Prolog - Newbie question...simple rule question...

Author

Newbie question...simple rule question...

mans

2005-08-11, 9:03 am

Dear All,

I am trying to understand the best way to write a rule which checks on
all the elements.

-------------
people(a).
people(c).
ok(b).
ok(a).

everybodyok_works :- findall(X,ok(X),List),maplist(people,Lis
t).

everybodyok_bad :- people(X),ok(X).

-------------

What am I missing? Is there a better way to write rule which says:
"Everybody is ok if for all people we know has said they are OK." ?

Thanks in Advance,
Mans

Jan Wielemaker

2005-08-11, 9:03 am

On 2005-08-11, mans <mshapshak@gmail.com> wrote:
> Dear All,
>
> I am trying to understand the best way to write a rule which checks on
> all the elements.
>
> -------------
> people(a).
> people(c).
> ok(b).
> ok(a).
>
> everybodyok_works :- findall(X,ok(X),List),maplist(people,Lis
t).
>
> everybodyok_bad :- people(X),ok(X).
>
> -------------
>
> What am I missing? Is there a better way to write rule which says:
> "Everybody is ok if for all people we know has said they are OK." ?

Prolog tries to find instances that satisfy some condition. Think
from the other side. Everybody is ok iff there is nobody who is not
ok.

Cheers --- Jan

mans

2005-08-11, 9:03 am

Dear Jan,

Thanks for you quick reply.

I am trying to make my prolog as similar to the natural language as
possible and hence deal with the way the actual user is
thinking...which in this case may be the reverse of the way prolog
thinks??.

But in any case how about this if I stick withe same input
declarations:

notok(X) :- \+ok(X).
everybodyok_new :- not((people(X),notok(X))).

Which seems to work..although I am not sure what the meta call means:

[trace] 125 ?- everybodyok_new.
Call: (7) everybodyok_new ? creep
^ Call: (8) not((people(_G506), notok(_G506))) ? creep
Call: (10) people(_G506) ? creep
Exit: (10) people(a) ? creep
Call: (10) notok(a) ? creep
Call: (11) ok(a) ? creep
Exit: (11) ok(a) ? creep
Fail: (10) notok(a) ? creep
Redo: (10) people(_G506) ? creep
Exit: (10) people(c) ? creep
Call: (10) notok(c) ? creep
Call: (11) ok(c) ? creep
Fail: (11) ok(c) ? creep
Exit: (10) notok(c) ? creep
Exit: (9) '<meta-call>'((people(c), notok(c))) ? creep
^ Fail: (8) not((people(_G506), notok(_G506))) ? creep
Fail: (7) everybodyok_new ? creep

Thanks again!
Mans

Jan Wielemaker

2005-08-11, 5:03 pm

On 2005-08-11, mans <mshapshak@gmail.com> wrote:
> Dear Jan,
>
> Thanks for you quick reply.
>
> I am trying to make my prolog as similar to the natural language as
> possible and hence deal with the way the actual user is
> thinking...which in this case may be the reverse of the way prolog
> thinks??.
>
> But in any case how about this if I stick withe same input
> declarations:
>
> notok(X) :- \+ok(X).
> everybodyok_new :- not((people(X),notok(X))).
>
> Which seems to work..although I am not sure what the meta call means:
>
> [trace] 125 ?- everybodyok_new.
> Call: (7) everybodyok_new ? creep
> ^ Call: (8) not((people(_G506), notok(_G506))) ? creep
> Call: (10) people(_G506) ? creep
> Exit: (10) people(a) ? creep
> Call: (10) notok(a) ? creep
> Call: (11) ok(a) ? creep
> Exit: (11) ok(a) ? creep
> Fail: (10) notok(a) ? creep
> Redo: (10) people(_G506) ? creep
> Exit: (10) people(c) ? creep
> Call: (10) notok(c) ? creep
> Call: (11) ok(c) ? creep
> Fail: (11) ok(c) ? creep
> Exit: (10) notok(c) ? creep
> Exit: (9) '<meta-call>'((people(c), notok(c))) ? creep

This is an artifact introduced by SWI-Prolog to deal with complex
goals (i.e. goals with control structures) given to call/1, once/1,
etc.

> ^ Fail: (8) not((people(_G506), notok(_G506))) ? creep
> Fail: (7) everybodyok_new ? creep

Now that you understand this, you are ready for the more
readable form:

everybodyok :- forall(people(X), ok(X)).

And for the definition of forall/2 (a built-in):

forall(Cond, Action) :-
\+ (Cond, \+ Action).

Enjoy --- Jan

mans

2005-08-11, 5:03 pm

Jan,
Thanks!
Mans
P=2ES. Just out of curiosity. Internally does prolog use go through the
list similar to findall(X,ok(X),List),maplist(=ADpeople,
List).?

Jan Wielemaker

2005-08-11, 5:03 pm

On 2005-08-11, mans <mshapshak@gmail.com> wrote:
> Jan,
> Thanks!
> Mans
> P.S. Just out of curiosity. Internally does prolog use go through the
> list similar to findall(X,ok(X),List),maplist(-people,List).?

No :-) \+/1 can be implemented as

\+(G) :- G, !, fail.
\+(_).

In fact the compiler handles \+, but that doesn't change things too much.

Cheers --- Jan

Mauro Di Nuzzo

2005-08-12, 9:01 am

Try this, and let me know if it works please:

everybodyok :- (
people(X),
( ok(X)
-> fail
; !, fail
)
; true
).

Regards,
M

"mans" <mshapshak@gmail.com> ha scritto nel messaggio
news:1123762484.841598.229320@g44g2000cwa.googlegroups.com...
> Dear All,
>
> I am trying to understand the best way to write a rule which checks on
> all the elements.
>
> -------------
> people(a).
> people(c).
> ok(b).
> ok(a).
>
> everybodyok_works :- findall(X,ok(X),List),maplist(people,Lis
t).
>
> everybodyok_bad :- people(X),ok(X).
>
> -------------
>
> What am I missing? Is there a better way to write rule which says:
> "Everybody is ok if for all people we know has said they are OK." ?
>
> Thanks in Advance,
> Mans
>

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