Metamorphic programming is an approach to extend the structured recursive programming discipline, which favors the use of fold operations over general recursion, to abstract data types. The key idea is to represent an ADT by two parts, a constructorand a destructor,which are essentially functions to/from a common representation. Then a fold can work on an ADT by applying parameter functions to values that are delivered by the ADT's own destructor. Fold operations that use as a parameter the constructor of another ADT, called ADT transformers,play an important role and offer a concise programming style. Several laws for ADT folds and transformers exist that can be used for program optimization and verification.
Jack W. Crenshaw wrote the Let's Build a Compiler article series from 1988 - 1995. This document is a formatted version of that excellent non-technical introduction to compiler construction. These web pages were created in 2005, and port Mr. Crenshaw's original Pascal code for the 68000 under SK*OS to the Forth language on a 80x86 CPU, under Windows XP. The text files were downloaded from http://compilers.iecc.com/crenshaw/. They are highly recommended. In this transcript I have assumed a 32-bit, byte-addressing Forth, with 8-bit characters. Division is symmetric, not floored, and two's complement is assumed throughout. iForth works splendidly for it, but other Forths can do it too.