New library thunk.el

thunk.el is extracted from stream.el in ELPA, with additional tests.

* lisp/emacs-lisp/thunk.el: New file.
* test/automated/thunk-tests.el: New file.
* etc/NEWS: Add information about thunk.el

lisp/emacs-lisp/thunk.el

@@ -0,0 +1,63 @@
+;;; 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 control the evaluation of
+;; forms.  Use `thunk-delay' to delay the evaluation of a form, and
+;; `thunk-force' to evaluate it. Evaluation is cached, and only
+;; happens once.
+
+;; 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