How to Access Variables from Outer Ranges in Go Templates?

Error: "Can't Evaluate Field X in Type Y" in Go Templates with Nested Ranges

The error "can't evaluate field X in type Y (X not part of Y but stuck in a {{range}} loop)" occurs when you attempt to access a field that is not a part of the struct being iterated over in a Go template. This can happen when you have nested ranges and attempt to access a variable that is defined in an outer range.

For instance, consider the following example:

type User struct {
    Username string
    Password []byte
    Email string
}

func main() {
    users := []User{{"user1", []byte("password"), "user1@example.com"}, {"user2", []byte("password"), "user2@example.com"}}
    renderer.HTML(w, http.StatusOK, "users/index", map[string]interface{}{
        "lang":  "en",
        "users": users,
    })
}

In the corresponding template file, the following code attempts to access the lang variable within the range loop:

{{ range .users }}
  <form action="/{{ .lang }}/users" method="POST">
    <input type="text" name="Username" value="{{ .Username }}">
    <input type="text" name="Email" value="{{ .Email }}">
  </form>
{{ end }}

However, since lang is not a field of the User struct, the template engine will raise the error mentioned above. To resolve this issue, you can use the $ variable to access the current context, which contains all the variables defined in the outer range. The updated template code would look like this:

{{ range .users }}
  <form action="/{{ $.lang }}/users" method="POST">
    <input type="text" name="Username" value="{{ .Username }}">
    <input type="text" name="Email" value="{{ .Email }}">
  </form>
{{ end }}

This will correctly access the lang variable and generate the desired output.

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