Current Restrictions

• The maximum arity for predicate and function symbols is 255.
• The maximum length of atoms that can be stored in an XSB object code file is in principle 2³² - 1, but in practice it is 2²⁸ - 1 (i.e., in 32-bit platforms it is bounded by the size of the maximum integer; see below).
• In the current version, you should never try to rename a byte code file generated for a module, though you can move it around in your file system. Since the module name is stored in the file, renaming it causes the system to load it into wrong places. However, byte code files for non-modules can be renamed at will.
• XSB allows up to 1 Gigabyte of address space for 32-bit SUNs and 512 Megabytes of address space for other 32-bit platforms. For SUNs the address space for integers is -2²⁸-- (2²⁸ - 1). For MIPS-based machines (e.g. Silicon Graphics machines), the address space for integers is -2²⁶-- (2²⁶ - 1). For all other machines it is -2²⁷-- (2²⁷ - 1). This restriction can cause unexpected results when numbers are computed. The amount of space allowed for floating point numbers is similar for each machine. For 64-bit platforms, addresses, integers, and floating point numbers are all stored in 60 bits. However, as the object code file format is the same as for the 32-bit versions, compiled constants are subject to 32-bit limitations.
• Indexing on floating-point numbers does not work, since, as implemented in XSB, the semantics of floating-point unification is murky in the best case. Therefore, it is advisable that if you use floating point numbers in the first argument of a procedure, that you explicitly index the predicate in some other argument.
• The XSB compiler cannot distinguish the occurrences of a 0-ary predicate and a name of a module (of an import declaration) as two different entities. For that reason it fails to characterise the same symbol table entry as both a predicate and a module at the same time. As a result of this fact, a compiler error is issued and the file is not compiled. For that reason we suggest the use of mutually exclusive names for modules and 0-ary predicates, though we will try to amend this restriction in future versions of XSB.
• Subsumption-based tabled predicates may not be delayed. Consequently,
  • the truth value of a negative call on a subsumptive predicate must be known at completion of the producing call, thus avoiding a negative delay of this negative call, and
  • only unconditional answers may be derived for a subsumptive predicate, thus avoiding the positive delay of calls which consume such an answer.
  Violations of either of these conditions raise an exception and abort the computation.