Mike Schaeffer's Weblog
Wed, 30 May 2012
Anaphoric Macros in Emacs Lisp
In my Lisp programming, I find myself using Anaphoric Macros quite a bit. My first exposure to this type of macro (and deliberate variable capture) was in Paul Graham's On Lisp. Since I haven't been able to find Emacs Lisp implementations of these macos, I wrote my own.

The first of the two macros is an anaphoric version of the standard if special form:
(defmacro aif (test if-expr &optional else-expr)
  "An anaphoric variant of (if ...). The value of the test
expression is locally bound to 'it' during execution of the
consequent clauses. The binding is present in both consequent
branches."
  (declare (indent 1))
  `(let ((it ,test))
     (if it ,if-expr ,else-expr)))
The second macro is an anaphoric version of while:
(defmacro awhile (test &rest body)
  "An anaphoric varient of (while ...). The value of the test
expression is locally bound to 'it' during execution of the body
of the loop."
  (declare (indent 1))
  (let ((escape (gensym "awhile-escape-")))
    `(catch ',escape
       (while t
         (let ((it ,test))
           (if it
               (progn ,@body)
             (throw ',escape ())))))))
What both of these macros have in common is that they emulate an existing conditional special form, while adding a local binding that makes it possible to access the result of the condition. This is particularly useful in scenarios where a predicate function returns a true value that contains useful information beyond t or nil.

reddit this! Digg Me!

[/tech/emacs] permanent link