How to Properly Manage Variable Scope in Go Templates' Range Loops?

Range Loop Variable Scope in Go Templates

In Go templating, it's important to understand how variables declared within a range loop interact with variables declared outside the loop.

Consider the following code snippet:

{{ $prevDate := "" }}
{{ range $post := .Posts }}
  {{ if ne $prevDate $post.Date }}
    <div>
<p>The goal is to compare the current post's date with the previous post's date to display a section header when the date changes. However, the value of $prevDate seems to be reset to an empty string at the beginning of each iteration.</p>
<p>The issue lies in the scope of variables within the range loop. Each iteration of the loop creates a new shadowed variable $prevDate. This means that the value of $prevDate declared outside the loop is not modified, while the shadowed variable is modified within the loop.</p>
<p>Therefore, the problem can be solved by using the following alternatives:</p>
<p><strong>Solution #1: Registered Function</strong></p>
<p>Register a function that takes the current index and returns the date of the previous post, if available:</p>
<pre class="brush:php;toolbar:false">func PrevDate(i int) string {
    if i == 0 {
        return ""
    }
    return posts[i-1].Date
}

Then, use the function in the template:

{{range $index, $post := .Posts}}
    {{$prevDate := PrevDate $index}}
{{end}}

Solution #2: Method of Posts Type

Add a method to the Posts type to get the previous date:

func (p *Posts) PrevDate(i int) string {
    if i == 0 {
        return ""
    }
    return (*p)[i-1].Date
}

Then, use the method in the template:

{{range $index, $post := .Posts}}
    {{$prevDate := $.Posts.PrevDate $index}}
{{end}}

By using these solutions, the $prevDate variable can retain its value across iterations of the range loop, allowing you to compare post dates effectively.