Replace Expression abstract type with a union type for union splitting optimizations

[ChrisRackauckas]

#62 (comment)

Problem right now is that Operation needs a Vector{Expression} so without JuliaLang/julia#269 we cannot have Expression have Operation in it.

[ChrisRackauckas]

Yingbo Ma [11:42 AM]
On #68, what should we replace Expression with Union{Int, UInt, ..., Symbol}?
ChrisRackauckas
#68 Replace Expression abstract type with a union type for union splitting optimizations
#62 (comment)

Problem right now is that Operation needs a Vector{Expression} so without JuliaLang/julia#269 we cannot have Expression have Operation in it.
JuliaDiffEq/ModelingToolkit.jlToday at 10:31 AMAdded by GitHub

Chris Rackauckas [11:42 AM]
that's pretty much the idea
but one of the things we need in there is Operation
but Operation needs Vector{Expression}
So we can't make Expression = Union{Int, UInt, ..., Symbol}
since we need the definition both before and after
we can do something like
Operation takes a Vector{Union{ExpressionComponents},Operation}

since recursive definitions like that are allowed
that's the whole

  x::Vector{Union{Nothing,M}}
end
M([M([nothing])])```

thing.
Anyways, with the current timings and profiles it is not the biggest issue right now, so I at least feel fine moving on from it

Yingbo Ma [11:45 AM]
>`Operation` takes a `Vector{Union{ExpressionComponents},Operation}`
Do you mean `Vector{Union{ExpressionComponents...,Operation}}`?

Chris Rackauckas [11:45 AM]
it's definitely something to play with
`Vector{Union{ExpressionComponents,Operation}}`
with `ExpressionComponents = Union{Int, UInt, ..., Symbol}`

Yingbo Ma [11:46 AM]
I see, thanks!

Chris Rackauckas [11:46 AM]
or call it expression primatives
so you basically have primatives
or groups of primatives which is the Operation