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;;; thunk.el --- Lazy form evaluation -*- lexical-binding: t -*-
;; Copyright (C) 2015 Free Software Foundation, Inc.
;; Author: Nicolas Petton <nicolas@petton.fr>
;; Keywords: sequences
;; Version: 1.0
;; Package: thunk
;; Maintainer: emacs-devel@gnu.org
;; This file is part of GNU Emacs.
;; GNU Emacs 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 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs 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 GNU Emacs. If not, see <http://www.gnu.org/licenses/>.
;;; Commentary:
;;
;; Thunk provides functions and macros to delay the evaluation of
;; forms.
;;
;; Use `thunk-delay' to delay the evaluation of a form, and
;; `thunk-force' to evaluate it. The result of the evaluation is
;; cached, and only happens once.
;;
;; Here is an example of a form which evaluation is delayed:
;;
;; (setq delayed (thunk-delay (message "this message is delayed")))
;;
;; `delayed' is not evaluated until `thunk-force' is called, like the
;; following:
;;
;; (thunk-force delayed)
;; Tests are located at test/automated/thunk-tests.el
;;; Code:
(defmacro thunk-delay (&rest body)
"Delay the evaluation of BODY."
(declare (debug t))
(let ((forced (make-symbol "forced"))
(val (make-symbol "val")))
`(let (,forced ,val)
(lambda (&optional check)
(if check
,forced
(unless ,forced
(setf ,val (progn ,@body))
(setf ,forced t))
,val)))))
(defun thunk-force (delayed)
"Force the evaluation of DELAYED.
The result is cached and will be returned on subsequent calls
with the same DELAYED argument."
(funcall delayed))
(defun thunk-evaluated-p (delayed)
"Return non-nil if DELAYED has been evaluated."
(funcall delayed t))
(provide 'thunk)
;;; thunk.el ends here
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