DiagnostiCar (3) - Question & Answer Interface

This post aims to explain how to create an interactive shell so that users may answer questions when the reasoning system need user information. The reasoning engine will access the ask engine by an predicate (I will call it ask). An naive implementation of the ask predicate could be:

ask(X) :-
 write(X), write(' is true?'), nl,
 read(yes).

It works, but askable predicates will be asked every time the reasoner will need an information, even if it was previously answered. Another specific problem from DiagnostiCar constraints, was how to separate which information is from which user, considering we're at an web application environment. The simultaneous users access to the reasoning engine couldn't simply be locked while an user was giving an answer back, so we must find a smarter way to ask.

To remember info in Prolog, we will need to use dynamic relations to store the asked questions, for those who don't know about dynamic relations I will give a short explanation. If you want to know more, read the "Building Expert Systems in Prolog"[1] book.

Dynamic relations works similarly as a relational table in a relational database [2], but without a type scheme (although it's strongly recommended to make a convention). The informations required to create a dynamic relation are just it's name and arity. To declare a dynamic predicate you just need to run dynamic(Predicate). Once the relation is a dynamic one, you can store information (assert/1 or asserta/1) or delete information (use retract/1 and retractall/1) eg.: assert(foo(bar)). To query a dynamic predicate you just have to call it (it allows unification!) eg.: ?- foo(bar).  -> true. I made the following basic but illustrative example about Prolog DB:

create_account(Id, Ammount) :-
    assert(account(Id, Ammount)).
    
deposit(Id, Ammount) :-
    retract(account(Id, PrevAmmount)),
    NewAmmount is Ammount + PrevAmmount,
    assert(account(Id, NewAmmount)).
    
withdraw(Id, Ammount) :-
    account(Id, PrevAmmount),
    PrevAmmount >= Ammount,
    retract(account(Id, PrevAmmount)),
    NewAmmount is PrevAmmount - Ammount,
    assert(account(Id, NewAmmount)).

To test the predicates do some queries:

?- create_account(1, 1000).
true.

?- deposit(1, 200).
true.

?- account(Id, Ammount).
Id = 1,
Ammount = 1200.

?- withdraw(1, 400).
true.

?- withdraw(1, 20000).
false.

?- account(Id, Ammount).
Id = 1,
Ammount = 800.

The best solution found, to solve the concurrence and the question tracking, was to create predicates to read and write, so the webapp could obtain such information from the expert system. To avoid repeating the same questions, we will have to maintain a table with questions made, one with the answered questions and another to keep the answers from the users. Note that each relation needs to keep an user id to know from which user each information belongs to. The expert system question interface is the following:

% asked: UserId | Question
:- dynamic asked/2.
% asked: UserId | Question
:- dynamic askfor/2.
% known: UserId | Question | Fact
:- dynamic known/3.

% Predicate used by the expert system to ask.
ask(Uid, Question, Answer) :-
    known(Uid, Question, Answer).
ask(Uid, Question, _ ) :-
    not(asked(Uid,Question)),
    not(askfor(Uid,Question)),
    assert(askfor(Uid, Question)),
    fail.

% Predicate used by the web app to input users answers for a given qeustion.
answer(Uid, Question, [Answer | Answers]) :-
    assert(known(Uid, Question, Answer)),
    answer(Uid, Question, Answers).
answer(Uid, Question, []) :-
    assert(asked(Uid, Question)),
    retractall(askfor(Uid,Question)).

% Routine called by the webapp to clean the data from a user whose 
% session was expired.
cleanup(Uid) :-
    retractall(askfor(Uid,_)),
    retractall(known(Uid,_,_)),
    retractall(asked(Uid,_)).

Not that hard! I will try to tell all about the abductive reasoning on the next post.

[1] "Building Expert Systems in Prolog"

[2] SWI Database