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

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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.