define-module : simple-sealers
  . #:use-module : srfi srfi-9
  . #:use-module : srfi srfi-9 gnu
  . #:export (make-sealer-triplet)

;; Make a sealer, unsealer, and brand-check predicate using
;; dynamic type generation.
define (make-sealer-triplet)
  define-record-type <seal>
    seal val        ; constructor (sealer)
    . sealed?       ; predicate   (brand-check)
    val unseal      ; accessor    (unsealer)

  ;; Prevents snooping on contents at REPL, etc
  define (print-seal _rec port)
    display "#<sealed>" port
  set-record-type-printer! <seal> print-seal

  ;; Return sealer, unsealer, sealed? predicate
  values seal unseal sealed?