;;; Beyond call/cc

;; Copyright (C) 2010 Free Software Foundation, Inc.

;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
;;;; License as published by the Free Software Foundation; either
;;;; version 3 of the License, or (at your option) any later version.
;;;; 
;;;; This library 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
;;;; Lesser General Public License for more details.
;;;; 
;;;; You should have received a copy of the GNU Lesser General Public
;;;; License along with this library; if not, write to the Free Software
;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

;;; Code:

(define-module (ice-9 control)
  #:re-export (call-with-prompt abort-to-prompt
               default-prompt-tag make-prompt-tag)
  #:export (% abort))

(define (abort . args)
  (apply abort-to-prompt (default-prompt-tag) args))

(define-syntax %
  (syntax-rules ()
    ((_ expr)
     (call-with-prompt (default-prompt-tag)
                       (lambda () expr)
                       default-prompt-handler))
    ((_ expr handler)
     (call-with-prompt (default-prompt-tag)
                       (lambda () expr)
                       handler))
    ((_ tag expr handler)
     (call-with-prompt tag
                       (lambda () expr)
                       handler))))

;; Each prompt tag has a type -- an expected set of arguments, and an unwritten
;; contract of what its handler will do on an abort. In the case of the default
;; prompt tag, we could choose to return values, exit nonlocally, or punt to the
;; user.
;;
;; We choose the latter, by requiring that the user return one value, a
;; procedure, to an abort to the prompt tag. That argument is then invoked with
;; the continuation as an argument, within a reinstated default prompt. In this
;; way the return value(s) from a default prompt are under the user's control.
(define (default-prompt-handler k proc)
  (% (default-prompt-tag)
     (proc k)
     default-prompt-handler))