bbb85eff49
This fixes Bug#28990. Also add a note to the file header that creating thunks requires lexical-binding. (thunk-delay): `cl-assert' lexical-binding.
75 lines
2.2 KiB
EmacsLisp
75 lines
2.2 KiB
EmacsLisp
;;; thunk.el --- Lazy form evaluation -*- lexical-binding: t -*-
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;; Copyright (C) 2015-2017 Free Software Foundation, Inc.
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;; Author: Nicolas Petton <nicolas@petton.fr>
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;; Keywords: sequences
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;; Version: 1.0
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;; Package: thunk
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;; Maintainer: emacs-devel@gnu.org
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;; This file is part of GNU Emacs.
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;; GNU Emacs is free software: you can redistribute it and/or modify
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;; it under the terms of the GNU General Public License as published by
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;; the Free Software Foundation, either version 3 of the License, or
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;; (at your option) any later version.
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;; GNU Emacs is distributed in the hope that it will be useful,
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;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;; GNU General Public License for more details.
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;; You should have received a copy of the GNU General Public License
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;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
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;;; Commentary:
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;;
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;; Thunk provides functions and macros to delay the evaluation of
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;; forms.
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;;
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;; Use `thunk-delay' to delay the evaluation of a form (requires
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;; lexical-binding), and `thunk-force' to evaluate it. The result of
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;; the evaluation is cached, and only happens once.
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;;
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;; Here is an example of a form which evaluation is delayed:
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;;
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;; (setq delayed (thunk-delay (message "this message is delayed")))
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;;
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;; `delayed' is not evaluated until `thunk-force' is called, like the
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;; following:
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;;
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;; (thunk-force delayed)
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;;; Code:
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(eval-when-compile (require 'cl-macs))
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(defmacro thunk-delay (&rest body)
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"Delay the evaluation of BODY."
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(declare (debug t))
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(cl-assert lexical-binding)
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(let ((forced (make-symbol "forced"))
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(val (make-symbol "val")))
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`(let (,forced ,val)
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(lambda (&optional check)
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(if check
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,forced
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(unless ,forced
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(setf ,val (progn ,@body))
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(setf ,forced t))
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,val)))))
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(defun thunk-force (delayed)
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"Force the evaluation of DELAYED.
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The result is cached and will be returned on subsequent calls
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with the same DELAYED argument."
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(funcall delayed))
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(defun thunk-evaluated-p (delayed)
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"Return non-nil if DELAYED has been evaluated."
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(funcall delayed t))
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(provide 'thunk)
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;;; thunk.el ends here
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