5 Debugging

This chapter describes the debugging facilities that are available in development systems. The purpose of these facilities is to provide information concerning the control flow of your program.

The main features of the debugging package are as follows:

• The Procedure Box model of Prolog execution, which provides a simple way of visualizing control flow, especially during backtracking. Control flow is viewed at the predicate level, rather than at the level of individual clauses.
• The ability to exhaustively trace your program or to selectively set spypoints. Spypoints allow you to nominate interesting predicates at which, for example, the program is to pause so that you can interact.
• The ability to set advice points. An advice point allows you to carry out some actions at certain points of execution, independently of the tracing activity. Advice points can be used, e.g. for checking certain program invariants (cf. the assert facility of the C programming language), or for gathering profiling or branch coverage information. Spypoints and advice points are collectively called breakpoints.
• The wide choice of control and information options available during debugging.

The Procedure Box model of execution is also called the Byrd Box model after its inventor, Lawrence Byrd.

Much of the information in this chapter is also in Chapter eight of [Clocksin & Mellish 81], which is recommended as an introduction.

Unless otherwise stated, the debugger prints goals using write_term/3 with the value of the Prolog flag debugger_print_options.

The debugger is not available in runtime systems and the predicates defined in this chapter are undefined; see Runtime Systems.