Add ast.Walk example demonstrating how to process links

ast/example_test.go

@@ -0,0 +1,39 @@
+package ast_test
+
+import (
+	"fmt"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+)
+
+func ExampleWalk() {
+	// Extract links from markdown text
+	src := []byte(`Some links: <https://golang.org/>, [goldmark repo](https://github.com/yuin/goldmark)`)
+
+	fn := func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+		switch n.Kind() {
+		case ast.KindLink, ast.KindAutoLink:
+		default:
+			return ast.WalkContinue, nil
+		}
+		if l, ok := n.(*ast.AutoLink); ok {
+			fmt.Printf("auto link: %s\n", l.URL(src))
+		}
+		if l, ok := n.(*ast.Link); ok {
+			fmt.Printf("link: %s\n", l.Destination)
+		}
+		return ast.WalkContinue, nil
+	}
+
+	node := goldmark.DefaultParser().Parse(text.NewReader(src))
+	_ = ast.Walk(node, fn)
+	// Output:
+	//
+	// auto link: https://golang.org/
+	// link: https://github.com/yuin/goldmark
+}