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

etc/NEWS

@@ -582,6 +582,11 @@ prefixed with `seq-' and work on lists, strings and vectors.
 The map library provides map-manipulation functions that work on alists,
 hash-table and arrays.  All functions are prefixed with "map-".
 
+** thunk
+*** New thunk library:
+Thunk provides functions and macros to control the evaluation of
+forms.
+
 ** Calendar and diary
 
 +++

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

test/automated/thunk-tests.el

@@ -0,0 +1,55 @@
+;;; 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