Prolog has been in use for many years now, both for research in artificial intelligence (AI) and other fields and for commercial and industrial work. Recently interest in it has swelled dramatically, in part because of its role in the Japanese Fifth Generation project and in part because good implementations are becoming widely available. Special-purpose Prolog workstations such as the Japanese Personal Sequential Inference (PSI) machine are beginning to appear, and some mainframe manufacturers are producing Prolog accelerators for their machines. Nevertheless it remains a difficult language to learn; the gap between acquiring a reading knowledge and a writing knowledge of it is much wider than in almost any other programming language currently in use. From my own experience of teaching a postgraduate course in it for the past few years, it is clear that those who have considerable experience of more conventional languages such as Pascal or FORTRAN are handicapped by their instincts - they still thirst after 'for' loops and 'while' loops, type declarations and assignment statements and the many other features common to most languages. Many go so far as to try, elaborately and usually unsuccessfully, to recreate these features in Prolog rather than make the effort to explore the different idioms that it encourages.
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