How to check if all template data is used by go template

php editor Apple here will introduce you to a useful trick, that is, how to check whether Go Template uses all template data. During the development process, sometimes we encounter situations where some template data is not used, which may result in incomplete data rendering or unnecessary performance loss. Through the following methods, we can quickly check and solve this problem. First, we need to understand the basic syntax and usage of Go Template. We can then use the go tool to parse the template file and generate an AST (Abstract Syntax Tree). By traversing this AST, we can find all template identifiers and compare them with the data source to determine whether there is unused template data. This method is simple and effective, I hope it will be helpful to everyone!

Question content

Suppose I have a template string

today i visited {{ .market }} to buy {{ .fruit }}

The template data provided is

map[string]string := { market: "whole foods", fruit: "bananas", veg: "celery" }

I want to panic here because .veg is not used in the template string. is it possible?

I'm using go template (text/template).

Solution

No built-in support. You can parse the parsed template, but it's unnecessarily complex. Also note that this kind of static analysis can never be done: templates can access data based on runtime parameters, and whether to use everything can only be decided at runtime and may vary from execution to execution (e.g. you can use index mapping value) {{index .someMap .someKey}} where someKey is the value provided at runtime).

An acceptable solution might be to pass a data structure that keeps track of which elements were accessed, you can check that after the template is executed and if not everything is used, you can do whatever you want.

For example, this structure keeps track of which elements have not been visited yet:

type Params struct {
    m, remaining map[string]string
}

func NewParams(m map[string]string) *Params {
    return &Params{
        m:         m,
        remaining: maps.Clone(m),
    }
}

func (p *Params) Get(key string) string {
    delete(p.remaining, key)
    return p.m[key]
}

Usage example:

const src = `today i visited {{ .Get "market" }} to buy {{ .Get "fruit" }}`
t := template.Must(template.New("").Parse(src))

m := NewParams(
    map[string]string{"market": "whole foods", "fruit": "bananas", "veg": "celery"},
)

if err := t.Execute(os.Stdout, m); err != nil {
    panic(err)
}

fmt.Println()
if len(m.remaining) > 0 {
    fmt.Println("Following entries were not used:", m.remaining)
}

This will output (try it on Go Playground):

today i visited whole foods to buy bananas
Following entries were not used: map[veg:celery]

Please note that I used 2 mappings (m and remaining) to allow multiple accesses to the same element. If each element can only be accessed once, then a simple map (and remove the element from that map) will also work.

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