Table of Contents

Visitor Pattern (Fluent)

TL;DR: Dispatch operations by runtime type using a fluent builder - add new operations without modifying visited types.

Quick Example

// Result visitor: produces a value
var formatter = Visitor<Node, string>.Create()
    .On<Add>(_ => "+")
    .On<Number>(n => $"#{n.Value}")
    .Default(_ => "?")
    .Build();

formatter.Visit(new Add(...));    // "+"
formatter.Visit(new Number(7));   // "#7"

// Action visitor: side effects only
var counter = ActionVisitor<Node>.Create()
    .On<Number>(_ => count++)
    .On<Add>(_ => Log("add"))
    .Default(_ => { })
    .Build();

counter.Visit(node);

What It Is

Visitor separates operations from the objects they operate on. PatternKit's fluent visitor provides type-safe dispatch by runtime type with a builder API.

Key characteristics:

  • Non-intrusive: No Accept method required on visited types
  • First-match-wins: Handlers evaluated in registration order
  • Type-safe: Strongly-typed handlers receive concrete types
  • Immutable: Thread-safe after Build()

Pattern Variants

Variant Description Use Case
Visitor<TBase, TResult> Sync visitor returning a value Formatting, transformation
ActionVisitor<TBase> Sync visitor for side effects Logging, counting
AsyncVisitor<TBase, TResult> Async visitor returning a value I/O operations
AsyncActionVisitor<TBase> Async visitor for side effects Async notifications

When to Use

  • Stable type hierarchies: AST nodes, payments, UI elements
  • Multiple operations: Add formatters, validators, analyzers
  • Avoid class modification: Operations external to types
  • Centralized behavior: One place for type-specific logic

When to Avoid

  • Few operations: Virtual methods may be simpler
  • Pattern matching suffices: C# switch expressions work well
  • Frequently changing types: Each new type needs new handlers

Diagram

classDiagram
    class Visitor~TBase,TResult~ {
        +Visit(in TBase) TResult
        +TryVisit(in TBase, out TResult) bool
    }
    class Builder {
        +On~T~(handler) Builder
        +Default(handler) Builder
        +Build() Visitor
    }

    Visitor --> Builder : Create()

    class Node {
        <<abstract>>
    }
    class Add
    class Number
    class Mul

    Node <|-- Add
    Node <|-- Number
    Node <|-- Mul
    Visitor --> Node : visits

See Also

Edit this page