(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?
Using an interned symbol pollutes the package you're currently in with symbols that are only used for their names anyway:
> *package* #<PACKAGE COMMON-LISP-USER> > (defpackage bar) #<PACKAGE BAR> > (find-symbol "BAR") BAR ; :INTERNAL
Uninterned symbols don't do that:
;; Uninterned symbols don't cause symbol pollution: > (defpackage #:foo) #<PACKAGE FOO> > (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:
> (defpackage "BARFOO") #<PACKAGE BARFOO> > (find-symbol "BARFOO") NIL ; NIL
To illustrate the problem, consider the following interaction:
> (defpackage hello (:use cl) (:export hello)) #<PACKAGE HELLO> ;; Let's write some FOO stuff... > (defpackage foo (:use cl)) #<PACKAGE FOO> > (in-package foo) #<PACKAGE FOO> ;; Oh, I forgot to import HELLO! ;; Let's fix that. FOO> (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.