Visitor Pattern: Add Operations Without Editing the Classes

Every compiler team hits the same wall. You have an AST: a dozen node types — Num, Add, FunctionCall, IfExpr — and they are done. The grammar is frozen; nobody’s inventing new node kinds this quarter. What is not done is the list of things you need to do to that tree.

First you eval() it. Then product wants a pretty-printer, so you add prettyPrint() to all twelve nodes. Then the type system lands, so you add typecheck() to all twelve. Then a bytecode target, so compile() — all twelve again. Each new operation is a shotgun edit across the entire hierarchy, and worse, one operation’s logic is now smeared across twelve files. The type-checker for Add lives in Add.java, the type-checker for IfExpr lives in IfExpr.java, and reading “how does typechecking work” means opening a dozen tabs.

The nodes are stable. The operations are what multiply. Your code is organized along the wrong axis — and the Gang of Four named the fix.

The intuition: the tax auditor

A tax auditor visits businesses all day — a restaurant, a farm, a software startup. The auditor is one operation (“audit”), but the audit behaves differently per business type: for the restaurant they check food-safety receipts, for the farm they check subsidy filings, for the startup they check stock-option accounting.

Notice who holds the logic. The restaurant doesn’t know how to audit itself; it just opens its books and says “here, I’m a restaurant — do your restaurant thing.” The knowledge of what an audit means lives entirely in the auditor. Next year a new auditor shows up — a fire-safety inspector — and none of the businesses change at all. They just accept a different visitor.

That auditor is the visitor. Each business is an element. The element’s only job is to announce its concrete type and hand itself over; the operation lives in the visitor.

From here the members-only walkthrough takes over: the expression-problem tradeoff spelled out, the full code (the two contracts, the accept() bounce, and two drop-in visitors), the class-diagram structure, a step-by-step trace, the real-world visitors you can open right now, and the interview corner with a hands-on AST refactor challenge and a quiz.