1;;; thunk.el --- Lazy form evaluation -*- lexical-binding: t -*- 2 3;; Copyright (C) 2015-2021 Free Software Foundation, Inc. 4 5;; Author: Nicolas Petton <nicolas@petton.fr> 6;; Keywords: sequences 7;; Version: 1.0 8;; Package: thunk 9 10;; Maintainer: emacs-devel@gnu.org 11 12;; This file is part of GNU Emacs. 13 14;; GNU Emacs is free software: you can redistribute it and/or modify 15;; it under the terms of the GNU General Public License as published by 16;; the Free Software Foundation, either version 3 of the License, or 17;; (at your option) any later version. 18 19;; GNU Emacs is distributed in the hope that it will be useful, 20;; but WITHOUT ANY WARRANTY; without even the implied warranty of 21;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 22;; GNU General Public License for more details. 23 24;; You should have received a copy of the GNU General Public License 25;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>. 26 27;;; Commentary: 28;; 29;; Thunk provides functions and macros to delay the evaluation of 30;; forms. 31;; 32;; Use `thunk-delay' to delay the evaluation of a form (requires 33;; lexical-binding), and `thunk-force' to evaluate it. The result of 34;; the evaluation is cached, and only happens once. 35;; 36;; Here is an example of a form which evaluation is delayed: 37;; 38;; (setq delayed (thunk-delay (message "this message is delayed"))) 39;; 40;; `delayed' is not evaluated until `thunk-force' is called, like the 41;; following: 42;; 43;; (thunk-force delayed) 44;; 45;; This file also defines macros `thunk-let' and `thunk-let*' that are 46;; analogous to `let' and `let*' but provide lazy evaluation of 47;; bindings by using thunks implicitly (i.e. in the expansion). 48 49;;; Code: 50 51(require 'cl-lib) 52 53(defmacro thunk-delay (&rest body) 54 "Delay the evaluation of BODY." 55 (declare (debug (def-body))) 56 (cl-assert lexical-binding) 57 `(let (forced 58 (val (lambda () ,@body))) 59 (lambda (&optional check) 60 (if check 61 forced 62 (unless forced 63 (setf val (funcall val)) 64 (setf forced t)) 65 val)))) 66 67(defun thunk-force (delayed) 68 "Force the evaluation of DELAYED. 69The result is cached and will be returned on subsequent calls 70with the same DELAYED argument." 71 (funcall delayed)) 72 73(defun thunk-evaluated-p (delayed) 74 "Return non-nil if DELAYED has been evaluated." 75 (funcall delayed t)) 76 77(defmacro thunk-let (bindings &rest body) 78 "Like `let' but create lazy bindings. 79 80BINDINGS is a list of elements of the form (SYMBOL EXPRESSION). 81Any binding EXPRESSION is not evaluated before the variable 82SYMBOL is used for the first time when evaluating the BODY. 83 84It is not allowed to set `thunk-let' or `thunk-let*' bound 85variables. 86 87Using `thunk-let' and `thunk-let*' requires `lexical-binding'." 88 (declare (indent 1) (debug let)) 89 (cl-callf2 mapcar 90 (lambda (binding) 91 (pcase binding 92 (`(,(pred symbolp) ,_) binding) 93 (_ (signal 'error (cons "Bad binding in thunk-let" 94 (list binding)))))) 95 bindings) 96 (cl-callf2 mapcar 97 (pcase-lambda (`(,var ,binding)) 98 (list (make-symbol (concat (symbol-name var) "-thunk")) 99 var binding)) 100 bindings) 101 `(let ,(mapcar 102 (pcase-lambda (`(,thunk-var ,_var ,binding)) 103 `(,thunk-var (thunk-delay ,binding))) 104 bindings) 105 (cl-symbol-macrolet 106 ,(mapcar (pcase-lambda (`(,thunk-var ,var ,_binding)) 107 `(,var (thunk-force ,thunk-var))) 108 bindings) 109 ,@body))) 110 111(defmacro thunk-let* (bindings &rest body) 112 "Like `let*' but create lazy bindings. 113 114BINDINGS is a list of elements of the form (SYMBOL EXPRESSION). 115Any binding EXPRESSION is not evaluated before the variable 116SYMBOL is used for the first time when evaluating the BODY. 117 118It is not allowed to set `thunk-let' or `thunk-let*' bound 119variables. 120 121Using `thunk-let' and `thunk-let*' requires `lexical-binding'." 122 (declare (indent 1) (debug let)) 123 (cl-reduce 124 (lambda (expr binding) `(thunk-let (,binding) ,expr)) 125 (reverse bindings) 126 :initial-value (macroexp-progn body))) 127 128;; (defalias 'lazy-let #'thunk-let) 129;; (defalias 'lazy-let* #'thunk-let*) 130 131 132(provide 'thunk) 133;;; thunk.el ends here 134