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"