Excerpt from draft of code using Structured Data

class Node(adt.Sum):
    __slots__ = ()

    Inner: adt.Ctor[BinOp, Node, Node]
    Leaf: adt.Ctor[int]

    @match.function
    def eval(self) -> int:
        """Evaluate this node to an int."""


@Node.eval.when(self=Node.Inner(match.pat.opr, match.pat.left, match.pat.right))
def _eval_inner(opr, left, right):
    return opr(left.eval(), right.eval())


@Node.eval.when(self=Node.Leaf(match.pat.value))
def _eval_leaf(value):
    return value