How Can I Extract All Actions from a Parsed HTML Template?

How to Inspect a Parsed Template for Embedded Actions

Problem Statement

Given a parsed template, how can we retrieve a list of all the actions defined within it?

Solution

Disclaimer: Internal API Usage

Note that the method we'll use, Template.Tree, is not intended for external use. It's exported solely for the internal workings of the html/template package. Relying on it in your code can lead to unexpected behavior in future releases of the package.

Walkthrough

A parsed template is represented internally as a tree (parse.Tree) comprising various nodes. Each node represents a specific element within the template, such as text, fields, function calls, and so on.

To find the actions, we'll traverse this tree recursively, looking for nodes of type parse.NodeAction. These nodes represent non-control actions, such as field evaluations.

Implementation

import (
    "fmt"
    "html/template"
    "text/template/parse"
)

func ListTemplateFields(t *template.Template) []string {
    return listNodeFields(t.Tree.Root, nil)
}

func listNodeFields(node parse.Node, results []string) []string {
    if node.Type() == parse.NodeAction {
        results = append(results, node.String())
    }

    if listNode, ok := node.(*parse.ListNode); ok {
        for _, n := range listNode.Nodes {
            results = listNodeFields(n, results)
        }
    }

    return results
}

Example Usage

t := template.Must(template.New("example").Parse(`{{ .name }} {{ .age }}`))
fields := ListTemplateFields(t)
fmt.Println(fields)

Output

[{{.name}} {{.age}}]

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