Brad Ediger

Paul Graham's Arc language has been causing a commotion in the Lisp and web application communities. One of the stand-out features of Arc is its tidy syntax for simple anonymous functions. In Arc, the increment function can be expressed thus, using square brackets:

[+ 1 _]

(lambda (x) (+ x 1))

Arc's implementation of the bracket syntax is in brackets.scm, written by Eli Barzilay. Using Eli's Scheme code as a guide, I wrote the following Common Lisp reader macro:

(defun square-bracket-reader (stream char)
  (declare (ignore char))
  `(lambda (&optional _ __)
    (declare (ignorable _ __)) ; don't warn about unused variables
    ,(read-delimited-list #\] stream t)))

(defun use-square-bracket-readtable ()
  ; Install #'square-bracket-reader as a reader macro for [
  (set-macro-character #\[ #'square-bracket-reader)
  
  ; Make ] behave like ) to the reader so that [+ 1 _] works,
  ; not just [+ 1 _ ].
  (set-macro-character #\] (get-macro-character #\) nil)))

Usage is very simple:

CL-USER> (use-square-bracket-readtable)
T
CL-USER> ([+ 1 _] 10)
11
CL-USER> ([+ _ __] 1 2)
3