I am not sure as to why 'find_identity(X)' is returning as false [Code Below] - prolog

Snippets of my code are shown below. After doing a lot of cumbersome tracing, I have reached a point where I think it's the external modules which are the source of my problem, as the findall in links_of_actor never gets called, but links_of_actor itself gets called. I would be very grateful to anyone who can help and can clarify/add more of my code to this question if needed.
find_identity(A) :-
all_actor_links(ALs),
find_identity(ALs,[],A).
find_identity([a(A,_)],_,A).
find_identity(ALs,Ms,A) :-
agent_ask_oracle(oscar,o(1),link,L),
( memberchk(L,Ms) -> find_identity(ALs,Ms,A)
; otherwise -> purge_actors(ALs,L,NewALs),
find_identity(NewALs,[L|Ms],A)
).
links_of_actor(A,Ls) :-
actor(A),
wp(A,WT),
findall(L,wt_link(WT,L),Ls1),
findall(L,link(L),Ls2),
intersection(Ls1,Ls2,Ls).
actor_links(A,a(A,Ls)) :-
links_of_actor(A,Ls).
all_actor_links(ALs) :-
findall(A,actor(A),As),
maplist(actor_links,As,ALs).
----------------------------------------------------- Support Functions ---------------------------------------------------
% wp(Q,WT) <- issue query Q to Wikipedia and return the page in wikitext format
wp(Q,WT):-
wp_cache(Q,WT),!.
wp(Q,WT):-
wp_query2URL(Q,URL),
http_get(URL,R,[]),
atom_json_term(R,RR,[]),
wt_get(RR,WT0),
( atomic_list_concat1(_L,'#REDIRECT',WT0) -> wt_link(WT0,QQ),wp(QQ,WT)
; otherwise -> WT=WT0
),
assert(wp_cache(Q,WT)).
------------------------------------------------------------ Edit ----------------------------------------------------------------
After using the guitracer that is available with prolog, I have found that the program fails at http_get(URL,R,[]) in the wp(Q,WT) predicate. However, I am still not sure as to why this isn't working - Could it be possibly because of my internet?
To clarify, the predicate actor is defined in my file: actor('Billy Bob Thornton'). etc. and so are the links : link('Barack Obama')..


Using these definitions, the error can be localized by adding a # in front of those goals that have to succeed.
...
wp(Q,WT):-
#wp_query2URL(Q,URL),
#http_get(URL,R,[]),
#atom_json_term(R,RR,[]),
#wt_get(RR,WT0),
...


Well, did you try to see if the URL you gave to http_open is actually valid? You can always test from the top-level if you can http_open an URL:
?- use_module(library(http/http_client)).
true.
?- http_open("stackoverflow.com", R, []).
R = % a lot of stuff

Related

How to repair error in Einstein Zebra in prolog >

I have a problem with zebra puzzle.
rightOf(A,B,(B,A,_,_,_)).
rightOf(A,B,(_,B,A,_,_)).
rightOf(A,B,(_,_,B,A,_)).
rightOf(A,B,(_,_,_,B,A)).
middleHouse(A,(_,_,A,_,_)).
firstHouse(A,(A,_,_,_,_)).
nextTo(A,B,(A,B,_,_,_)).
nextTo(A,B,(_,A,B,_,_)).
nextTo(A,B,(_,_,A,B,_)).
nextTo(A,B,(_,_,_,A,B)).
nextTo(A,B,(B,A,_,_,_)).
nextTo(A,B,(_,B,A,_,_)).
nextTo(A,B,(_,_,B,A,_)).
nextTo(A,B,(_,_,_,B,A)).
house[Nationality,Pet,Sport,Drinks,Colour].
zebra_owner(Owner) :-
houses(Houses),
exists(house(Owner,zebra,_,_,_), Houses).
length(5,Houses),
exists(house(british,_,_,_,red),Houses),
exists(house(spanish,dog,_,_,_),Houses),
exists(house(_,_,_,coffe,green),Houses),
exists(house(ukrainian,_,_,tea,_),Houses),
rightOf(house(_,_,_,_,green),house(_,_,_,_,ivory),Houses),
exists(house(_,snail,tennis,_,_),Houses),
exists(house(_,_,chess,_,yellow),Houses),
middleHouse(house(_,_,_,mlik,_),Houses),
firstHouse(house(norwegian,_,_,_,_),Houses),
nextTo(house(_,_,rugby,_,_),house(_,fox,_,_,_),Houses),
nextTo(house(_,_,chess,_,_),house(_,horse,_,_,_),Houses),
exists(house(_,_,volleyball,orangejuice,_),Houses),
exists(house(japanese,_,_,_,_),Houses),
nextTo(house(norwegian,_,_,_,_),house(_,_,_,_,blue),Houses),
nextTo(house(_,_,_,tea,_),house(_,_,_,milo,_),Houses),
exists(house(zebra_owner,zebra,_,_,_),Houses).
Under
house[Nationality,Pet,Smokes,Drinks,Colour].
I see the error: Singleton variables: house[Nationality,Pet,Smokes,Drinks,Colour].
And when I write ?- zebra_owner(Owner)
I see the error: procedure 'houses(A)' does not exist. Reachable from:zebra_owner(A).`

