#lang racket/base

(require racket/contract racket/list racket/match)

(provide metadata? metadata-ref metadata-ref+)

(define metadata?
  (listof (or/c
            (list/c 'date integer? integer? integer?)
            (list/c 'title string?)
            (list/c 'slug string?)
            (cons/c 'tags (listof string?))
            (list/c 'summary string?)
            (cons/c 'authors (listof string?)))))

(define (metadata-ref md key [default (λ () (error "no such key"))])
  (match (assoc key md)
    [#f (if (procedure? default) (default) default)]
    [(cons _ rst) rst]))

(define (metadata-ref+ md key [default (λ () (error "no such key"))])
  (first (metadata-ref md key default)))