Like a lot of other languages, Common Lisp uses the colon (:) as a
scope-resolution operator. FOO:BAR means the symbol named BAR in the
package named FOO. But one can also have FOO::BAR (two colons) and
:BAR. What's the difference?
A Lisp package is a collection of symbols. Some of those symbols are
"exported," meaning they're meant to be used by functions in other
packages. Exported symbols are like public methods in an
object-oriented language. They're the public API that users of your
package should follow. "Unexported" symbols are like private methods or
private member variables, except that in Lisp they're not really private.
If (and only if) a symbol has been exported from a package, you can
refer to it from another package with a single colon, e.g.
PACKAGE:SYMBOL. Trying to use an unexported symbol this way results in
an error.
You can always get at any symbol, regardless of whether or not it was
exported, by using the double-colon (::) syntax, PACKAGE::SYMBOL.
The double-colon is a marker in the code that you're using an unexported
symbol. If that symbol comes from someone else's library, then you
probably shouldn't be doing that, because it's not part of the API.
You can modify the list of exported symbols in a package with EXPORT and
UNEXPORT. But normally a package's exported symbols are given in the
DEFPACKAGE macro:
(defpackage stuff
(:use common-lisp)
(:export foo bar))
Now you can use STUFF:FOO and STUFF:BAR. This is a quick way to learn
about the interface to a new Lisp library: find all the DEFPACKAGE
expressions and look at what they export.
As a special case, a colon before a symbol name with no package name,
e.g. :FOO, specifies a symbol in the KEYWORD package. Keyword symbols
always evaluate to themselves (they can't store a value). Every time
you use a new keyword symbol, it is automatically added to and exported
from the KEYWORD package. So :FOO is identical to KEYWORD:FOO. Don't
try to write it ::FOO.
Another note: unlike namespaces in some languages, Lisp packages cannot
be nested. You cannot have a symbol FOO::BAR::BAZ.
-Stuart
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