ast-visitor-pattern

# 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: ```typescript // 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: ```typescript 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. ```typescript // ✅ 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.ts` - `MongoDdlCommandVisitor` 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`