What's "|...|" stand for?

Question

What's the meaning of :|temp files| in the following code snippet?

(defmethod acceptor-remove-session ((acceptor my-site-acceptor) (session t))
    (declare (ignore acceptor))
    (loop for path in (session-value :|temp files|)
     do
         (ignore-errors (delete-file path))))

Answer 1

In Common Lisp, |...| may be used to quote characters in a symbol name; it may escape ordinarily disallowed characters such as spaces, and it also disables case conversion.

• foo or :foo have the symbol-name of "FOO"
• |temp files| or :|temp files| have the symbol-name of "temp files"
• || is named "", the empty string, which is otherwise impossible to produce

(The colon here has the usual meaning of a keyword symbol (a symbol in the KEYWORD package) and is independent of the bars.)

|...| is useful when a task lends itself to the use of symbols, but not ones named according to Common Lisp conventions. I would imagine in this case the text "temp files" occurs in the program's output somewhere, or is used for a filename, or some other case where seeing TEMP-FILES would be annoying.

Answer 2

| is a multiple escape character in Common Lisp symbols:

? 'aaBBcc|DDeeFFgg|hhII|jjKK|LL

-> |AABBCCDDeeFFggHHIIjjKKLL|

The vertical bar does not need to surround the whole symbol name. It can also escape parts of the symbol.