The option command allows the user to set options in the CHR\ compiler.
| Option | ::= | option(Option, On_or_off). |
Options can be switched on or off. Default is on. Advanced users may switch an option off to improve the efficiency of the handler at the cost of safety. Options are:
check_guard_bindings :
When executing a guard , it is checked that no global variables
(variables of the rule heads) are touched (see subsection on how
CHRs work). If the option is on, guards involving cut, if-then-else
or negation may not work correctly if a global variable has been touched before.
If switched off, guard checking may be significantly
faster, but only safe if the user makes sure that global variables are
not touched. To ensure that the variables are sufficiently bound,
tests like nonvar/1 or delays can be added to the predicates
used in the guards.already_in_store :
Before adding a user-defined constraint to the constraint store, it is
checked if there is an identical one already in the store. If there
is, the new constraint needs not to be added. The handling of the
duplicate constraint is avoided. This option can be set to off,
because the checking may be too expensive if duplicate constraints
rarely occur. Specific duplicate constraints can still be removed by
a simpagation rule of the form Constraint \ Constraint <=> true.already_in_heads :
In two-headed simplification rules, the intention is often to simplify
the two head constraints into a stronger version of one of the
constraints. However, a straightforward encoding of the rule may
include the case where the new constraint is identical to the
corresponding head constraint. Removing the head constraint and
adding it again in the body is inefficient and may cause termination
problems. If the already_in_heads option is on, in such a case
the head constraint is kept and the body constraint ignored. Note
however, that this optimization currently only works if the body
constraint is the only goal of the body or the first goal in the
conjunction comprising the body of the rule (see the example handler
for domains). The option may be too expensive if identical head-body
constraints rarely occur.debug_compile (set and
unset with dbgcomp and nodbgcomp) is also taken into
account by the CHR compiler. The default is on.
If switched off, the resulting code is more
efficient, but cannot be debugged anymore (see section 5.8).