NOT (Negation)
A negation of a pattern requires that pattern to be false. A negation is itself a pattern.
Syntax
A negation of a pattern is of the form:
not "{" <pattern> "}" ;where <pattern> can be any valid pattern. Note that at least one variable of <pattern> must be bound outside the negation.
Behavior
A negation is true, if its negated pattern is false.
Negations can be used in a match clause or a condition of a rule
to set a constraint of not matching the negated pattern.
When using negations in rules, complicated logical behavior can arise: we use the theory of stratified negation to compute the behavior of patterns in these cases.
Negation of statement
For this example, use a database with the IAM schema and sample data loaded.
The most direct approach to using negation is to specify a pattern for all results that need to be excluded.
Let’s try to get all users except those who have a full-name attribute with a particular value:
match
$u isa user;
not {$u has full-name "Kevin Morrison";};
fetch
$u as "Not Kevin": full-name;See example output
{
"Not Kevin": {
"full-name": [ { "value": "Bob Fake", "type": { "label": "full-name", "root": "attribute", "value_type": "string" } } ],
"type": { "label": "person", "root": "entity" }
}
}Negation in rules
For this example, use a database with the IAM schema and sample data loaded.
Negation can be used in a rule condition in the same way as it can be used in a match clause.
There is no need to negate the conclusion of a rule, as the rule will not repeat itself if there is nothing to change.
Let’s create a rule to create complementary valued attributes for the validity of permission relations:
define
rule permission-non-validity: when {
$permission isa permission;
not { $permission has validity true; };
} then {
$permission has validity false;
};Negation with cycles
The following example yields an error!!
In contrast to the previous example, we cannot write a rule of the following form:
define
rule give-all-permissions: when {
$permission isa permission;
not { $permission has validity true; };
} then {
$permission has validity true;
};The rule creates a logical cycle, which violates the principles of of stratified negation.