--- /dev/null
+;;; 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
--- /dev/null
+;;; thunk-tests.el --- Tests for thunk.el -*- lexical-binding: t -*-
+
+;; Copyright (C) 2015 Free Software Foundation, Inc.
+
+;; Author: Nicolas Petton <nicolas@petton.fr>
+;; 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:
+
+;; Tests for thunk.el
+
+;;; Code:
+
+(require 'ert)
+(require 'thunk)
+
+(ert-deftest thunk-should-be-lazy ()
+ (let (x)
+ (thunk-delay (setq x t))
+ (should (null x))))
+
+(ert-deftest thunk-can-be-evaluated ()
+ (let* (x
+ (thunk (thunk-delay (setq x t))))
+ (should-not (thunk-evaluated-p thunk))
+ (should (null x))
+ (thunk-force thunk)
+ (should (thunk-evaluated-p thunk))
+ (should x)))
+
+(ert-deftest thunk-evaluation-is-cached ()
+ (let* ((x 0)
+ (thunk (thunk-delay (setq x (1+ x)))))
+ (thunk-force thunk)
+ (should (= x 1))
+ (thunk-force thunk)
+ (should (= x 1))))
+
+(provide 'thunk-tests)
+;;; thunk-tests.el ends here