Function sub typing of Functors

General Usage Performance
1

When defining a functor should the struct be subtyped from Function?

For instance

struct foo <: Function 
   a
   b
end

function (functor::foo)(x, y) 
    return functor.a*x + functor.b*y
end

or just is fine?

struct foo 
   a
   b
end
2

No, the functor is like a closure, it is a function and the struct is a parameter, which can be of any type.