You cannot write
house[Nationality,Pet,Sport,Drinks,Colour].
that's non-existent syntax.
To express that predicate house/1 is valid for list [Nationality,Pet,Sport,Drinks,Colour], write:
house([Nationality,Pet,Sport,Drinks,Colour]).
You also must pay attention to predicate naming: houses/1 is not house/1. Predicate houses/1 is defined nowhere (the error message is wrong, it should really say predicate houses/1 is undeclared).

how to solve the error in this prolog code

What is wrong in this code
frequency(f1).
frequency(f2).
frequency(f3).
allocation(z1,z2,z3,z4,z5,z6,z7,z8) :-
frequency(z1), frequency(z2), frequency(z3), frequency(z4), frequency(z5), frequency(z6), frequency(z7), frequency(z8),
z1\==z2, z1\==z3, z2\==z4, z2\==z3, z3\==z4, z3\==z6, z4\==z5, z4\==z6, z4\==z7, z6\==z7, z6\==z8.
?- allocation(f1,f2,f3,f1,f2,f3,f2,f3).
syntax error: . or operator expected after expression

The following code highlighting should make it clear why the goal allocation(f1,f2,f3,f1,f2,f3,f2,f3) cannot succeed:
allocation(Z1,Z2,Z3,Z4,Z5,Z6,Z7,Z8) :-
dif(Z1,Z2), dif(Z1,Z3),
dif(Z2,Z3), dif(Z2,Z4),
dif(Z3,Z4), dif(Z3,Z6),
dif(Z4,Z5), dif(Z4,Z6), dif(Z4,Z7),
dif(Z6,Z7), dif(Z6,Z8),
maplist(frequency, [Z1,Z2,Z3,Z4,Z5,Z6,Z7,Z8]).
Note the use of dif/2 instead of (\==)/2 in the code above?
prolog-dif preserves logical-purity, for easier debugging!

SWI-Prolog ERROR: No permission to open websocket

I tried the code below and I got:
ERROR: No permission to open websocket `'ws://localhost:9999''.
Why?
If I use root(.), it's ok.
UPDATED (correct code):
:- use_module(library(http/websocket)).
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_path)).
:- use_module(library(http/http_authenticate)).
:- initialization main.
main :-
run.
:- http_handler(root(ws), http_upgrade_to_websocket(echo, []), [spawn([])]).
echo(WebSocket) :-
ws_receive(WebSocket, Message),
( Message.opcode == close
-> true
; ws_send(WebSocket, Message),
echo(WebSocket)
).
run :-
run(9999).
run(Port) :-
http_server(http_dispatch, [port(Port)]).
stop :-
stop(9999).
stop(Port) :-
http_stop_server(Port, []).

In abstract terms, If you use root(X), then this means that the path where this is available is /X.
So, if you specify root(ws), then in your case, you would have to access:
ws://localhost:9999/ws
If you specify root(.) then / suffices etc.
Next time, please specify a complete example, so that others can actually try out your code without having to guess the remainder of your snippet.

Simple error for someone who knows prolog. There is no fullstop to be causing the error

likes(a,b).
likes(b,d).
likes(c,e).
likes(d,f).
likes(e,h).
likes_trans(X,Y) :- likes(X,Z),likes_trans(Z,Y).
likes_trans(X,Y) :- likes(X,Y).
listfriends(X,R) :- findall(Y,likes_trans(X,Y),R).
likes_both(X,Y,R) := listfriends(X,P), listfriends(Y,S), member(R,P), member(S,P).
Why does the last line 'likes_both' give me the error (when loading the file)
Full stop in clause-body? Cannot redefine
Any suggestions much appreciated, I know I'm missing something fundamental here.

It looks like the := on the last line should be replaced with :-
likes_both(X,Y,R) :- listfriends(X,P), listfriends(Y,S), member(R,P), member(S,P).
/* Here ---------^^ */

Raise an error in SWI Prolog

I'd like to print a message and stop the evaluation of the predicate. How do I do that?

Take a look at this link where the catch/3 and throw/1 mechanisms in Prolog are described.
It is possible to throw an exception or handle an exception using this mechanism.
The example (given on the site) is:
p:- true.
p:- throw(b).
q:- catch(p, B, write('hellop')), r(c).
r(X) :- throw(X).
Then the call:
?- catch(p, X, (write('error from p'), nl)).
will illustrate the ecxeption handling.

I played with a few other examples which I found. This might be usefull.
p :- throw(b).
r(X) :- throw(X).
q:- catch(p, B, format('Output1: Error from throwing p')), r(B).
catch(throw(exit(1)), exit(X), format('Output: `w', [X])).
Output: 1
1 is thrown to catcher 'exit(X)' and recovered by format('Output: ~w', [X])),
catch(p, C, format('hellop')).
Output: hellop
p throws 'b' which is recovered by writing 'hellop'
catch(q, C, format('Output2, Recovering q: helloq ')).
Output1: Error from throwing p
Output2, Recovering q: helloq
Ben

The stderr stream is aliased to user_error (see https://www.swi-prolog.org/pldoc/man?section=streamalias )
You can write a term (like a string) to a stream using write/2.
You can stop the program using halt.
For example:
% Happy case. Comes first, uses ! to avoid taking the alternative branch
foo(X) :- bar(X, Y), baz(Y), !.
% Fallback branch, in case the above failed. Writes a message and halts.
foo(X) :-
write(user_error, "Couldn't foo"),
halt.

Resources