Task 2: Reading the Answer

Solution to Task 1

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

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


if __name__ == "__main__":
    main()

loaded: {'facts': 26}
grandparent: {'status': 'true'}

Theory: Four Shapes of Answer

Every query returns a dictionary with a status. There are four values:

The bindings key is present only when status is bindings:

{'status': 'true'}
{'status': 'bindings', 'bindings': [{'G': 'elena'}]}

So result["bindings"] raises KeyError on a question without variables. Read status first, then reach for bindings.

bindings is a list of dictionaries, one per match. The keys are the variable names without the ? marker: ?G in the query comes back as G. An empty list means the question had variables but nothing matched.

unknown is not false. Under world open. a question the program can neither support nor refute stays open, and the engine says so rather than guessing. See World Assumption.

Task

Add a function describe(result) above main that turns a result dictionary into a short readable string:

• when status is bindings, report how many matches there were and show them
• otherwise, report the status itself

Then use it to ask four questions and print each answer:

1. ?- dario[hasGrandparent -> elena]. — a fact the rules derive
2. ?- dario[hasGrandparent -> ?G]. — the same question with a variable
3. ?- dario[hasParent -> anna]. — something the program never states
4. ?- ?X[hasSibling -> ?Y]. — two variables at once

Before running, predict which of the four comes back unknown, and how many matches the last one finds.

Optional Extra Task

The sibling question returns Anna and Bruno twice, once in each direction. Print the matches as one line each instead of one long list, and explain in a comment why both directions appear.