With each reprinting of this book, a dozen minor errors have silently disappeared. But a reprinting is no occasion for making improvements, however valuable, that would affect the page numbering: we should then have several slightly different, incompatible editions. An accumulation of major changes (and the Editor's urgings) have prompted this second edition.

As luck would have it, changes to ml have come about at the same time. ml has a new standard library and the language itself has been revised. It is worth stressing that the changes do not compromise ml's essential stability. Some obscure technical points have been simplified. Anomalies in the original definition have been corrected. Existing programs will run with few or no changes. The most visible changes are the new character type and a new set of top level library functions.

The new edition brings the book up to date and greatly improves the presentation. Modules are now introduced early — in Chapter 2 instead of Chapter 7 — and used throughout. This effects a change of emphasis, from data structures (say, binary search trees) to abstract types (say, dictionaries). A typical section introduces an abstract type and presents its ml signature. Then it explains the ideas underlying the implementation, and finally presents the code as an ml structure. Though reviewers have been kind to the first edition, many readers have requested such a restructuring.