Change default reader in common lisp

Question

I wrote some function that could replace the function read of common lisp

(defun my-read (stream &rest args)
  (declare (ignore args))
  (funcall (my-get-macro-character (read-char stream))))

Is there a way to use this function as default reader?

Answer 1

You can't redefine the built in functions¹, but you can define a package that shadows cl:read and defines a new function my:read, so that when you use that package, it looks like it's the default read function. E.g., something like this:

CL-USER> (defpackage #:my-package 
           (:use "COMMON-LISP")
           (:shadow #:read)
           (:export #:read))
;=> #<PACKAGE "MY-PACKAGE">

CL-USER> (defun my-package:read (&rest args)
           (declare (ignore args))
           42)
;=> MY-PACKAGE:READ

CL-USER> (defpackage #:another-package
           (:use #:my-package "COMMON-LISP")
           (:shadowing-import-from #:my-package #:read))
;=> #<PACKAGE "ANOTHER-PACKAGE">

CL-USER> (in-package #:another-package)
;=> #<PACKAGE "ANOTHER-PACKAGE">

ANOTHER-PACKAGE> (read)
;=> 42

---

1. Actually, as Rainer Joswig noted in the comments, even though it's undefined behavior (see 11.1.2.1.2 Constraints on the COMMON-LISP Package for Conforming Programs), there often are ways to redefine some of the Common Lisp function, For instance, in SBCL you can use unlock-package, as shown in redefining built-in function. CLISP has package locks. Other implementations may have similar functionality.

Answer 2

One approach is to use set-macro-character on all "valid" input characters in a readtable. (This is okay if you only accept ASCII input, but I don't know if it would be practical for full Unicode.)

Something like this:

(defun replace-default-read-behavior (rt fn)
  (loop for c across 
        " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~"
        do (set-macro-character c fn t rt)))

(defun my-parser (stream char)
  (format t "custom read: ~A~A" char (read-line stream)))

(defun my-get-macro-character (char)
  (declare (ignore char))
  #'my-parser)

(defun my-read (stream char)
  (funcall (my-get-macro-character char) stream char))

(defvar *my-readtable* (copy-readtable ()))

(replace-default-read-behavior *my-readtable* #'my-read)

(let ((*readtable* *my-readtable*))
  (read-from-string "foo"))
custom read: foo  ; printed
NIL               ; returned
3