Parsing and Canonical Unparsing

Parsing and formatting have often been identified as problems that do not have elegant and widely accepted solutions in ANSI Common Lisp. Format strings tend to be parsimonious and can be very difficult to read. Destructuring primitives exist only for lists and their utility in parsing is further limited by the lack of non-deterministic features in Common Lisp. This paper demonstrates a simple implementation of non-deterministic parsing and canonical unparsing in Common Lisp. This method can provide a solution to both of these problems in a simple unified way. A single relation is constructed between an input string and a data-structure. This relation can provide a data-structure associated with a string, or construct a canonical string from the data-structure. While this type of solution may not meet performance requirements in some situations it is elegant, quite general and is easy to work with.