Common Lisp the Language, 2nd Edition

27.5. Compiling Format Control Strings

The control strings used by format are essentially programs that perform printing. The macro formatter provides the efficiency of using a compiled function for printing without losing the visual compactness of format strings.

[Macro]
formatter control-string

The control-string must be a literal string. An error is signaled if control-string is not a valid format control string. The macro formatter expands into an expression of the form (function (lambda (stream &rest args) ...)) that does the printing specified by control-string. The lambda created accepts an output stream as its first argument and zero or more data values as its remaining arguments. The value returned by the lambda is the tail (if any) of the data values that are not printed out by control-string. (For example, if the control-string is "~A~A", the cddr (if any) of the data values is returned.) The form (formatter "~%~2@{~S, ~}") is equivalent to the following:

#'(lambda (stream &rest args) 
    (terpri stream) 
    (dotimes (n 2) 
      (if (null args) (return nil)) 
      (prin1 (pop args) stream) 
      (write-string ", " stream)) 
    args)

In support of the above mechanism, format is extended so that it accepts functions as its second argument as well as strings. When a function is provided, it must be a function of the form created by formatter. The function is called with the appropriate output stream as its first argument and the data arguments to format as its remaining arguments. The function should perform whatever output is necessary and return the unused tail of the arguments (if any). The directives and ~{~} with no body are also extended so that they accept functions as well as control strings. Every other standard function that takes a format string as an argument (for example, error and warn) is also extended so that it can accept functions of the form above instead.