Visitor design
- TypeScript course
- Advanced
- Lesson 50/57
- 5 min
Write the AST visitor with the visitor pattern.
Visitor design
The visitor pattern allows performing a separate operation for each node type while traversing a data structure. An AST visitor contains a visit method for each node type; the traverse function takes a node and calls the appropriate visit method.
interface Visitor {
visitNumber(value: number): void;
visitString(value: string): void;
visitAdd(): void;
}
function traverse(n: Node, v: Visitor): void {
switch (n.kind) {
case "number": v.visitNumber(n.value); break;
case "string": v.visitString(n.value); break;
case "add": v.visitAdd(); break;
}
}Walking with traverse
The traverse function recursively walks the AST. For each node the relevant visit method is called; for nodes with children (e.g. add) the children are also traversed. This processes the whole tree.
function traverse(n: Node, v: Visitor): void {
switch (n.kind) {
case "number": v.visitNumber(n.value); break;
case "string": v.visitString(n.value); break;
case "add":
v.visitAdd();
traverse(n.left, v);
traverse(n.right, v);
break;
}
}Practice Walk the AST with a visitor
Define the Node discriminated union type ("number" (value: number), "string" (value: string), "add" (left: Node, right: Node)). Define the Visitor interface: with visitNumber(value: number): void, visitString(value: string): void, visitAdd(): void methods. Write a traverse(n: Node, v: Visitor): void function using switch. Create a Visitor implementation: visitNumber does console.log("n:" + value), visitString does console.log("s:" + value), visitAdd does console.log("add"). Call traverse({ kind: "add", left: { kind: "number", value: 1 }, right: { kind: "string", value: "x" } }, visitor).
Starter code
// Node discriminated union türünü tanımla (kind: number, string, add)
// Visitor interface tanımla (visitNumber, visitString, visitAdd)
// traverse fonksiyonunu switch ile yaz
// Visitor uygulaması oluştur ve traverse çağırSample solution
type Node =
| { kind: "number"; value: number }
| { kind: "string"; value: string }
| { kind: "add"; left: Node; right: Node };
interface Visitor {
visitNumber(value: number): void;
visitString(value: string): void;
visitAdd(): void;
}
function traverse(n: Node, v: Visitor): void {
switch (n.kind) {
case "number":
v.visitNumber(n.value);
break;
case "string":
v.visitString(n.value);
break;
case "add":
v.visitAdd();
traverse(n.left, v);
traverse(n.right, v);
break;
}
}
const visitor: Visitor = {
visitNumber: (value) => console.log("n:" + value),
visitString: (value) => console.log("s:" + value),
visitAdd: () => console.log("add"),
};
traverse({ kind: "add", left: { kind: "number", value: 1 }, right: { kind: "string", value: "x" } }, visitor);Expected output
add n:1 s:x
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the core purpose of the visitor pattern?
- To hide runtime errors
- To merge node types
- To perform a separate operation for each node type Correct answer
- To delete nodes
Explanation: The visitor pattern defines a separate visit method for each node type; the traverse function calls the correct method based on the node type.
2 Visitor interface method
interface Visitor {
visitNumber(value: number): void;
visitString(value: string): void;
visit____(): void;
}interface Visitor {
visitNumber(value: number): void;
visitString(value: string): void;
visitAdd(): void;
}Explanation: The visitor pattern contains a separate visit method for each node type; visitAdd is called for the add node.
3 Visitor design parts
| Term | Meaning |
|---|---|
Visitor interface |
visit methods |
traverse fonksiyonu |
walks the tree |
switch (n.kind) |
calls correct visit |
özyinelemeli traverse |
processes children |