trace/2

Availability::- use_module(library(prolog_trace)).(can be autoloaded)

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. Tracing a predicate is achieved by wrapping the predicate using wrap_predicate/4.

Each time a port (of the 4-port model) is passed that has a trace point set, the goal is printed. 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.

Examples

Examples for scanl/N

?- scanl(plus, [1,2,3,4,5], 0, Sums).
Sums = [0, 1, 3, 6, 10, 15].

When considering the lists columns of a table, scanl/5 combines the accumulator with each row to produce the next value in the result list. We illustrate this using plus/4 which adds the accumulator with the values from the row to produce the next accumulator. We use trace/2 to illustrate how the values are combined.

plus(A,B,C,Result) :-
    Result is A+B+C.

?- trace(plus/4, exit).
%         plus/4: [exit]
?- scanl(plus, [1,2,3,4,5], [10,20,30,40,50], 0, Sums).
 T Exit: plus(1, 10, 0, 11)
 T Exit: plus(2, 20, 11, 33)
 T Exit: plus(3, 30, 33, 66)
 T Exit: plus(4, 40, 66, 110)
 T Exit: plus(5, 50, 110, 165)
Sums = [0, 11, 33, 66, 110, 165].