183 lines
6 KiB
EmacsLisp
183 lines
6 KiB
EmacsLisp
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;;; let-alist.el --- Easily let-bind values of an assoc-list by their names -*- lexical-binding: t; -*-
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;; Copyright (C) 2014-2017 Free Software Foundation, Inc.
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;; Author: Artur Malabarba <emacs@endlessparentheses.com>
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;; Package-Requires: ((emacs "24.1"))
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;; Version: 1.0.5
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;; Keywords: extensions lisp
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;; Prefix: let-alist
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;; Separator: -
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;; This is an Elpa :core package. Don't use functionality that is not
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;; compatible with Emacs 24.1.
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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 <http://www.gnu.org/licenses/>.
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;;; Commentary:
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;; This package offers a single macro, `let-alist'. This macro takes a
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;; first argument (whose value must be an alist) and a body.
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;;
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;; The macro expands to a let form containing body, where each dotted
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;; symbol inside body is let-bound to their cdrs in the alist. Dotted
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;; symbol is any symbol starting with a `.'. Only those present in
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;; the body are let-bound and this search is done at compile time.
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;;
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;; For instance, the following code
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;;
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;; (let-alist alist
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;; (if (and .title .body)
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;; .body
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;; .site
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;; .site.contents))
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;;
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;; essentially expands to
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;;
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;; (let ((.title (cdr (assq 'title alist)))
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;; (.body (cdr (assq 'body alist)))
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;; (.site (cdr (assq 'site alist)))
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;; (.site.contents (cdr (assq 'contents (cdr (assq 'site alist))))))
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;; (if (and .title .body)
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;; .body
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;; .site
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;; .site.contents))
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;;
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;; If you nest `let-alist' invocations, the inner one can't access
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;; the variables of the outer one. You can, however, access alists
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;; inside the original alist by using dots inside the symbol, as
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;; displayed in the example above by the `.site.contents'.
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;;
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;;; Code:
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(defun let-alist--deep-dot-search (data)
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"Return alist of symbols inside DATA that start with a `.'.
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Perform a deep search and return an alist where each car is the
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symbol, and each cdr is the same symbol without the `.'."
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(cond
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((symbolp data)
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(let ((name (symbol-name data)))
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(when (string-match "\\`\\." name)
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;; Return the cons cell inside a list, so it can be appended
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;; with other results in the clause below.
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(list (cons data (intern (replace-match "" nil nil name)))))))
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((not (consp data)) nil)
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((eq (car data) 'let-alist)
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;; For nested ‘let-alist’ forms, ignore symbols appearing in the
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;; inner body because they don’t refer to the alist currently
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;; being processed. See Bug#24641.
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(let-alist--deep-dot-search (cadr data)))
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(t (append (let-alist--deep-dot-search (car data))
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(let-alist--deep-dot-search (cdr data))))))
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(defun let-alist--access-sexp (symbol variable)
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"Return a sexp used to access SYMBOL inside VARIABLE."
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(let* ((clean (let-alist--remove-dot symbol))
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(name (symbol-name clean)))
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(if (string-match "\\`\\." name)
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clean
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(let-alist--list-to-sexp
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(mapcar #'intern (nreverse (split-string name "\\.")))
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variable))))
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(defun let-alist--list-to-sexp (list var)
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"Turn symbols LIST into recursive calls to `cdr' `assq' on VAR."
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`(cdr (assq ',(car list)
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,(if (cdr list) (let-alist--list-to-sexp (cdr list) var)
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var))))
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(defun let-alist--remove-dot (symbol)
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"Return SYMBOL, sans an initial dot."
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(let ((name (symbol-name symbol)))
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(if (string-match "\\`\\." name)
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(intern (replace-match "" nil nil name))
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symbol)))
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;;; The actual macro.
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;;;###autoload
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(defmacro let-alist (alist &rest body)
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"Let-bind dotted symbols to their cdrs in ALIST and execute BODY.
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Dotted symbol is any symbol starting with a `.'. Only those present
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in BODY are let-bound and this search is done at compile time.
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For instance, the following code
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(let-alist alist
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(if (and .title .body)
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.body
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.site
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.site.contents))
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essentially expands to
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(let ((.title (cdr (assq \\='title alist)))
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(.body (cdr (assq \\='body alist)))
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(.site (cdr (assq \\='site alist)))
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(.site.contents (cdr (assq \\='contents (cdr (assq \\='site alist))))))
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(if (and .title .body)
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.body
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.site
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.site.contents))
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If you nest `let-alist' invocations, the inner one can't access
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the variables of the outer one. You can, however, access alists
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inside the original alist by using dots inside the symbol, as
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displayed in the example above."
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(declare (indent 1) (debug t))
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(let ((var (make-symbol "alist")))
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`(let ((,var ,alist))
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(let ,(mapcar (lambda (x) `(,(car x) ,(let-alist--access-sexp (car x) var)))
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(delete-dups (let-alist--deep-dot-search body)))
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,@body))))
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;;;; ChangeLog:
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;; 2015-12-01 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; packages/let-alist: Define it as a :core package
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;;
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;; 2015-06-11 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; * let-alist (let-alist--deep-dot-search): Fix cons
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;;
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;; 2015-03-07 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; let-alist: Update copyright
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;;
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;; 2014-12-22 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; packages/let-alist: Use `make-symbol' instead of `gensym'.
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;;
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;; 2014-12-20 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; packages/let-alist: Enable access to deeper alists
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;;
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;; 2014-12-14 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; let-alist.el: Add lexical binding. Version bump.
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;;
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;; 2014-12-11 Artur Malabarba <bruce.connor.am@gmail.com>
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;;
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;; let-alist: New package
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;;
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(provide 'let-alist)
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;;; let-alist.el ends here
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