Consider using classes rather than types with factory functions

[rlivings39]

Writing

export type UnaryMinus = { kind: "unary-minus"; expr: Expr; op: "-" };
export function UnaryMinus(expr: Expr): UnaryMinus {
  return { kind: "unary-minus", expr, op: "-" };
}

instead of

export class UnaryMinus {
  readonly kind = "unary-minus";
  readonly op = "-";
  constructor(public expr: Expr) {}
}

seems more verbose and wasteful. See if there are tradeoffs and consider switching.