalent of type classes with a single function. I've worked around this by defining an alias for the function signature, manually making sure that the would-be type class instances have the same function signature, and then referring to them by the alias.

As an example, the book describes a generic Hittable class with a single function hit. It then describes the Sphere class, which implements Hittable, and describes the world as a HittableList.

In Mojo, Hittable is a function signature:

# Function must be capturing because it is a struct method and closes
# over the struct instance.
alias Hittable = fn (Ray3, Interval, /) capturing -> HitRecord
"""A function signature for a hittable object."""

Then, Sphere is a struct with a method hit:

@value
@register_passable("trivial")
struct Sphere:
    """A sphere in 3D space."""

    var center: Point3
    var radius: F

    fn hit(self, r: Ray3, ray_t: Interval, /) -> HitRecord:
        ...

And our HittableList is a wrapper around DynamicVector[Hittable]:

@value
struct HittableList:
    """A list of hittable objects."""

    var value: DynamicVector[Hittable]
    ...

Problem

I'd like to construct my world like so:

fn setup_world() -> HittableList:
    let green_horizon: Sphere = Sphere(Point3(y=-100.5, z=-1.0), 100.0)
    let normal_mapped_sphere: Sphere = Sphere(Point3(z=-1.0), 0.5)

    var world = HittableList()
    world.add(green_horizon.hit)
    world.add(normal_mapped_sphere.hit)

    return world

However, that doesn't work. I get the following error:

/Users/connorbaker/Packages/mojo_rt/main.mojo:17:14: error: invalid call to 'add': method argument #0 cannot be converted from unknown overload to 'fn(Ray3, Interval, /) capturing -> HitRecord'
    world.add(green_horizon.hit)
    ~~~~~~~~~^~~~~~~~~~~~~~~~~~~
/Users/connorbaker/Packages/mojo_rt/main.mojo:17:15: note: try resolving the overloaded function first
    world.add(green_horizon.hit)
              ^~~~~~~~~~~~~~~~~
Included from /Users/connorbaker/Packages/mojo_rt/main.mojo:3:
Included from /Users/connorbaker/Packages/mojo_rt/mojo_rt/camera.mojo:3:
/Users/connorbaker/Packages/mojo_rt/mojo_rt/hittable_list.mojo:16:5: note: function declared here
    fn add(inout self, h: Hittable) -> None:
    ^
/Users/connorbaker/Packages/mojo_rt/main.mojo:18:14: error: invalid call to 'add': method argument #0 cannot be converted from unknown overload to 'fn(Ray3, Interval, /) capturing -> HitRecord'
    world.add(normal_mapped_sphere.hit)
    ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/connorbaker/Packages/mojo_rt/main.mojo:18:15: note: try resolving the overloaded function first
    world.add(normal_mapped_sphere.hit)
              ^~~~~~~~~~~~~~~~~~~~~~~~
Included from /Users/connorbaker/Packages/mojo_rt/main.mojo:3:
Included from /Users/connorbaker/Packages/mojo_rt/mojo_rt/camera.mojo:3:
/Users/connorbaker/Packages/mojo_rt/mojo_rt/hittable_list.mojo:16:5: note: function declared here
    fn add(inout self, h: Hittable) -> None:
    ^
mojo: error: failed to parse the provided Mojo

I find it odd that the following does work:

fn setup_world() -> HittableList:
    let green_horizon: Hittable = Sphere(Point3(y=-100.5, z=-1.0), 100.0).hit
    let normal_mapped_sphere: Hittable = Sphere(Point3(z=-1.0), 0.5).hit

    var world = HittableList()
    world.add(green_horizon)
    world.add(normal_mapped_sphere)

    return world

My setup_world function is here: https://github.com/ConnorBaker/mojo_rt/blob/9602d9d86c11ac8249806a421ef7cac47c8a7665/main.mojo#L12-L31.

Why does one variant work while the other doesn't?