'ruleset' Reference

A Nero rule set is defined within a Joe script using the ruleset expression, which returns a RuleSet value.

The ruleset Expression

A ruleset expression has this form:

ruleset { body }

where body consists of Nero axioms and rules, just like a standalone Nero program, but may also contain any number of export declarations.

Axioms

An axiom has this form:

relation (term, [term...]);

where

  • relation is an identifier, conventionally beginning with an uppercase letter.
  • The terms are constant terms: Joe keywords, strings, numbers, true, false, or null.

Rules

A rule has this form:

head :- bodyAtom [, bodyAtom...] [where constraints] ;

The rule head is an atom whose terms are constant or variable terms:

relation (term, [term...])

  • All variable terms in the head must be bound in the rule's body.

A body atom is an atom, possibly negated, whose terms may be constants, variables, or wildcards:

[not] relation (term, [term...])

  • The first body atom must not be negated.
  • Variables used in negated atoms and constraints must be bound by non-negated atoms to their left in the rule's body.

A rule may have zero or more optional constraints using the where clause:

where constraint [, constraint...]

A constraint has this form:

variable op term

where

  • variable must be a variable bound in a non-negated body atom.
  • op is a comparison operator, one of: ==, !=, >, >=, <, <=
  • term is a constant or variable term; if a variable, it must be bound in a non-negated body atom.

Export Declarations

By default, all facts inferred via axioms or rules are returned to the script as Fact values. An export declaration directs Nero to return all facts having a given relation as some other kind of Joe value.

An export declaration has this form:

export relation [as expression];

where

  • relation is a relation used in an axiom or in the head of a rule.
  • If no as clause is given, relation must also be the name of a type that is in scope whose initializer takes the same number of arguments as the relevant atoms have terms.
  • If the as clause is given, expression must be a Joe expression that evaluates to a callable in the current scope.

However specified, the callable will be used to convert all facts having the given relation to Joe values on output from the rule set.