Visitor pattern

In short: The visitor pattern separates an operation from the object structure it works on, so you can add new operations without changing the classes.

In more detail: Each element offers an accept(visitor) method and calls visitor.visit(this) inside it. The visitor holds the matching code per element type (“double dispatch”).

In Depth

• Typical example: A syntax tree of nodes such as number, addition, variable. Visitors are “evaluate”, “print”, “optimise”.
• Advantage: A new operation is a new visitor class, without touching the node classes.
• Disadvantage: New node types force every visitor to change. The pattern pays off when operations change more often than types.
• Alternatives: In languages with pattern matching over sum types (Rust, Kotlin, TypeScript, modern Java) a match often replaces the visitor.
• Known from: Compilers, file-system walks, ast.NodeVisitor in Python.

See also: Iterator pattern