+;;;; READER
+
+;;; It is a basic Lisp reader. It does not use advanced stuff
+;;; intentionally, because we want to use it to bootstrap a simple
+;;; Lisp. The main entry point is the function `ls-read', which
+;;; accepts a strings as argument and return the Lisp expression.
+(defun make-string-stream (string)
+ (cons string 0))
+
+(defun %peek-char (stream)
+ (if (streamp stream)
+ (peek-char nil stream nil)
+ (and (< (cdr stream) (length (car stream)))
+ (char (car stream) (cdr stream)))))
+
+(defun %read-char (stream)
+ (if (streamp stream)
+ (read-char stream nil)
+ (and (< (cdr stream) (length (car stream)))
+ (prog1 (char (car stream) (cdr stream))
+ (incf (cdr stream))))))
+
+(defun whitespacep (ch)
+ (or (char= ch #\space) (char= ch #\newline) (char= ch #\tab)))
+
+(defun skip-whitespaces (stream)
+ (loop for ch = (%peek-char stream)
+ while (and ch (whitespacep ch))
+ do (%read-char stream)))
+
+(defun terminalp (ch)
+ (or (null ch) (whitespacep ch) (char= #\) ch)))
+
+(defun read-until (stream func)
+ (let ((string ""))
+ (loop for ch = (%peek-char stream)
+ until (funcall func ch)
+ do (setq string (concat string (string ch)))
+ do (%read-char stream))
+ string))
+
+(defun skip-whitespaces-and-comments (stream)
+ (let (ch)
+ (skip-whitespaces stream)
+ (setq ch (%peek-char stream))
+ (while (and ch (eql ch #\;))
+ (read-until stream (lambda (x) (eql x #\newline)))
+ (skip-whitespaces stream)
+ (setq ch (%peek-char stream)))))
+
+(defun %read-list (stream)
+ (skip-whitespaces-and-comments stream)
+ (let ((ch (%peek-char stream)))
+ (cond
+ ((char= ch #\))
+ (%read-char stream)
+ nil)
+ ((char= ch #\.)
+ (%read-char stream)
+ (skip-whitespaces-and-comments stream)
+ (prog1 (ls-read stream)
+ (unless (char= (%read-char stream) #\))
+ (error "')' was expected."))))
+ (t
+ (cons (ls-read stream) (%read-list stream))))))
+
+(defvar *eof* (make-symbol "EOF"))
+(defun ls-read (stream)
+ (skip-whitespaces-and-comments stream)
+ (let ((ch (%peek-char stream)))
+ (cond
+ ((null ch)
+ *eof*)
+ ((char= ch #\()
+ (%read-char stream)
+ (%read-list stream))
+ ((char= ch #\')
+ (%read-char stream)
+ (list 'quote (ls-read stream)))
+ ((char= ch #\`)
+ (%read-char stream)
+ (list 'backquote (ls-read stream)))
+ ((char= ch #\")
+ (%read-char stream)
+ (prog1 (read-until stream (lambda (ch) (char= ch #\")))
+ (%read-char stream)))
+ ((char= ch #\,)
+ (%read-char stream)
+ (if (eql (%peek-char stream) #\@)
+ (progn (%read-char stream) (list 'unquote-splicing (ls-read stream)))
+ (list 'unquote (ls-read stream))))
+ ((char= ch #\#)
+ (%read-char stream)
+ (ecase (%read-char stream)
+ (#\'
+ (list 'function (ls-read stream)))))
+ (t
+ (let ((string (read-until stream #'terminalp)))
+ (if (every #'digit-char-p string)
+ (parse-integer string)
+ (intern (string-upcase string))))))))
+
+(defun ls-read-from-string (string)
+ (ls-read (make-string-stream string)))
+
+
+;;;; Compiler
+