trace/2

Availability:built-in

trace(+Pred, +Ports)

Put a trace point on all predicates satisfying the predicate specification Pred. Ports is a list of port names (call, redo, exit, fail). The atom all refers to all ports. If the port is preceded by a - sign, the trace point is cleared for the port. If it is preceded by a +, the trace point is set.

The predicate trace/2 activates debug mode (see debug/0). Each time a port (of the 4-port model) is passed that has a trace point set, the goal is printed as with trace/0. Unlike trace/0, however, the execution is continued without asking for further information. Examples:

`?- trace(hello).`	Trace all ports of hello with any arity in any module.
`?- trace(foo/2, +fail).`	Trace failures of foo/2 in any module.
`?- trace(bar/1, -all).`	Stop tracing bar/1.

The predicate debugging/0 shows all currently defined trace points.