Why colons precede variables in Common Lisp

Question

What does the syntax, colons preceding variable in Common Lisp, mean? I've seen programs with such, and I'll present some sample code here, out of a large set of functions.

(defun expand (successorf node)
    (mapcar (lambda (action-state-cost)
          (let ((action (car action-state-cost))
                (state (cadr action-state-cost))
                (cost (caddr action-state-cost)))
            (make-node :state state :parent node
                       :action action :path-cost (+ (node-path-cost node) cost)
                       :depth (1+ (node-depth node)))
            ))
      (funcall successorf (node-state node))
      ))

Answer 1

They're not variables, actually; those are keywords. They're a special kind of efficient token, similar to “atoms” in other languages. It's a convenient, built-in way to pass named (and, almost always, optional) parameters into a function call.

http://www.gigamonkeys.com/book/functions.html describes the syntax of function calls.

Answer 2

Keyword Symbols

:foo is a keyword symbol.

• interned in and exported from the KEYWORD package
• constantly bound to itself

Usage

Keyword symbols are used when one needs the combination of the following properties:

• a symbol is the right data structure
• symbols with the same name should be unique (by interning them in a package) -> package KEYWORD
• different packages are not needed or wanted -> package KEYWORD
• writing the symbol should be easy by not needing to quote them -> :foo better than ':foo
• the ability to act as a variable with different values is not needed -> :foo evaluates to :foo itself and only to :foo

In Common Lisp generally symbols can be in a package (kind of a namespace).

An unexported symbol bar in a package foo is written as foo::bar. The double colon is between the package name and the symbol name.

An exported symbol then is written as foo:bar. A single colon is used.

If the symbol is available in the current package then is written as bar without the package.

The package KEYWORD

There is a special package called KEYWORD. A symbol bar in that package is simply and always written as :bar.

Examples

These keyword symbols have also these interesting properties: the symbols are automatically exported from the package KEYWORD (so keyword::bar, keyword:bar, ::bar and :bar are all the same symbol) and they evaluate to themselves:

CL-USER 5 > :bar
:BAR

CL-USER 6 > (describe :bar)

:BAR is a SYMBOL
NAME          "BAR"
VALUE         :BAR
FUNCTION      #<unbound function>
PLIST         NIL
PACKAGE       #<The KEYWORD package, 0/4 internal, 5830/8192 external>

CL-USER 7 > (eq 'keyword::bar ':bar)
T

CL-USER 8 > (eq :bar ':bar)  ; quoted or unquoted, each subform evaluates to :bar
T

Usage

Keyword symbols are used for example as names in named arguments:

(defun foo (&key bar) (+ bar 10))

(foo :bar 7)

Typically they are also used in arguments to instance and structure construction.

(defstruct node state parent action)

DEFSTRUCT is a Common Lisp macro and it generates several functions. One of them is a function MAKE-NODE, which can be used as:

(make-node :state 'open
           :parent some-parent
           :action an-action)

Note: sometimes the data might also be a keyword. For example in above form, the state might be :open and not open:

(make-node :state :open
           :parent some-parent
           :action an-action)