I have just started with prolog (don't ask me why), and I have written the following simple program:
member(X,[X|_Tail]).
member(X,[_Head|Tail]) :-
member(X,Tail).
list = [bob,carol,ted,alice].
when I try the following query
member(bob,l)
(I am using SWISH prolog, so I don't need to use the question mark),I get the error:
No permission to modify static procedure `(=)/2'
I've looked around and not sure what the issue is, nor what this means. Any spots?
You are getting the error:
No permission to modify static procedure '(=)/2'
because this line in your source code:
list = [bob,carol,ted,alice].
[tries to] define the predicate =/2. It is exactly identical to
'='( list , [bob, carol, ted, alice] ) .
Which, if compiled, would override the unification operator =/2 with a version that would always fail (because the atom list cannot unify with the list [bob, carol, ted, alice].
At the prolog prompt, however, one could query list = [bob, carol, ted, alice]. and have it fail. Or more likely query List = [bob, carol, ted, alice]. and have it succeed and report the new value of List.
If you want to specify a goal to run at startup, then something like this:
start_my_program :- writeln(hello) .
:- start_my_program .
would do you, specifying the initial goal to be evaluated.
Related
I've never written in Prolog before. I have to provide facts so that when it runs it displays:
?- subset([a,b],[a,c,d,b]).
true.
?-include([],[a,b]).
true.
So far, I've written this:
subset([],_Y).
subset([X|T],Y):- member(X,Y),subset(T,Y).
But include doesn't work when I write include([],[a,b]). . It shows this:
ERROR: toplevel: Undefined procedure: include/2 (DWIM could not correct goal)
Any help would be appreciated. Thanks
You get the error because you didn't define the predicate include/2. Your given example looks like include/2 should be describing the same relation as subset/2. So you can either rename your definition from subset/2 to include/2 and then run the query or you can use subset/2 to define include/2:
include(X,Y) :-
subset(X,Y).
Note that in order to use member/2 you have to use library(lists). However, in some Prolog systems (e.g. SWI) this library includes a predicate subset/2 thus leading to a warning when you consult your source file:
Warning: ...
Local definition of user:subset/2 overrides weak import from lists
If you want to implement your own version of subset/2 anyway and not get this warning, you can rename your predicate or not use library(lists) and implement your version of member/2, for example:
subset([],_Y).
subset([X|T],Y) :-
element_in(X,Y),
subset(T,Y).
element_in(X,[X|_]).
element_in(X,[Y|Ys]) :-
dif(X,Y),
element_in(X,Ys).
anyone of you could tell me how to turn off "Redefined static procedure" warnings?
I red online documentation of swi-prolog and i found this predicate no_style_check(ultimate) that in principle should turn off these warnings, but when i execute this predicate
main:-
no_style_check(singleton),
no_style_check(discontiguous),
no_style_check(multiple),
require,
test_all.
i received this error
ERROR: Domain error: style_name' expected, foundmultiple'
Anyone knows an alternative way to do this or could tell me why i receive this error ?
Thanks in advance!
Prolog is a pretty loosey-goosey language, so by default it warns you when you do certain things that are not wrong per se, but tend to be a good indication that you've made a typo.
Now, suppose you write something like this:
myfoo(3, 3).
myfoo(N, M) :- M is N*4+1.
Then from the prompt you write this:
?- asserta(myfoo(7,9)).
ERROR: asserta/1: No permission to modify static procedure `myfoo/2'
ERROR: Defined at user://1:9
What's happening here is that you haven't told Prolog that it's OK for you to modify myfoo/2 so it is stopping you. The trick is to add a declaration:
:- dynamic myfoo/2.
myfoo(3, 3).
myfoo(N, M) :- M is N*4+1.
Now it will let you modify it just fine:
?- asserta(myfoo(7,9)).
true.
Now suppose you have three modules and they each advertise themselves by defining some predicate. For instance, you might have three files.
foo.pl
can_haz(foo).
bar.pl
can_haz(bar).
When you load them both you're going to get a warning:
?- [foo].
true.
?- [bar].
Warning: /home/fox/HOME/Projects/bar.pl:1:
Redefined static procedure can_haz/1
Previously defined at /home/fox/HOME/Projects/foo.pl:1
true.
And notice this:
?- can_haz(X).
X = bar.
The foo solution is gone.
The trick here is to tell Prolog that clauses of this predicate may be defined in different files. The trick is multifile:
foo.pl
:- multifile can_haz/1.
can_haz(foo).
bar.pl
:- multifile can_haz/1.
can_haz(bar).
In use:
?- [foo].
true.
?- [bar].
true.
?- can_haz(X).
X = foo ;
X = bar.
:- discontiguous does the same thing as multifile except in a single file; so you define clauses of the same predicate in different places in one file.
Again, singleton warnings are a completely different beast and I would absolutely not modify the warnings on them, they're too useful in debugging.
I wrote the following simple code, and I expect that when I write 'male.', this code ask me once "is it male?" and if i input 'No' it write on screen "she is female".
male :- ( print('is it male ? '),read(yes)) -> true; asserta( not(male)),female.
female:- not(male),print('she is female').
not(P) :- (call(P) -> fail ; true) .
but this code has following error:
uncaught exception: error(permission_error(modify,static_procedure,not/1),asserta/1);
the error in swi-prolog is :
ERROR: asserta/1: No permission to modify static_procedure `not/1'
As gusbro said, not/1 is a predefined static procedure (and therefore it is not a good idea to use the same name). However, this is not the reason you get the error in swi-prolog as you can overwrite the standard definition:
?- assert(not(42)).
true.
