Why are uninterned symbols used for package names and exports in Common Lisp?

Question

In a screen cast on Common List the author uses uninterned symbols for package names and exports.

(defpackage #:foo
  (:use :cl)
  (:export #:bar
           #:ba         


        

Answer 1

Using an interned symbol pollutes the package you're currently in with symbols that are only used for their names anyway:

[1]> *package*
#
[2]> (defpackage bar)
#
[3]> (find-symbol "BAR")
BAR ;
:INTERNAL

Uninterned symbols don't do that:

;; Uninterned symbols don't cause symbol pollution:
[4]> (defpackage #:foo)
#
[5]> (find-symbol "FOO")
NIL ;
NIL

You can also use strings directly, but since you're usually dealing with uppercase symbol names, they are less convenient to write:

[6]> (defpackage "BARFOO")
#
[7]> (find-symbol "BARFOO")
NIL ;
NIL

Example

To illustrate the problem, consider the following interaction:

[1]> (defpackage hello (:use cl) (:export hello))
#

;; Let's write some FOO stuff...
[2]> (defpackage foo (:use cl))
#
[3]> (in-package foo)
#

;; Oh, I forgot to import HELLO!
;; Let's fix that.
FOO[4]> (defpackage foo (:use cl hello))
*** - (COMMON-LISP:USE-PACKAGE (# #)
      #): 1 name conflicts remain
      Which symbol with name "HELLO" should be accessible in #?

;; Oops.