How to Programmatically Extract Field Actions from Parsed Go Templates?

Understanding Template Fields Extraction from Parsed Templates

Introduction:

Programmatically retrieving a list of template actions defined in parsed Go templates is a common task in certain scenarios. This article explores a technique to accomplish this using the Template.Tree field.

Inspection of Parsed Templates

The parsed representation of a template is stored in the Template.Tree field, accessible as a parse.Tree instance. This tree holds all the components of a template, including actions, texts, and control structures.

Identifying Template Actions

Template actions, such as field evaluation and function calls, are represented by nodes with the parse.NodeAction type. To extract a list of these actions from the tree, we can recursively walk its structure and check the type of each node.

Sample Implementation

The following code demonstrates an example implementation that retrieves a list of template actions:

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

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

Output

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

Caveats:

While this technique generally allows for extracting field actions from templates, it has limitations. For instance, it doesn't handle all control structures and complex template syntax. It is intended as a demonstration of the underlying concepts and should be adapted accordingly.

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