{{- $url := .Permalink -}}
{{- with .Params.canonical_url -}}
  {{- $candidate := . -}}
  {{- if or (strings.HasPrefix $candidate "http://") (strings.HasPrefix $candidate "https://") -}}
    {{- $url = $candidate -}}
  {{- else -}}
    {{- $url = $candidate | absURL -}}
  {{- end -}}
{{- end -}}
{{- return $url -}}