extroonie.com/layouts/partials/writing.html

{{- $pages := where site.RegularPages "Section" "writing" -}}
{{- range $pages -}}
	{{- if not (in (slice "post" "essay") .Params.format) -}}
		{{- errorf "%s: format must be post or essay" .File.Path -}}
	{{- end -}}
{{- end -}}
{{- return $pages -}}