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| ;;; Beyond call/cc | |
| ;; Copyright (C) 2010, 2011, 2013 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 shift reset shift* reset* | |
| call-with-escape-continuation call/ec | |
| let-escape-continuation let/ec)) | |
| (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)) | |
| ;; Kindly provided by Wolfgang J Moeller <[email protected]>, modelled | |
| ;; after the ones by Oleg Kiselyov in | |
| ;; http://okmij.org/ftp/Scheme/delim-control-n.scm, which are in the | |
| ;; public domain, as noted at the top of http://okmij.org/ftp/. | |
| ;; | |
| (define-syntax-rule (reset . body) | |
| (call-with-prompt (default-prompt-tag) | |
| (lambda () . body) | |
| (lambda (cont f) (f cont)))) | |
| (define-syntax-rule (shift var . body) | |
| (abort-to-prompt (default-prompt-tag) | |
| (lambda (cont) | |
| ((lambda (var) (reset . body)) | |
| (lambda vals (reset (apply cont vals))))))) | |
| (define (reset* thunk) | |
| (reset (thunk))) | |
| (define (shift* fc) | |
| (shift c (fc c))) | |
| (define (call-with-escape-continuation proc) | |
| "Call PROC with an escape continuation." | |
| (let ((tag (list 'call/ec))) | |
| (call-with-prompt tag | |
| (lambda () | |
| (proc (lambda args | |
| (apply abort-to-prompt tag args)))) | |
| (lambda (_ . args) | |
| (apply values args))))) | |
| (define call/ec call-with-escape-continuation) | |
| (define-syntax-rule (let-escape-continuation k body ...) | |
| "Bind K to an escape continuation within the lexical extent of BODY." | |
| (let ((tag (list 'let/ec))) | |
| (call-with-prompt tag | |
| (lambda () | |
| (let ((k (lambda args | |
| (apply abort-to-prompt tag args)))) | |
| body ...)) | |
| (lambda (_ . results) | |
| (apply values results))))) | |
| (define-syntax-rule (let/ec k body ...) | |
| (let-escape-continuation k body ...)) | |