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134 lines
4.1 KiB
EmacsLisp
134 lines
4.1 KiB
EmacsLisp
;;; thunk.el --- Lazy form evaluation -*- lexical-binding: t -*-
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;; Copyright (C) 2015-2020 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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;;
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;; This file also defines macros `thunk-let' and `thunk-let*' that are
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;; analogous to `let' and `let*' but provide lazy evaluation of
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;; bindings by using thunks implicitly (i.e. in the expansion).
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;;; Code:
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(require 'cl-lib)
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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
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(val (lambda () ,@body)))
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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 (funcall val))
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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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(defmacro thunk-let (bindings &rest body)
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"Like `let' but create lazy bindings.
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BINDINGS is a list of elements of the form (SYMBOL EXPRESSION).
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Any binding EXPRESSION is not evaluated before the variable
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SYMBOL is used for the first time when evaluating the BODY.
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It is not allowed to set `thunk-let' or `thunk-let*' bound
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variables.
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Using `thunk-let' and `thunk-let*' requires `lexical-binding'."
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(declare (indent 1) (debug let))
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(cl-callf2 mapcar
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(lambda (binding)
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(pcase binding
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(`(,(pred symbolp) ,_) binding)
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(_ (signal 'error (cons "Bad binding in thunk-let"
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(list binding))))))
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bindings)
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(cl-callf2 mapcar
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(pcase-lambda (`(,var ,binding))
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(list (make-symbol (concat (symbol-name var) "-thunk"))
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var binding))
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bindings)
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`(let ,(mapcar
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(pcase-lambda (`(,thunk-var ,_var ,binding))
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`(,thunk-var (thunk-delay ,binding)))
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bindings)
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(cl-symbol-macrolet
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,(mapcar (pcase-lambda (`(,thunk-var ,var ,_binding))
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`(,var (thunk-force ,thunk-var)))
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bindings)
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,@body)))
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(defmacro thunk-let* (bindings &rest body)
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"Like `let*' but create lazy bindings.
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BINDINGS is a list of elements of the form (SYMBOL EXPRESSION).
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Any binding EXPRESSION is not evaluated before the variable
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SYMBOL is used for the first time when evaluating the BODY.
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It is not allowed to set `thunk-let' or `thunk-let*' bound
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variables.
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Using `thunk-let' and `thunk-let*' requires `lexical-binding'."
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(declare (indent 1) (debug let))
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(cl-reduce
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(lambda (expr binding) `(thunk-let (,binding) ,expr))
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(nreverse bindings)
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:initial-value (macroexp-progn body)))
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;; (defalias 'lazy-let #'thunk-let)
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;; (defalias 'lazy-let* #'thunk-let*)
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(provide 'thunk)
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;;; thunk.el ends here
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