I first began programming in Standard ML in 1989. I had always enjoyed implementing efficient data structures, so I immediately set about translating some of my favorites into Standard ML. For some data structures, this was quite easy, and to my great delight, the resulting code was often both much clearer and much more concise than previous versions I had written in C or Pascal or Ada. However, the experience was not always so pleasant. Time after time, I found myself wanting to use destructive updates, which are discouraged in Standard ML and forbidden in many other functional languages. I sought advice in the existing literature, but found only a handful of papers. Gradually, I realized that this was unexplored territory, and began to search for new ways of doing things.

Eight years later, I am still searching. There are still many examples of data structures that I just do not know how to implement efficiently in a functional language. But along the way, I have learned many lessons about what does work in functional languages. This book is an attempt to codify these lessons. I hope that it will serve as both a reference for functional programmers and as a text for those wanting to learn more about data structures in a functional setting.