proposal: text/template/parse: traversal utility

[tylermmorton]

When using the package text/template/parse to analyze a template ParseTree, it is common to want to visit all Nodes in a tree, even when they are nested in other nodes.

I've implemented a Traverse function that works recursively from a tree root:

package traverse

import (
	"text/template/parse"
)

func Traverse(cur parse.Node, visitor func(parse.Node)) {
	switch node := cur.(type) {
	case *parse.ActionNode:
		if node.Pipe != nil {
			Traverse(node.Pipe, visitor)
		}
	case *parse.BoolNode:
	case *parse.BranchNode:
		if node.Pipe != nil {
			Traverse(node.Pipe, visitor)
		}
		if node.List != nil {
			Traverse(node.List, visitor)
		}
		if node.ElseList != nil {
			Traverse(node.ElseList, visitor)
		}
	case *parse.BreakNode:
	case *parse.ChainNode:
	case *parse.CommandNode:
		if node.Args != nil {
			for _, arg := range node.Args {
				Traverse(arg, visitor)
			}
		}
	case *parse.CommentNode:
	case *parse.ContinueNode:
	case *parse.DotNode:
	case *parse.FieldNode:
	case *parse.IdentifierNode:
	case *parse.IfNode:
		Traverse(&node.BranchNode, visitor)
	case *parse.ListNode:
		if node.Nodes != nil {
			for _, child := range node.Nodes {
				Traverse(child, visitor)
			}
		}
	case *parse.NilNode:
	case *parse.NumberNode:
	case *parse.PipeNode:
		if node.Cmds != nil {
			for _, cmd := range node.Cmds {
				Traverse(cmd, visitor)
			}
		}
		if node.Decl != nil {
			for _, decl := range node.Decl {
				Traverse(decl, visitor)
			}
		}
	case *parse.RangeNode:
		Traverse(&node.BranchNode, visitor)
	case *parse.StringNode:
	case *parse.TemplateNode:
		if node.Pipe != nil {
			Traverse(node.Pipe, visitor)
		}
	case *parse.TextNode:
	case *parse.VariableNode:
	case *parse.WithNode:
		Traverse(&node.BranchNode, visitor)
	}
	visitor(cur)
}

Is this something that could be added to text/template/parse as a utility function? I believe that a cleaned up version of this or something similar to the ast.Walk interface could increase the utility of the parse package and open the door for static analysis tooling around go templates.

[seankhliao]

cc @robpike

[zokrezyl]

This could have saved me couple of days of research. Thanks a lot
Some additional features could be:

• either have two visitors, or the visitor should receive a bool flag for pre-visiting and post-visiting
• also a generic context object would be great
• for situations where one would do tree transformation output should be also someway built in
• the visitors shall be able to return either bool or pointer. Depending of the values, the visit should or not continue deeper