Disallow more subtypes

General Usage
1 
abstract type Foo end

struct FooA <: Foo 
x
end

struct FooB <: Foo
x
end

I want to disallow any other concrete types of Foo. Is that possible?

2

Not really. What is your use case for that?

3

if you are using Foo for dispatch, you could do:

struct FooA
  x
end

struct FooB
  x
end

const Foo = Union{FooA,FooB}

that will make Foo impossible to extend, but available for dispatch