Successful Lisp - Chapter 24: FORMAT Speaks a Different Language

Overview

This chapter covers Lisp's FORMAT function, a text formatting system inspired by FORTRAN that implements its own programming language for output formatting.

Key Capabilities

The FORMAT function can:

• Print numbers as words or Roman numerals
• Generate columnar output
• Create conditional text based on formatted variables
• Handle various numeric representations and formatting styles

Basic Syntax

FORMAT requires three components: a destination argument, a format control string, and optional arguments for the control string.

Destination options:

• T: sends output to *STANDARD-OUTPUT*
• NIL: returns formatted output as a string
• String with fill pointer: appends output to the string
• Output stream: directs to specific stream

Formatting Directives

Directive	Purpose
~%	newline
~&	fresh line (newline if not already at line start)
~\|	page break
~T	tab stop
~C	character
~D	decimal integer
~B, ~O, ~X	binary, octal, hexadecimal
~R	spell integer as words
~P	plural handling
~F, ~E, ~G, ~$	floating-point formats
~A	legible output (no escapes)
~S	readable output (with escapes)
~~	literal tilde

Special Features

Case conversion

• ~(text~): lowercase
• ~:@(text~): uppercase
• ~:(text~): capitalize each word
• ~@(text~): capitalize first word only

Number formatting modifiers

• ~wD: right-justify in field width w
• ~w,'cD: pad with character c
• ~@D: include plus sign for positive numbers
• ~:D: insert commas between digit groups

Roman numerals

~@R converts integers to Roman numeral representation.

Plurals

~P appends "s"; ~@P handles "y"/"ies"; ~:P reuses previous argument.

List Iteration

Use ~{format-control~} to iterate over list elements. Include ~^ to terminate early if arguments exhaust.

Conditionals

Three forms exist:

• Ordinal: ~[format-0~;format-1~;...~] selects clause by integer index
• Binary: ~:[false~;true~] tests for NIL
• Conditional: ~@[format~] tests argument; processes only if non-NIL

Dynamic Parameters

Use V to specify parameters from the argument list at runtime, allowing field widths and other values to vary during execution.