The CHR debugging facilities are currently
rather limited. Only tracing is currently available. To use the CHR
debugging facilities for a CHR file it must be compiled for debugging.
Generating debug info is controlled by the CHR option debug,
whose default is derived from the SWI-Prolog flag generate_debug_info.
Therefore debug info is provided unless the -nodebug is
used.
For CHR constraints the four standard ports
are defined:
- call
-
A new constraint is called and becomes active.
- exit
-
An active constraint exits: it has either been inserted in the store
after trying all rules or has been removed from the constraint store.
- fail
-
An active constraint fails.
- redo
-
An active constraint starts looking for an alternative solution.
In addition to the above ports, CHR constraints have five additional
ports:
- wake
-
A suspended constraint is woken and becomes active.
- insert
-
An active constraint has tried all rules and is suspended in the
constraint store.
- remove
-
An active or passive constraint is removed from the constraint store.
- try
-
An active constraint tries a rule with possibly some passive
constraints. The try port is entered just before committing to the rule.
- apply
-
An active constraint commits to a rule with possibly some passive
constraints. The apply port is entered just after committing to the
rule.
Tracing is enabled with the chr_trace/0
predicate and disabled with the chr_notrace/0
predicate.
When enabled the tracer will step through the call,
exit, fail, wake and apply
ports, accepting debug commands, and simply write out the other ports.
The following debug commands are currently supported:
CHR debug options:
<cr> creep c creep
s skip
g ancestors
n nodebug
b break
a abort
f fail
? help h help
Their meaning is:
- creep
-
Step to the next port.
- skip
-
Skip to exit port of this call or wake port.
- ancestors
-
Print list of ancestor call and wake ports.
- nodebug
-
Disable the tracer.
- break
-
Enter a recursive Prolog top level. See break/0.
- abort
-
Exit to the top level. See abort/0.
- fail
-
Insert failure in execution.
- help
-
Print the above available debug options.
The library(chr) module
contains several predicates that allow inspecting and printing the
content of the constraint store.
- chr_trace
-
Activate the CHR tracer. By default the CHR tracer is activated and
deactivated automatically by the Prolog predicates trace/0
and
notrace/0.
- chr_notrace
-
Deactivate the CHR tracer. By default the CHR tracer is activated and
deactivated automatically by the Prolog predicates trace/0
and
notrace/0.
- chr_leash(+Spec)
-
Define the set of CHR ports on which the CHR tracer asks for user
intervention (i.e. stops). Spec is either a list of ports as
defined in section 8.4.1 or a
predefined `alias'. Defined aliases are:
full to stop at
all ports, none or off to never stop, and default
to stop at the call,
exit, fail, wake and apply
ports. See also leash/1.
- chr_show_store(+Mod)
-
Prints all suspended constraints of module Mod to the
standard output. This predicate is automatically called by the
SWI-Prolog top level at the end of each query for every CHR module
currently loaded. The Prolog flag
chr_toplevel_show_store controls whether the top level
shows the constraint stores. The value true enables it. Any
other value disables it.
- find_chr_constraint(-Constraint)
-
Returns a constraint in the constraint store. Via backtracking, all
constraints in the store can be enumerated.
