Add let-alist macro and package.

* let-alist.el: Add new package and macro.

2 files changed, 114 insertions, 0 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index d5f22541dcf..e65835cfb93 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,7 @@
+2014-12-10  Artur Malabarba  <bruce.connor.am@gmail.com>
+
+        * let-alist.el: Add new package and macro.
+
 2014-12-10  Eric S. Raymond  <esr@snark.thyrsus.com>
 
         * vc/vc-dispatcher.el, vc/vc-hooks.el, vc/vc-rcs.el,
diff --git a/lisp/let-alist.el b/lisp/let-alist.el
new file mode 100644
index 00000000000..1cf5c27af3e
--- /dev/null
+++ b/lisp/let-alist.el
@@ -0,0 +1,110 @@
+;;; let-alist.el --- Easily let-bind values of an assoc-list by their names.
+
+;; Copyright (C) 2014 Free Software Foundation, Inc.
+
+;; Author: Artur Malabarba <bruce.connor.am@gmail.com>
+;; Maintainer: Artur Malabarba <bruce.connor.am@gmail.com>
+;; Version: 1.0
+;; Keywords: extensions lisp
+;; Prefix: let-alist
+;; Separator: -
+
+;;; Commentary:
+;;
+;; This package offers a single macro, `let-alist'.  This macro takes a
+;; first argument (whose value must be an alist) and a body.
+;;
+;; The macro expands to a let form containing body, where each dotted
+;; symbol inside body is let-bound to their cdrs in the alist.  Dotted
+;; symbol is any symbol starting with a `.'.  Only those present in
+;; the body are let-bound and this search is done at compile time.
+;;
+;; For instance, the following code
+;;
+;;   (let-alist alist
+;;     (if (and .title .body)
+;;         .body
+;;       .site))
+;;
+;; expands to
+;;
+;;   (let ((.title (cdr (assq 'title alist)))
+;;         (.body (cdr (assq 'body alist)))
+;;         (.site (cdr (assq 'site alist))))
+;;     (if (and .title .body)
+;;         .body
+;;       .site))
+;;
+;; Note that only one level is supported.  If you nest `let-alist'
+;; invocations, the inner one can't access the variables of the outer
+;; one.
+
+;;; License:
+;;
+;; 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/>.
+
+;;; Code:
+
+
+(defun let-alist--deep-dot-search (data)
+  "Return alist of symbols inside DATA which start with a `.'.
+Perform a deep search and return an alist where each car is the
+symbol, and each cdr is the same symbol without the `.'."
+  (cond
+   ((symbolp data)
+    (let ((name (symbol-name data)))
+      (when (string-match "\\`\\." name)
+        ;; Return the cons cell inside a list, so it can be appended
+        ;; with other results in the clause below.
+        (list (cons data (intern (replace-match "" nil nil name)))))))
+   ((not (listp data)) nil)
+   (t (apply #'append
+        (remove nil (mapcar #'let-alist--deep-dot-search data))))))
+
+;;;###autoload
+(defmacro let-alist (alist &rest body)
+  "Let-bind dotted symbols to their cdrs in ALIST and execute BODY.
+Dotted symbol is any symbol starting with a `.'.  Only those
+present in BODY are let-bound and this search is done at compile
+time.
+
+For instance, the following code
+
+  (let-alist alist
+    (if (and .title .body)
+        .body
+      .site))
+
+expands to
+
+  (let ((.title (cdr (assq 'title alist)))
+        (.body (cdr (assq 'body alist)))
+        (.site (cdr (assq 'site alist))))
+    (if (and .title .body)
+        .body
+      .site))"
+  (declare (indent 1) (debug t))
+  `(let ,(mapcar (lambda (x) `(,(car x) (cdr (assq ',(cdr x) ,alist))))
+                 (delete-dups (let-alist--deep-dot-search body)))
+     ,@body))
+
+(provide 'let-alist)
+
+;; Local Variables:
+;; sentence-end-double-space: t
+;; End:
+
+;;; let-alist.el ends here