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# Visitors, Walkers, and Rewriters
Use `MarkupVisitor` to transform, walk, and rewrite markup trees.
## Markup Visitor
The core ``MarkupVisitor`` protocol provides the basis for transforming, walking, or rewriting a markup tree.
```swift
public protocol MarkupVisitor {
associatedtype Result
}
```
Using its ``MarkupVisitor/Result`` type, you can transform a markup tree into anything: another markup tree, or perhaps a tree of XML or HTML elements. There are two included refinements of `MarkupVisitor` for common uses.
## Markup Walker
The first refinement of `MarkupVisitor`, ``MarkupWalker``, has an associated `Result` type of `Void`, so it's meant for summarizing or detecting aspects of a markup tree. If you wanted to append to a string as elements are visited, this might be a good tool for that.
```swift
import Markdown
/// Counts `Link`s in a `Document`.
struct LinkCounter: MarkupWalker {
var count = 0
mutating func visitLink(_ link: Link) {
if link.destination == "https://swift.org" {
count += 1
}
descendInto(link)
}
}
let source = "There are [two](https://swift.org) links to <https://swift.org> here."
let document = Document(parsing: source)
print(document.debugDescription())
var linkCounter = LinkCounter()
linkCounter.visit(document)
print(linkCounter.count)
// 2
```
## Markup Rewriter
The second refinement, ``MarkupRewriter``, has an associated `Result` type of an optional ``Markup`` element, so it's meant to change or even remove elements from a markup tree. You can return `nil` to delete an element, or return another element to substitute in its place.
```swift
import Markdown
/// Delete all **strong** elements in a markup tree.
struct StrongDeleter: MarkupRewriter {
mutating func visitStrong(_ strong: Strong) -> Markup? {
return nil
}
}
let source = "Now you see me, **now you don't**"
let document = Document(parsing: source)
var strongDeleter = StrongDeleter()
let newDocument = strongDeleter.visit(document)
print(newDocument!.debugDescription())
// Document
// └─ Paragraph
// └─ Text "Now you see me, "
```
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