Nodes

COL programs are stored in a tree-like immutable data structure. All nodes are defined in src/col/vct/col/ast/Node.scala. Other than some primitive types and collection types like Int, String, Seq[_] and tuples (_, _), all types in the tree are descendants of Node.

Nodes are split into two kinds:

• Descendants of NodeFamily. These are regular nodes, like expressions and statements.
  • They have structural equality, meaning comparing them constitutes comparing their fields.
  • It makes sense to say that each family of nodes should rewrite to itself: one Statement should always be rewritten to another Statement.
• Descendants of Declaration. These are nodes to which we can refer.
  • They have reference equality, which means a new Variable(TInt) != new Variable(TInt).
  • It make sense to ask in which scope the declaration can be referred to. For example, a reference to an argument makes no sense outside the method it is declared in.
  • We should be able to rewrite a declaration to zero, one or more successors of itself.

We will not exhaustively describe all kinds of node here, but we highlight the ones that have special Properties.

Expr

All expressions have a type:

trait ExprImpl[G] { this: Expr[G] =>
	def t: Type[G]
}

You may presume that the node is checked before its type is queried, and may crash otherwise. The type of expressions is closely related to coercion.

Type

The relations between types must be defined in Types.leastCommonSuperType and CoercionUtils.getCoercion.