lispcommon-lispsymbols

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


In a screen cast (archived blog) on Common List the author (Alexander Lehmann) uses uninterned symbols for package names and exports.

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

(in-package #:foo)

He also uses the sharp sign in front of anonymous functions.

(defun transposed (m)
  (make-instance 'matrix
                 :rows (matrix-cols m)
                 :cols (matrix-rows m)
                 :generator #'(lambda (i j) (matrix-at m j i))))

In the book Practical Common Lisp the sharp sign isn't used for package names and exports as far as I have read.

What's the reason for using the uninterned symbols (the sharp sign) in these cases?


Solution

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

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

    Uninterned symbols don't do that:

    ;; Uninterned symbols don't cause symbol pollution:
    [4]> (defpackage #:foo)
    #<PACKAGE 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")
    #<PACKAGE BARFOO>
    [7]> (find-symbol "BARFOO")
    NIL ;
    NIL
    

    Example

    To illustrate the problem, consider the following interaction:

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