examples.md

Visitor — where it lives outside this repo

Cited, real implementations to study (or point an agent at) when designing or reviewing visitor-shaped code.

Python standard library

• ast.NodeVisitor / ast.NodeTransformer. The classic form as a supported API: subclass, implement visit_ClassName per node, call generic_visit to recurse. When the tree is Python source, this is the right tool — the stdlib owns the node family, you own the operation. docs.python.org/3/library/ast.html#ast.NodeVisitor
• functools.singledispatch. The deletion of the pattern's plumbing: dispatch on argument type, registered by annotation — what this unit's Operation wraps with a strict default. docs.python.org/3/library/functools.html#functools.singledispatch

Major ecosystems

• pylint checkers. Every lint rule is a visitor: checkers implement visit_<nodetype> methods over the parsed tree, and new rules ship without touching the node classes — the open-operation-set promise at ecosystem scale. pylint.readthedocs.io
• LibCST. Concrete-syntax-tree visitors and transformers powering large-scale codemods (Instagram's refactors); the visitor as a production migration tool. libcst.readthedocs.io
• SQLAlchemy's visitors module. SQL compilation walks clause trees with visitor machinery (ClauseVisitor, traversal utilities) — the pattern deep inside a library most Python services already depend on. docs.sqlalchemy.org/en/latest/core/visitors.html

What to notice across all of them

The pattern appears wherever the node family is stable and owned by someone else (Python's grammar, SQL clauses) while operations multiply (lint rules, compilers, codemods). None of the Python examples hand-write accept() — dispatch is either a naming convention (visit_X) or singledispatch. When reviewing, check the unknown-node policy: ast's generic_visit deliberately recurses past unknown nodes, lint rules deliberately skip — an exporter that skips unknown nodes is losing data silently.