CLHS: Section 22.3

22.3 Formatted Output

format is useful for producing nicely formatted text, producing good-looking messages, and so on. format can generate and return a string or output to destination.

The control-string argument to format is actually a format control. That is, it can be either a format string or a function, for example a function returned by the formatter macro.

If it is a function, 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 compilation process performed by formatter produces a function that would do with its arguments as the format interpreter would do with those arguments.

The remainder of this section describes what happens if the control-string is a format string.

Control-string is composed of simple text (characters) and embedded directives.

format writes the simple text as is; each embedded directive specifies further text output that is to appear at the corresponding point within the simple text. Most directives use one or more elements of args to create their output.

A directive consists of a tilde, optional prefix parameters separated by commas, optional colon and at-sign modifiers, and a single character indicating what kind of directive this is. There is no required ordering between the at-sign and colon modifier. The case of the directive character is ignored. Prefix parameters are notated as signed (sign is optional) decimal numbers, or as a single-quote followed by a character. For example, ~5,'0d can be used to print an integer in decimal radix in five columns with leading zeros, or ~5,'*d to get leading asterisks.

In place of a prefix parameter to a directive, V (or v) can be used. In this case, format takes an argument from args as a parameter to the directive. The argument should be an integer or character. If the arg used by a V parameter is nil, the effect is as if the parameter had been omitted. # can be used in place of a prefix parameter; it represents the number of args remaining to be processed. When used within a recursive format, in the context of ~? or ~{, the # prefix parameter represents the number of format arguments remaining within the recursive call.

Examples of format strings:

"~S"        ;This is an S directive with no parameters or modifiers.  
"~3,-4:@s"  ;This is an S directive with two parameters, 3 and -4,    
            ; and both the colon and at-sign flags.                   
"~,+4S"     ;Here the first prefix parameter is omitted and takes     
            ; on its default value, while the second parameter is 4.

Figure 22-6. Examples of format control strings

format sends the output to destination. If destination is nil, format creates and returns a string containing the output from control-string. If destination is non-nil, it must be a string with a fill pointer, a stream, or the symbol t. If destination is a string with a fill pointer, the output is added to the end of the string. If destination is a stream, the output is sent to that stream. If destination is t, the output is sent to standard output.

In the description of the directives that follows, the term arg in general refers to the next item of the set of args to be processed. The word or phrase at the beginning of each description is a mnemonic for the directive. format directives do not bind any of the printer control variables (*print-...*) except as specified in the following descriptions. Implementations may specify the binding of new, implementation-specific printer control variables for each format directive, but they may neither bind any standard printer control variables not specified in description of a format directive nor fail to bind any standard printer control variables as specified in the description.