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Task 5: Revising Safely

Solution to Task 4

Show the full program and what it prints 
"""Drive a small family knowledge base from Python."""

from framex import Client

PROGRAM = """world open.

Person { hasName: String }.
Man extends Person.
Woman extends Person.

elena : Woman.
anna : Woman.
bruno : Man.
dario : Man.
felix : Man.

anna[hasParent -> elena].
bruno[hasParent -> elena].
dario[hasParent -> bruno].
felix[hasParent -> anna].

?X[hasSibling -> ?Y] <- ?X[hasParent -> ?P] AND ?Y[hasParent -> ?P] AND ?X != ?Y.
?C[hasGrandparent -> ?G] <- ?C[hasParent -> ?P] AND ?P[hasParent -> ?G].
"""


def describe(result):
    status = result["status"]
    if status == "bindings":
        rows = result["bindings"]
        return f"{len(rows)} match(es): {rows}"
    return status


NEW_FACTS = """carla : Woman.
carla[hasParent -> elena].
"""


def main():
    with Client() as client:
        loaded = client.load(PROGRAM)
        print("loaded:", loaded)

        result = client.query("?- dario[hasGrandparent -> elena].")
        print("grandparent:", result)

        print("ground :", describe(client.query("?- dario[hasGrandparent -> elena].")))
        print("binding:", describe(client.query("?- dario[hasGrandparent -> ?G].")))
        print("open   :", describe(client.query("?- dario[hasParent -> anna].")))
        print("many   :", describe(client.query("?- ?X[hasSibling -> ?Y].")))

        change = client.add(source=NEW_FACTS)
        print("added  :", change["added"])
        print("removed:", change["removed"])
        print("carla  :", describe(client.query("?- carla[hasSibling -> ?Y].")))

        print()
        print(client.explain("dario[hasGrandparent -> elena]"))

        blocked = client.why_not("dario[hasParent -> anna]")
        print("why_not status:", blocked["nodes"][0]["status"])
        print("why_not scope :", blocked["scope"])


if __name__ == "__main__":
    main()

Running it prints:

loaded: {'facts': 26}
grandparent: {'status': 'true'}
ground : true
binding: 1 match(es): [{'G': 'elena'}]
open   : unknown
many   : 2 match(es): [{'X': 'anna', 'Y': 'bruno'}, {'X': 'bruno', 'Y': 'anna'}]
added  : ['carla:Person', 'anna[hasSibling -> carla]', 'bruno[hasSibling -> carla]', 'carla[hasSibling -> anna]', 'carla[hasSibling -> bruno]']
removed: []
carla  : 2 match(es): [{'Y': 'anna'}, {'Y': 'bruno'}]

World assumption: open
dario[hasGrandparent -> elena]
  Derived by rule #1: ?C[hasGrandparent -> ?G] <- ?C[hasParent -> ?P] AND ?P[hasParent -> ?G].
  Bindings: C = dario, G = elena, P = bruno
  Because:
    dario[hasParent -> bruno]
      Asserted fact
        Source: <json-source> | time: unspecified | weight: None | <json-source>:15:1
    bruno[hasParent -> elena]
      Asserted fact
        Source: <json-source> | time: unspecified | weight: None | <json-source>:14:1

why_not status: unknown
why_not scope : bounded first-blocker inspection; not exhaustive minimal repairs

Theory: Look Before You Retract

Removing a stated fact also removes everything derived from it. dependents shows that reach before you commit to it:

client.dependents("dario[hasParent -> bruno]")

It returns a plain list of fact strings — the facts that would lose their support:

['dario[hasGrandparent -> elena]']

retract(fact=...) then withdraws the stated fact and answers with the same added / removed shape that add used. The derived facts appear in removed; you withdrew one fact and the engine dropped its consequences.

diff() repeats the most recent change. It takes no arguments and is useful when the call that caused the change is far from the code inspecting it.

After a retraction the question that used to answer true answers unknown, not false. Withdrawing evidence removes a reason to believe something. It does not create a reason to disbelieve it.

Task

Take Dario’s parent away and watch the consequences.

  1. Print the dependents of dario[hasParent -> bruno] before touching it.
  2. Retract the fact, and print the removed list from the result.
  3. Print the removed list from diff() as well, and compare the two.
  4. Ask ?- dario[hasGrandparent -> elena]. again and print the answer.

Predict the final status before you run it. It was true in Task 1.

Optional Extra Task

retract_source(source) removes everything that came from one source at once, rather than one fact at a time. compare(facts=..., before=..., after=...) checks a set of facts across two revisions.

Use dependents on bruno[hasParent -> elena] instead, and predict how many facts that retraction would remove before you try it. It sits one step higher in the chain than the fact you just withdrew.