GPRINT: A LISP Pretty Printer Providing Extensive User Format Control Mechanism

Abstract

A Lisp pretty printer is presented which makes it easy for a user to control the format of the output produced. The printer can be used as a general mechanism for printing data structures as well as programs. It is divided into two parts: a set of formatting functions and an output routine. The user specifies how a particular type of object should be formatted by creating a formatting function for the type. When passed an object of that type, the formatting function creates a sequence of directions which specify how the object should be printed if it can fit on one line and how it should be printed if it must be broken up across multiple lines. A simple template language makes it easy to specify these directions. Based on the line length available, the output routine decides what structures have to be broken up across multiple lines and produces the actual output following the directions created by the formatting functions. The paper concludes with a discussion of how the pretty printing method presented could be applied to languages other than Lisp.