5.2.1 Janus classed Undefined and TruthVal

The class janus.Undefined() represents an undefined result under the Well Founded Semantics.

Undefined janus.Undefined(term=None)

Instances are never created explicitly by the user. They are created by the calls to Prolog initiated from janus.once() and janus.Query().

The class has a single property class term that represents either the delay list or the residual program. See janus.TruthVal() for details.

Enum janus.TruthVal()

This class is a Python enumeration. Its values are passed as the optional truth parameter to janus.once() and janus.Query(). The defined instances are

NO_TRUTHVALS

Undefined results are reported as True. This is quite pointless in the current design and this may go.

PLAIN_TRUTHVALS

Return undefined results as janus.undefined, a unique instance of the class janus.Undefined().

DELAY_LISTS

Return undefined results as an instance of class janus.Undefined(). thats holds the delay list in Prolog native representation. See call_delays/2.

RESIDUAL_PROGRAM

Return undefined results as an instance of class janus.Undefined(). thats holds the residual program in Prolog native representation. See call_residual_program/2.

The instances of this enumeration are available as attributed of the janus module.

For example, given Russel's paradox defined in Prolog as below.

:- module(russel, [shaves/2]).

:- table shaves/2.

shaves(barber,P) :- person(P),  tnot(shaves(P,P)).
person(barber).
person(mayor).

From Python, we may ask who shaves the barber in four ways as illustrated below. Note that the Prolog representations for janus.DELAY_LISTS and janus.RESIDUAL_PROGRAM use the write_canonical/1 notation. They may later be changed to use a more human friendly notation.

# Using NO_TRUTHVALS
>>> janus.once("russel:shaves(barber, X)", truth=janus.NO_TRUTHVALS)
{'truth': True, 'X': 'barber'}

# Using default PLAIN_TRUTHVALS (default)
>>> janus.once("russel:shaves(barber, X)")
{'truth': Undefined, 'X': 'barber'}

# Using default DELAY_LISTS
>>> janus.once("russel:shaves(barber, X)", truth=janus.DELAY_LISTS)
{'truth': :(russel,shaves(barber,barber)), 'X': 'barber'}

# Using default RESIDUAL_PROGRAM
>>> janus.once("russel:shaves(barber, X)", truth=janus.RESIDUAL_PROGRAM)
{'truth': [:-(:(russel,shaves(barber,barber)),tnot(:(russel,shaves(barber,barber))))], 'X': 'barber'}