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Task 4: Making the Engine Explain Itself

Solution to Task 3

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].")))


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'}]

Theory: Asking Why

Two calls ask the engine about its own reasoning.

explain(fact) returns a string, not a dictionary. It is a ready-formatted proof tree, so print it directly:

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

It names the rule that fired, the bindings it used, and the asserted facts underneath, down to the source line each came from. Note the argument: a bare fact, with no ?- and no trailing period.

why_not(fact) handles the opposite case and returns a dictionary:

{'fact': ..., 'nodes': [...], 'complete': True, 'scope': '...'}

Each entry in nodes carries a goal and its status. The scope field is a disclaimer the engine states about its own answer:

bounded first-blocker inspection; not exhaustive minimal repairs

It reports the first thing that blocked the derivation. It does not enumerate every possible repair, and a returned explanation is not evidence that a claim holds in the real world.

Task

Make the program account for two of its earlier answers.

  1. Print the explanation for dario[hasGrandparent -> elena].
  2. Call why_not for dario[hasParent -> anna] — the question that came back unknown in Task 2.
  3. From the result, print the status of the first entry in nodes, and print the scope field.

Read the proof tree against the program. The rule it names should be the grandparent rule, and the two facts underneath should be the two hasParent facts that chain through Bruno.

Optional Extra Task

Combine a query with an explanation. Ask ?- ?X[hasSibling -> carla]., then loop over the bindings and print an explanation for each match. Build each fact string from the binding you got back.