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