language: `scoped model` definition

[Jozott00]

I'm not sure how feasible it is, but it would be nice if one could specify models that are only available in the current model scope.

model A(): Ex = {
  scoped model Expr(): Ex = { 2 }
  $Expr()
}

constant X = $A()

$Expr() // error: Unknown Model `$Expr` 

This would allow one to dynamically create a model within a model without causing name conflicts between them.

@flofriday How easy would it be to implement this?

[flofriday]

I thought that's already how they work, since they are quite deeply involved in the SymbolTable (because if they aren't scoped what's even the point to use the SymbolTable at all 😅)

Will investigate this and come back to you.
But I wouldn't add a new keyword for it all models should be properly scoped, I don't see a need for escape hatches here.

[Jozott00]

The default behavior is that they are not scoped. This allows us to write model factories:
https://openvadl.github.io/openvadl/tutorial.html#autotoc_md16

model BinExFactory(binExName: Id, op: BinOp): IsaDefs = {
  model $binExName(left: Ex, right: Ex): Ex = { 
    $left $op $right
  }
}
$BinExFactory(Addition ; +)
instruction ADD : RType = X(rd) := $Addition(X(rs1) ; X(rs2))

[AndreasKrall]

Currently all model names have a global scope and they are visible everywhere after their definition which can be either directly or indirectly via a model generating a model. I think it could be a useful feature, but it is some work. Instead of using the keyword scoped I would use a keyword or annotation called local. For model factories it is necessary that the default scope is global. Usually models are used globally, it is rare but useful to have local models too which can be only invoked inside the generating model.