;; guile-lib
;; Copyright (C) 2003,2004 Andy Wingo <wingo at pobox dot com>

;; This program is free software; you can redistribute it and/or    
;; modify it under the terms of the GNU General Public License as   
;; published by the Free Software Foundation; either version 2 of   
;; the License, or (at your option) any later version.              
;;                                                                  
;; This program is distributed in the hope that it will be useful,  
;; but WITHOUT ANY WARRANTY; without even the implied warranty of   
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the    
;; GNU General Public License for more details.                     
;;                                                                  
;; You should have received a copy of the GNU General Public License
;; along with this program; if not, contact:
;;
;; Free Software Foundation           Voice:  +1-617-542-5942
;; 59 Temple Place - Suite 330        Fax:    +1-617-542-2652
;; Boston, MA  02111-1307,  USA       gnu@gnu.org

;;; Commentary:
;;
;;A delay tree is a superset of a nodal tree (see (container
;;nodal-tree)). It extends nodal trees to allow any entry of the node to
;;be a promise created with the @code{delay} operator.
;;
;;; Code:

(define-module (container delay-tree)
  #:use-module (container nodal-tree)
  #:export (force-ref))

(define (force-ref node field)
  "Access a field in a node of a delay tree. If the value of the field
is a promise, the promise will be forced, and the value will be replaced
with the forced value."
  (let ((val (node-ref node field)))
    (and val
         (if (promise? val)
             (begin
               (node-set! node field (force val))
               (node-ref node field))
             val))))