The `html/template` package in Go

In Go’s html/template package, there are several types of placeholders and actions you can use inside your HTML template to manipulate data, control flow, and format content. Below is an example template that demonstrates many of these actions, along with an explanation of what they do.

Summary of Template Actions:

{{.FieldName}}: Insert data from the context.
{{if condition}}...{{else}}...{{end}}: Conditional rendering based on a value.
{{range .Slice}}...{{end}}: Loop through a slice or array.
{{with .Field}}...{{end}}: Set a new context and work with a specific field.
{{define "name"}}...{{end}}: Define a reusable sub-template.
{{template "name" .}}: Render a defined sub-template.

Template Example with Various Placeholders

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>{{.Title}}</title>
</head>
<body>
    <h1>{{.Header}}</h1>
    <p>{{.Message}}</p>

    <p><strong>Uppercase Title:</strong> {{.Title | upper}}</p> <!-- Custom function applied -->

    <h2>Conditional Example</h2>
    {{if .IsAdmin}}
        <p>You have admin privileges!</p>
    {{else}}
        <p>You are a regular user.</p>
    {{end}}

    <h2>Loop Example</h2>
    <ul>
    {{range .Items}}
        <li>{{.}}</li>
    {{end}}
    </ul>

    <h2>With Example</h2>
    {{with .Contact}}
        <p>Email: {{.Email}}</p>
        <p>Phone: {{.Phone}}</p>
    {{end}}

    <h2>Template Action Example</h2>
    {{define "footer"}}
        <footer>
            <p>Footer content goes here</p>
        </footer>
    {{end}}

    {{template "footer" .}} <!-- Includes the footer template -->
</body>
</html>

Explanation of the Template Placeholders

{{.Title}}:
- Placeholder that inserts the value of the Title field from the data passed to the template. The dot (.) represents the current context or data passed in.
{{if .IsAdmin}}...{{else}}...{{end}}:
- Conditional statement to check if IsAdmin is true. If it is, the first block (<p>You have admin privileges!</p>) is rendered. If not, the block under else is rendered.
{{range .Items}}...{{end}}:
- Loops over the Items field, which is expected to be a slice or array. For each item, the block is executed, and the current item is inserted where {{.}} is placed. It creates a list (<li>) for each item.
{{with .Contact}}...{{end}}:
- The with action sets the context to Contact temporarily. Inside the block, you can refer to fields of Contact directly (e.g., {{.Email}}). This is useful when you want to extract and use sub-fields in a structured object.
{{define "footer"}}...{{end}} and {{template "footer" .}}:
- define creates a named template (footer in this case) that can be reused later with template. The content inside {{define "footer"}} is the footer content, and {{template "footer" .}} will insert that defined template into the page.
{{.Title | upper}}:
- This applies a custom function (like upper to convert text to uppercase). You would need to define the upper function when parsing the template in Go using Funcs method.
Example in Go:
```
tmpl, err := template.New("index.html").Funcs(template.FuncMap{
    "upper": strings.ToUpper,
}).ParseFiles("index.html")
```

Example Data Structure in Go

Here’s the Go data structure that would work with this template:

package main

import (
    "html/template"
    "log"
    "net/http"
    "strings"
)

type Contact struct {
    Email string
    Phone string
}

type PageData struct {
    Title    string
    Header   string
    Message  string
    IsAdmin  bool
    Items    []string
    Contact  Contact
}

func handler(w http.ResponseWriter, r *http.Request) {
    data := PageData{
        Title:   "Go Templates Demo",
        Header:  "Welcome to Go Templates!",
        Message: "This page demonstrates various template features.",
        IsAdmin: true,
        Items:   []string{"Item 1", "Item 2", "Item 3"},
        Contact: Contact{
            Email: "contact@example.com",
            Phone: "123-456-7890",
        },
    }

    tmpl, err := template.New("index.html").Funcs(template.FuncMap{
        "upper": strings.ToUpper,
    }).ParseFiles("index.html")
    if err != nil {
        log.Fatal(err)
    }

    err = tmpl.Execute(w, data)
    if err != nil {
        log.Fatal(err)
    }
}

func main() {
    http.HandleFunc("/", handler)
    log.Fatal(http.ListenAndServe(":8080", nil))
}