;;;; superfluous-parentheses.lisp
;;;; Adds support for [] and {} in place of normal parentheses
(defpackage superfluous-parentheses
  (:use common-lisp))
(in-package superfluous-parentheses)

(define-condition simple-reader-error (simple-condition reader-error)
  ())

(defun read-bracketed-list (stream begin-char)
  (let* ((end-char (cond ((char= begin-char #\[) #\])
                         ((char= begin-char #\{) #\})
                         (t (error 'simple-reader-error
                                   :stream stream
                                   :format-control "Invalid bracket ~S"
                                   :format-arguments (list begin-char)))))
         (list-body (loop for next-char = (peek-char t stream t nil t)
                          until (or (char= next-char end-char) (char= next-char #\.))
                          collect (read stream t nil t)))
         (last-char (read-char stream t nil t))) ;; discard the last peeked character
    (cond ((char= last-char end-char) list-body)
          ((char= last-char #\.)
           (unless list-body
             (error 'simple-reader-error
                    :stream stream
                    :format-control "Nothing before . in list."
                    :format-arguments nil))
           ;; set the CDR of the list
           (rplacd (last list-body) (read stream t nil t))
           ;; skip past whitespace
           (peek-char t stream t nil t)
           (unless (char= (read-char stream t nil t) end-char)
             (error 'simple-reader-error
                    :stream stream
                    :format-control "Multiple objects after . in list."
                    :format-arguments nil))
           list-body)
          (t (error 'simple-reader-error
                    :stream stream
                    :format-control "Expected ~S or ., but got ~S."
                    :format-arguments (list end-char last-char))))))

(set-macro-character #\[ #'read-bracketed-list)
(set-macro-character #\] (get-macro-character #\)))

(set-macro-character #\{ #'read-bracketed-list)
(set-macro-character #\} (get-macro-character #\)))