How to Extract Template Field Names from a Parsed Go Template?

Retrieving a List of Template Actions from a Parsed Template

How can you extract the templates defined in a template as a slice of strings? Consider a template like:

<h1 id="name-age">{{ .name }} {{ .age }}</h1>

You wish to obtain []string{"name", "age"}.

Inspecting the Parsed Template Tree

The parsed template is represented by a template.Tree containing details about the template structure. Each node of this tree has a Node.Type() method that provides information about its type. Relevant types include:

• NodeAction: Non-control actions such as field evaluations
• NodeField: Field or method names

Iterating the Tree

To identify actions within the template, you can iterate through the tree and explore the nodes. The following sample function demonstrates this process:

import (
    "text/template/parse"
)

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

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

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

Example Usage

To determine the fields required for the template:

t := template.Must(template.New("cooltemplate").
    Parse(`<h1 id="name-age">{{ .name }} {{ .age }}</h1>`))
fmt.Println(ListTemplFields(t))

The output will be:

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

Note: This demonstration is not comprehensive and may not handle all cases. However, it illustrates the concept of extracting template actions by introspecting the parsed template tree.

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