◊(Local Yarn Code "Diff")

;; Provides common helper functions used throughout the project

(provide maybe-meta     ; Select from (current-metas) or default value ("") if not available
         maybe-attr     ; Return an attribute’s value or a default ("") if not available
         here-output-path
         here-id
         listing-context
         series-metas-noun    ; Retrieve noun-singular from current 'series meta, or ""
         series-metas-title   ; Retrieve title of series in current 'series meta, or ""
         metas-series-pagenode
         invalidate-series
         make-tag-predicate
         tx-strs
         ymd->english

(define (here-output-path)
  (cond [(current-metas)
         (define-values (_ rel-path-parts)
           (drop-common-prefix (explode-path (current-project-root))
                               (explode-path (string->path (select-from-metas 'here-path (current-metas))))))
         (->output-path (apply build-path rel-path-parts))]
        [else (string->path ".")]))

(define listing-context (make-parameter ""))

;; Checks current-metas for a 'series meta and returns the pagenode of that series,
;; or '|| if no series is specified.
(define (metas-series-pagenode)
  (define maybe-series (or (select-from-metas 'series (current-metas)) ""))
  (cond
    [(non-empty-string? maybe-series)