ANSI Common Lisp 22 Printer 22.2 The Lisp Pretty Printer 22.2.1 Pretty Printer Concepts
188.8.131.52 Pretty Print Dispatch Tables
A pprint dispatch table is a mapping from keys to pairs of values. Each key is a type specifier. The values associated with a key are a "function" (specifically, a function designator or nil) and a "numerical priority" (specifically, a real). Basic insertion and retrieval is done based on the keys with the equality of keys being tested by equal.
When *print-pretty* is true, the current pprint dispatch table (in *print-pprint-dispatch*) controls how objects are printed. The information in this table takes precedence over all other mechanisms for specifying how to print objects. In particular, it has priority over user-defined print-object methods because the current pprint dispatch table is consulted first.
The function is chosen from the current pprint dispatch table by finding the highest priority function that is associated with a type specifier that matches the object; if there is more than one such function, it is implementation-dependent which is used.
However, if there is no information in the table about how to pretty print a particular kind of object, a function is invoked which uses print-object to print the object. The value of *print-pretty* is still true when this function is called, and individual methods for print-object might still elect to produce output in a special format conditional on the value of *print-pretty*.