Trait example from "Rust By Example"

Here is a Julia implementation from this trait example in Rust By Example.

## Define the object

mutable struct Sheep
    name::String
    naked::Bool
end
Sheep(name) = Sheep(name, false)

is_naked(x::Sheep) = x.naked
name(x::Sheep) = x.name
noise(x::Sheep) = is_naked(x) ? "baaaah?" : "baaah!"

## For now, let's not define the custom version of `talk` and let it use
## the trait version.
# talk(x::Sheep) = println(name(x), " pauses briefly... ", noise(x))

function shear(x::Sheep) 
    if is_naked(x)
        println(name(x), " is already naked...")
    else
        x.naked = true
        println(name(x), " gets a haircut!")
    end
end

## Define the trait

struct Animal end
struct NonAnimal end

isanimal(x::T) where T =
    # The Animal trait has to have the following methods:
    if hasmethod(name, Tuple{T}) &&
       hasmethod(noise, Tuple{T})
        Animal()
    else 
        NonAnimal()
    end

talk(x::T) where T = talk(isanimal(x), x)
talk(::Animal, x) = println(name(x), " says ", noise(x))

## Let's try it

dolly = Sheep("Dolly")

talk(dolly)
shear(dolly)
talk(dolly)

Right now, this uses dynamic dispatch because of hasmethod. For a static version of hasmethod, see Tricks.jl.

For more on Julia traits, see Lyndon White's blog post and the Julia manual here.

tshort/Trait example from Rust.md