Datalog Code

The datalog code is a list of literals, separated by commas (,). As in Datalog or Prolog, this will be interpreted as a conjunction of the literals. To use disjunction, the code attribute has to be specified multiple times.

As literals, all literals that may be used in standard rules and queries, may also be used in datalog queries (see section 2.2 for a list). An argument of a literal may be one the following:

• an object name: If the object name includes special characters such as ! or ", it must be written in single quotes ('). This also holds for string object, e.g. "a string" must be written as '"a string"'.
• a predefined variable, defined by the context of the query or rule: If the query has a super class, the variable this refers to the instances of this class. If the query has parameters or computed attributes, variables with the names of the parameters or computed attributes will be predefined. In a DatalogInRule, the variable this refers to the instances of the concerned class. In a DatalogAttrRule, the variables src and dst refer to the source and destination object of the attribute. Note, that all predefined variables have to be prefixed with ~ and must be encoded in single quotes ('), e.g. '~this', '~src', '~param'.
• existential variables: These variables must be declared in a special literal vars(list), which has to be the first literal of the code. For example, the literal vars([x,y]) defines the variables x and y.

A query expression of the form query(q) may be also used as literal, or as second argument of an In-Literal. q may be any valid query expression, e.g. just the name of a query class, or a derive expression including the specification of parameters (for example, find_instances[Class/class]).

In addition, PROLOG predicates can be used as literals. You can define your own PROLOG predicates in a LPI-file (see section 5.2.2 for an example).