ClearRelation function needed in the PHP-environment.

[stefjoosten]

When I need a helper relation (e.g. with transitive closures), I sometimes need to delete the entire population of that relation. It would be very nice to have a PHP function for that. To clear out the entire relation, e.g. rel[A*A], I would like to write

TXT "{EX} ClearRelation;rel;A;A"

Use case for this feature

To edit a relation means to insert and delete pairs at arbitrary times during the lifetime of that relation. Suppose I want to edit a relation that is used in a transitive closure. E.g.

RELATION rel[A*A]
RELATION relPlus[A*A]

ENFORCE relPlus >: (rel \/ relPlus;rel) - I[A]

(Side note: - I[A] is necessary to avoid infinite loops, in particular cases.)

Suppose I want to edit rel, including the deletion of pairs, maintaining that relPlus = rel*. Now, I have to change my code to:

RELATION rel[A*A]
RELATION relPlus[A*A]

RELATION relCopy[A*A]
ENFORCE relCopy >: rel
ENFORCE relPlus >: (relCopy \/ relPlus;relCopy) - I[A]  -- compute the transitive closure

RELATION relCopy[A*A]
RULE rel |- relCopy
VIOLATION (TXT "{EX} ClearPop;relCopy;A;A"
          ,TXT "{EX} ClearPop;relPlus;A;A")

We use relCopy for deleting the contents of relPlus when deleting one or more pairs from rel.