ANSI Common Lisp 22 Printer 22.2 The Lisp Pretty Printer
22.2.1 Pretty Printer ConceptsThe facilities provided by the pretty printer permit programs to redefine the way in which code is displayed, and allow the full power of pretty printing to be applied to complex combinations of data structures.
Whether any given style of output is in fact "pretty" is inherently a somewhat subjective issue. However, since the effect of the pretty printer can be customized by conforming programs, the necessary flexibility is provided for individual programs to achieve an arbitrary degree of aesthetic control.
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 detection of circularity and sharing, and abbreviation based on length and nesting depth to be supported by the function.
The pretty printer is driven entirely by dispatch based on the value of *print-pprint-dispatch*. The function set-pprint-dispatch makes it possible for conforming programs to associate new pretty printing functions with a type.