Why Quaternion isn't type of Number?

[garazha-ilya]

1. It's just a matter of curiosity. But why is a Quaternion not a subtype of Number?
   Like in complex.jl:

struct Complex{T<:Real} <: Number
    re::T
    im::T
end

2. Should the following function be implemented?

-(q::Quaternion, x::Real) = Quaternion(real(q) - x, imag(q))

[ronisbr]

Hi @garazha-ilya !

It's just a matter of curiosity. But why is a Quaternion not a subtype of Number?
Like in complex.jl:

For reference frame rotations (and attitude control theory), a quaternion is historically represented by a 4 x 1 vector. That's why I decided to not make our implementation an extension of a number.

Should the following function be implemented?

I have to think, but it seems we can define +, -, and * against a quaternion and a number. However, what is the use case? I have never seen something like that useful for reference frame rotations.