Ast Visitor Pattern

prisma/ast-visitor-pattern

>- Use the frozen-class/visitor pattern for discriminated unions that have multiple dispatch sites. Use when creating a new set of variants (commands, IR nodes, factory calls) that will be switched over in 2+ places, or when refactoring an existing union type that has grown multiple switch sites.

805 tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

418

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/prisma/prisma --skill ast-visitor-pattern

The instruction itself

6 sections, as written by the author

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, always in the same file:

// 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 / MongoDdlCommandVisitor — packages/2-mongo-family/4-query/query-ast/src/ddl-commands.ts
OpFactoryCall / OpFactoryCallVisitor — packages/3-mongo-target/1-mongo-target/src/core/op-factory-call.ts

How to use it

Copy the folder

Take prisma/ast-visitor-pattern from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

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