AST Class/Visitor Pattern
When a discriminated union has 3+ variants and 2+ dispatch sites (renderers, serializers, classifiers, etc.), replace plain union + switch with frozen subclasses and a visitor interface. This makes adding a new variant a compiler error at every consumer, instead of a silent omission.
Structure
Four pieces, usually in one file — the Mongo DDL set below spreads them over three:
// 1. Abstract base (not exported — consumers use the union type)
abstract class FooNode {
abstract readonly kind: string;
abstract accept<R>(visitor: FooVisitor<R>): R;
protected freeze(): void { Object.freeze(this); }
}
// 2. Visitor interface
export interface FooVisitor<R> {
bar(node: BarNode): R;
baz(node: BazNode): R;
}
// 3. Concrete subclasses — readonly fields, freeze() in constructor
export class BarNode extends FooNode {
readonly kind = 'bar' as const;
readonly value: string;
constructor(value: string) {
super();
this.value = value;
this.freeze();
}
accept<R>(visitor: FooVisitor<R>): R { return visitor.bar(this); }
}
export class BazNode extends FooNode {
readonly kind = 'baz' as const;
readonly count: number;
constructor(count: number) {
super();
this.count = count;
this.freeze();
}
accept<R>(visitor: FooVisitor<R>): R { return visitor.baz(this); }
}
// 4. Union type
export type Foo = BarNode | BazNode;
Consuming
Define a visitor object (or class) per concern:
const renderVisitor: FooVisitor<string> = {
bar(node) { return node.value; },
baz(node) { return String(node.count); },
};
function render(node: Foo): string {
return node.accept(renderVisitor);
}
Always construct instances, never frozen object literals
This holds everywhere a node is built — tests and production construction surfaces (contract-free factories, builders). A factory must return new BarNode(...), never Object.freeze({ kind: 'bar', value: 'x' }). A frozen plain object has no prototype, so instanceof fails, accept() is missing, and a downstream shallow-copy ({ ...node }) silently strips the type back to an anonymous bag; constructor-time invariants are skipped too.
// ✅
const call = new BarNode('x');
export function bar(value: string): BarNode { return new BarNode(value); }
// ❌
const call: Foo = { kind: 'bar', value: 'x' };
export function bar(value: string): Foo { return Object.freeze({ kind: 'bar', value }); }
When NOT to use
- Single dispatch site → plain union + switch is simpler
- Fewer than 3 variants with no expected growth → not worth the boilerplate
Codebase examples
MongoAstNode base — packages/2-mongo-family/4-query/query-ast/src/ast-node.tsMongoDdlCommandVisitor interface — packages/2-mongo-family/4-query/query-ast/src/ddl-visitors.ts- Concrete DDL commands and their union —
packages/2-mongo-family/4-query/query-ast/src/ddl-commands.ts