CLHS: Section 22.3.7.6

22.3.7.6 Tilde Question-Mark: Recursive Processing

The next arg must be a format control, and the one after it a list; both are consumed by the ~? directive. The two are processed as a control-string, with the elements of the list as the arguments. Once the recursive processing has been finished, the processing of the control string containing the ~? directive is resumed. Example:

 (format nil "~? ~D" "<~A ~D>" '("Foo" 5) 7) =>  "<Foo 5> 7"
 (format nil "~? ~D" "<~A ~D>" '("Foo" 5 14) 7) =>  "<Foo 5> 7"

Note that in the second example three arguments are supplied to the format string "<~A ~D>", but only two are processed and the third is therefore ignored.

With the @ modifier, only one arg is directly consumed. The arg must be a string; it is processed as part of the control string as if it had appeared in place of the ~@? construct, and any directives in the recursively processed control string may consume arguments of the control string containing the ~@? directive. Example:

 (format nil "~@? ~D" "<~A ~D>" "Foo" 5 7) =>  "<Foo 5> 7"
 (format nil "~@? ~D" "<~A ~D>" "Foo" 5 14 7) =>  "<Foo 5> 14"