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Common Lisp the Language, 2nd Edition
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Control Up: Pretty
Printing Previous: Pretty
Printing
Pretty printing has traditionally been a black box process, displaying
program code using a set of fixed layout rules. Its utility can be
greatly enhanced by opening it up to user control. The facilities
described in this chapter provide general and powerful means for
specifying pretty-printing behavior.
By providing direct access to the mechanisms within the pretty
printer that make dynamic decisions about layout, the macros and
functions pprint-logical-block,
pprint-newline, and pprint-indent make it
possible to specify pretty printing layout rules as a part of any
function that produces output. They also make it very easy for the
function to support detection of circularity and sharing and
abbreviation based on length and nesting depth. Using the function
set-pprint-dispatch, one can associate a user-defined
pretty printing function with any type of object. A small set of new
format directives allows concise implementation of
user-defined pretty-printing functions. Together, these facilities
enable users to redefine the way code is displayed and allow the full
power of pretty printing to be applied to complex combinations of data
structures.
Implementation note: This chapter describes the interface of the XP pretty printer. XP is described fully in [54], which also explains how to obtain a portable implementation. XP uses a highly efficient linear-time algorithm. When properly integrated into a Common Lisp, this algorithm supports pretty printing that is only fractionally slower than ordinary printing.
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