10.9.5 Caveats and Limitations

Following are some general statements about the constraints and predicates of this library module.

Aliasing

In case of variable aliasing, i.e., if a variable occurs more than once in a constraint that is being posted, or if two variables of a constraint are unified after posting, then any guarantee to maintain a particular level of consistency no longer holds, and idempotency is almost always lost.

Termination

All constraints and propagators terminate. However, due to the combinatorial nature of constraint solving, and to the fact that constraint solving is based on filtering domains, which can be huge, pathological cases where termination takes extremely long time are easily constructed. After about 15,000 years on a 64-bit machine, the following query terminates with a representation error, when the lower bound of X exceeds the small integer range:

| ?- X #> abs(X).
[... ... ...]
! Representation error in user:'t=<u+c'/3
! CLPFD integer overflow
! goal:  't=<u+c'(_245,_247,-1)

Anyway, if you find non-pathological cases that take longer than reasonable time to terminate, then please write to sicstus-support@ri.se.

Error Checking

Contrary to most library modules, CLPFD constraints and predicates check their arguments to almost the same extent as built-in predicates. If you find a case where reasonable error checking is missing, then please write to sicstus-support@ri.se.

Copying Terms

If a term containing domain variables is written, copied, asserted, gathered as a solution to findall/3 and friends, or raised as an exception, then those domain variables will be replaced by brand new variables in the copy. To retain the domains and any attached constraints, you can use copy_term/3 with clpfd:full_answer asserted (see ref-lte-cpt and Answer Constraints). API change wrt. release 3.

Limited Support for Reals

The support for real variables is somewhat limited in this release and is likely to be extended in future releases.