1. ”assert” and ”retract”
Lecture 2
11/04/2017
”assert” and ”retract”
A Simple Chatbot
Modifying Programs Dynamically
Prolog’s Execution Tree
~ryang/prolog/top.html

2. A Simple Chatbot – only producing random responses
gen_reply(S, R):- % totally random
respones_db(random, Res),
% create a random number
length(Res, Length),
Upper is Length+1,
random(1, Upper, Rand),
nth_item(Res, Rand, R).
respones_db(random, [
[hello, '!'],
[hi, there , '.', this, is, not, a, hello_world, program, '!'],
[it, is, a, nice, day, '.'],
[have, you, talked, to, me, before, '?'],
['.', '.', '.', what, do, you, mean, '?']
]).
chatbot:-
print_welcome,
conversations.
conversations:-
repeat, % prolog built-in
print_prompt(you),
readin(S), % defined in file
% readin.pl
gen_reply(S,R),
print_prompt(me),
write_list(R),
is_quit(S).
is_quit(S):- member(bye, S).

3. (cont. ) | bot1 : sorry i cannot remember everything you said
(cont.) | bot1 : sorry i cannot remember everything you said . user |: ok. bot1 : sorry , i am only a single minded chatbot . user |: you said that. bot1 : sorry i cannot remember everything you said . user |: no problem. bot1 : oh ok user |: good. bot1 : can you say it again ? user |: good!!!! bot1 : have you talked to me before ? user |: getting boring now, bye. bot1 : bye !
| ?- chat. Welcome! I am a chatbot Please finish your line with a full stop, ?, or ! user Hi. bot1 : hello ! user |: what is your name? bot1 : have you talked to me before ? user |: no, I don't think so. bot1 : hi there . this is not a hello_world program ! user |: cool. bot1 : sorry , i am only a single minded chatbot . user |: ok, i don't mind. bot1 : what do you mean ? user |: I mean I like you.

4. A simple way to link chatbot to ‘cango’ program
In get_reply(S,R)
check if S has a pattern ‘to ... from ...’
if yes, call your ‘cango’, then generate the reply.
pattern_tofm([to, X, from, Y|_], X, Y). % yes!
pattern_tofm([_|T], X, Y):- % carry on checking
pattern_tofm(T, X, Y).

5. Built-ins to Modify Prolog Programs
The following built-ins allow modification of
the loaded program as it is actually running.
assert(C) - adds a prolog code (clause) C to program
asserta(C) - C is added to front
assertz(C) = assert(C) - C is added to end
They always succeed.
retract(C) - deletes a prolog code that matches C
It fails if there is nothing to delete

6. Examples of using assert/retract
?-nice.
?-disgusting.
?-retract(fog).
?-assert(sunshine).
?-funny.
?-retract(raining).
nice:- sunshine,
\+ raining.
% “\+ “ means not
funny:- sunshine,
raining.
disgusting:- raining, fog.
fog.

7. Examples of using assert/retract
Fibonacci numbers: 1,1,2,3,5,8,13,21,34,…
fib(N,F) - F is the N'th Fibonacci number.  
fib(1,1).
fib(2,1).
fib(N,F) :- N > 2, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1 + F2.

8. Execution Tree to Compute fib(5,F)
fib(4, F1) fib(3, F2) F is F1+F2
fib(3, F1’) fib(2,F2’) F’ is F1’+F2’
fib(2, F1’’) fib(1,F2’’) F’’ is F1’’+F2’’

9. To improve the above program, we can use “assert” to save intermediate results
:- dynamic myfib/2.
myfib(1,1).
myfib(2,1).
myfib(N,F) :- N > 2, N1 is N-1, N2 is N-2, myfib(N1,F1), myfib(N2,F2),
F is F1 + F2,
% remember n-th Fibonacci number asserta( myfib(N,F) ).

10. assert/retract plays two roles
Adding new information to the database
(ref. previous examples)
We will use this in our chatbot for collecting user’s information
Communicating between or-branches
An example: ?- binary(X),binary(Y).
binary(0).
binary(1).

11. the following program collects a list of boy names
:- dynamic tmp/1.
find_all_boy_names(NameList):-
% initialise the name list as an empty list
assert(tmp([])),
boy_names(X, _, _), % return solution one by one
retract(tmp(CurrentList)),
assert(tmp([X|CurrentList])), % add new solution
fail.
find_all_boy_names(NameList):- retract(tmp(NameList)).

12. findall( P, cango3(X,Y,P), AllRoutes).
How to use “findall”
Prolog has provided a built-in predicate findall(Vars, Goal, S) which collects all solutions of Goal into S.
Here is an example:
test(X,Y, AllRoutes):-
findall( P, cango3(X,Y,P), AllRoutes).
This returns
AllRoutes = [path1,path2,…]
where each pathi is a solution

13. Back to chatbot
If you want to remember some lines from user, you can have the following initially
respones_db(echo, []).
In the program, after reading a sentence S from user, you can do
retract(respones_db(echo, CurrentList)),
assert(respones(echo,([S|CurrentList])),
If your user says, I am from London,....
assert(user_info(from, london)),