For example:
Frontmatter:
item:
creators: ['Eric', 'John']
Shortcode:
{{ $num_creators := len .Page.Params.item.creators }}
Results in
failed to process shortcode: “M:\repos\REMOVED\layouts\shortcodes\focusbox.html:41:28”: execute of template failed at <len .Page.Params.item.creators>: error calling len: reflect: call of reflect.Value.Type on zero Value
This in the shortcode:
{{ printf "%#v" .Page.Params.item.creators }}
Outputs this on the page:
[]interface {}{"Eric", "John"}
So clearly the front matter data is being accessed correctly.