Julia type system and behavior-CodePudding

I'm still trying to wrap my head around Julia's types. For example assuming I wanted to define the structure of an Animal object. Taking as example typescript I would do something like this:

interface Animal {
    sound: string;
}

function sing(animal: Animal) {
    console.log(animal.sound);
}

const dog: Animal = {
    sound: 'woof',
};

// OR
class Dog implements Animal {
    sound = 'woof';
    // other Dog methods here
    run() {}
}

// Wrong implementations are caught immediately
const cow: Animal = {
    noise: 'bark'
}

// Error: Type '{ noise: string; }' is not assignable to type 'Animal'.
// Object literal may only specify known properties, and 'noise' does not exist in type 'Animal'.

How can I do this in Julia? In other words, Animal needs to have a .sound. This is what I tried:

abstract type Animal end

function sing(animal::Animal) 
    println(animal.sound)
end

struct Dog <: Animal
    sound::AbstractString
end

function run(dog::Dog) end

struct Cow <: Animal
    noise::AbstractString
end

But:

I don't get any warning from IDE that sing will not work with all the Animal implementation
When I try to run sing(Cow("moo")) I do get an error, but at the function sing!: ERROR: type Cow has no field sound

What am I missing? How can I maintain a large codebase if I get no warning before runtime? How can I define behaviour since I can't subtype concrete classes?

CodePudding user response：

Julia is not object oriented so taking OO is usually not comfortable.

Most often you will want to use multiple dispatch to define behavior for different object types eg.:

abstract type Animal end

getsound(::T) where T <: Animal = throw(ErrorException("not implemented for $T"))
struct Dog <: Animal end
struct Cow <: Animal end
struct Cat <: Animal
   specialsound::String
end
getsound(::Dog) = "hau"
getsound(cat::Cat) = cat.specialsound
sing(animal::Animal) = println(getsound(animal))

And here is the usage:

julia> sing(Dog())
hau

julia> sing(Cat("mew"))
mew

julia> sing(Cow())
ERROR: not implemented for Cow

Now regarding IDE - commenting out getsound(::T) in VS Codium does not underline the cow. However this indeed looks like something that could be added as compiler detects this as an error (assuming getsound(::T) where T <: Animal is not defined) :

julia> @code_typed sing(Cow())
CodeInfo(
1 ─     Main.getsound(animal)::Union{}
└──     unreachable
) => Union{}