InterTypes variants referring to locally defined variants

[p-stokes]

It might be useful to be able to define recursive sum types in InterTypes. Currently, the below example is not possible.

@sum Var begin
  Untyped(var::Symbol)
  Typed(var::Symbol, type::Symbol)
end

@sum UWDTerm begin
  Statement(relation::Symbol, variables::Vector{Var})
  UWDExpr(context::Vector{Var}, statements::Vector{Statement})
  UWDModel(header::String, uwd::UWDExpr)
end

[jpfairbanks]

The problem isn't the recursion. The following test case shows recursion support

@sum Const begin
  ConstStr(val::String)
  ConstInt(val::Int32)
  ConstBool(val::Bool)
end

@sum Term begin
  Constant(val::Const)
  Plus(terms::Vector{Term})
  IfThenElse(ifcase::Term, thencase::Term, elsecase::Term)
end

struct Assignment
  lhs::String
  rhs::Term
end

The problem is that a sum type variant can't refer to a previous variant. I identified that by deleting parts of the following code until it worked.

@sum UWDTerm begin
  Statement(relation::Symbol, variables::Vector{Var})
  UWDExpr(context::Vector{Var}, statements::Vector{Statement})
  UWDModel(header::String, uwd::UWDExpr)
end

The problem is that when processing UWDTerm the InterTypes.parse_intertype_decl is looking for a type called Statement which doesn't exist yet.

@olynch, this is a pretty big reason for using the @data macro. We had the ability to write really compact type specifications. You would be able to workaround it as follows. Except that InterTypes doesn't support subtyping relations.

@sum Var begin
  Untyped(var::Symbol)
  Typed(var::Symbol, type::Symbol)
end

struct Statement # <: UWDTerm
  relation::Symbol
  variables::Vector{Var}
end

struct UWDExpr # <: UWDTerm
  context::Vector{Var}
  statements::Vector{Statement}
end

struct UWDModel # <: UWDTerm
  header::String
  uwd::UWDExpr
end

[p-stokes]

Sorry for the poor phrasing/characterization. Yes, I made the same replacement with UWDTerm and it works in that fashion. I wasn't sure if that was the intended behavior and/or if there might be downstream effects, so I made an issue.