Shortcode with html like boolean attribute

In HTML, you can have boolean attributes, an example is the checked attribute of the input element. According to the HTML specification, the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

I tried to mimic a HTML like attribute with a Hugo shortcode, with partial success only.
What I did so far: I authored a shortcode checkbox.html, this shortcode is supposed to get a html-like checked attribute :

{{ with .Params  }}
  {{- if index . "checked" -}}
    ☑ Checked item
  {{- else -}}
    ☑ Checked item
{{- else -}}
  ☐ Unchecked item
{{ end }}

I now use this shortcode in my pages like this:

{{% checkbox checked=true %}}

{{% checkbox checked="true" %}}

{{% checkbox %}}

and it correctly prints out

☑ Checked item
☑ Checked item
☐ Unchecked item

However, if I specify the checked attribute as wanted (html like, without any value):

{{% checkbox checked %}}

I’m running into this error:

failed to process shortcode: "my\path\checkbox.html:2:9":
execute of template failed: template: shortcodes/checkbox.html:2:9:
executing "shortcodes/checkbox.html" at <index . "checked">:
error calling index: cannot index slice/array with type string

Any way to make this work?

Shortcode attributes are either named or positional:

{{ $checked := .Get 0 | default "" }}

See Create Your Own Shortcodes | Hugo