如何在文本/模板中获取循环切片中的键？

For example

{{ range $k, $v := .rec}}
{{ $k }} :: {{ $v }}
{{ end }}

Catch error panic: template: let:2:19: executing "let" at <.rec>: can't evaluate field rec in type []string.

{{ range $k, $v := .}}
{{ $k }} :: {{ $v }}
{{ end }}