Visitor

M4 · Trees and RecursionSession 8-9BehavioralReshaped in KotlinOutline

Intent. Add a new operation over an object structure without modifying the structure.

In Kotlin. A sealed hierarchy with an exhaustive when is the opposite trade-off, not a replacement.

Add an operation to a structure you are not allowed to change. This is where the Expression Problem shows up: Visitor makes new operations cheap and new types expensive, while a sealed class with an exhaustive when makes exactly the opposite trade. Neither one replaces the other, and knowing which side you are on is the point of this session.

Structure

classDiagram
    class Visitor {
        <<interface>>
        +visitCircle(c)
        +visitSquare(s)
    }
    class AreaVisitor
    class RenderVisitor
    class Shape {
        <<interface>>
        +accept(v)
    }
    class Circle {
        +accept(v)
    }
    class Square {
        +accept(v)
    }
    Visitor <|.. AreaVisitor
    Visitor <|.. RenderVisitor
    Shape <|.. Circle
    Shape <|.. Square
    Circle ..> Visitor : accept(v) calls back