?- not(42).
true.
the problem is that you have already defined not/1 in your code and, when you do not declare a predicate explicitly as dynamic, swi-prolog will assume that it's static and will not allow you to change it.
You can declare it as dynamic by inserting this line in your code:
:-dynamic(not/1).
I think that this will not solve the problem in other prolog implementations (eg gnu-prolog) as the error message says permission_error(modify,static_procedure,not/1); in any case it is highly recommended to change the name.
By the way, it would be simpler and cleaner to simply test what the argument is and print the corresponding message. If, however, you want to create something more complex (using a state maybe) you could use global variables since they are more efficient that assert/retract.
How do I define a rule that the user cannot query?
I only want the program itself to call this rule through another rule.
Ex:
rule1():- rule2().
rule2():- 1<5.
?-rule1().
true
?-rule2().
(I don't know what the answer will be, I just want this query to fail!)
Use a Logtalk object to encapsulate your predicates. Only the predicates that you declare public can be called (from outside the object). Prolog modules don't prevent calling any predicate as using explcit qualification bypasses the list of explicitly exported predicates.
A simple example:
:- object(rules).
:- public(rule1/1).
rule1(X) :-
rule2(X).
rule2(X) :-
X < 5.
:- end_object.
After compiling and loading the object above:
?- rules::rule1(3).
true.
?- rules::rule2(3).
error(existence_error(predicate_declaration,rule2(3)),rules::rule2(3),user)
If you edit the object code and explicitly declare rule2/1 as private you would get instead the error:
?- rules::rule2(3).
error(permission_error(access,private_predicate,rule2(3)),rules::rule2(3),user)
More information and plenty of examples at http://logtalk.org/
First, some notes:
I think you mean "predicate" instead of "rule". A predicate is a name/k thing such as help/0 (and help/1 is another) and can have multiple clauses, among them facts and rules, e.g. length([], 0). (a fact) and length([H|T], L) :- ... . (a rule) are two clauses of one predicate length/2.
Do not use empty parenthesis for predicates with no arguments – in SWI-Prolog at least, this will not work at all. Just use predicate2 instead of predicate2() in all places.
If you try to call an undefined predicate, SWI-Prolog will say ERROR: toplevel: Undefined procedure: predicate2/0 (DWIM could not correct goal) and Sicstus-Prolog will say {EXISTENCE ERROR: predicate2: procedure user:predicate2/0 does not exist}
Now, to the answer. Two ideas come to my mind.
(1) This is a hack, but you could assert the predicate(s) every time you need them and retract them immediately afterwards:
predicate1 :-
assert(predicate2), predicate2, retractall(predicate2).
If you want a body and arguments for predicate2, do assert(predicate2(argument1, argument2) :- (clause1, clause2, clause3)).
(2) Another way to achieve this would be to introduce an extra argument for the predicate which you do not want to be called by the user and use it for an identification that the user cannot possibly provide, but which you can provide from your calling predicate. This might be a large constant number which looks random, or even a sentence. This even enables you to output a custom error message in case the wrong identification was provided.
Example:
predicate1 :-
predicate2("Identification: 2349860293587").
predicate2(Identification) :-
Identification = "Identification: 2349860293587",
1 < 5.
predicate2(Identification) :- Identification \= "Identification: 2349860293587",
write("Error: this procedure cannot be called by the user. Use predicate1/0 instead."),
fail.
I don't use the equivalent predicate2("Identification: 2349860293587") for the first clause of predicate2/0, because I'm not sure where the head of the clause might appear in Prolog messages and you don't want that. I use a fail in the end of the second clause just so that Prolog prints false instead of true after the error message. And finally, I have no idea how to prevent the user from looking up the source code with listing(predicate2) so that will still make it possible to simply look up the correct identification code if s/he really wants to. If it's just to keep the user from doing accidental harm, it should however suffice as a protection.
This reminds me to facility found in Java. There one can query the
curent call stack, and use this to regulate permissions of calling
a method. Translated to Prolog we find in the old DEC-10 Prolog the
following predicate:
ancestors(L)
Unifies L with a list of ancestor goals for the current clause.
The list starts with the parent goal and ends with the most recent
ancestor coming from a call in a compiled clause. The list is printed
using print and each entry is preceded by the invocation number in
parentheses followed by the depth number (as would be given in a
trace message). If the invocation does not have a number (this will
occur if Debug Mode was not switched on until further into the execution)
then this is marked by "-". Not available for compiled code.
Since the top level is usually a compiled predicate prolog/0, this could be
used to write a predicate that inspects its own call stack, and then decides
whether it wants to go into service or not.
rule2 :- ancestors(L), length(L,N), N<2, !, write('Don't call me'), fail.
rule2 :- 1<5.
In modern Prologs we don't find so often the ancestors/1 predicate anymore.
But it can be simulated along the following lines. Just throw an error, and
in case that the error is adorned with a stack trace, you get all you need:
ancestors(L) :- catch(sys_throw_error(ignore),error(ignore,L),true).
But beware stack eliminiation optimization might reduce the stack and thus
the list returned by ancestors/1.
Best Regards
P.S.: Stack elimination optimization is already explained here:
[4] Warren, D.H.D. (1983): An Abstract Prolog Instruction Set, Technical Note 309, SRI International, October, 1983
A discussion for Jekejeke Prolog is found here:
http://www.jekejeke.ch/idatab/doclet/prod/en/docs/10_pro08/13_press/03_bench/05_optimizations/03_stack.html
I am new to learning prolog, and I want to know, if we have some procedure like
father("Nic","Adam").
and I want to write a function that it will add new value to this
father("Nic","Adam","something"..)
how can I do this? Using list? Or what?
Quick answer: You don't want to do that.
Longer answer: The father/2 predicate has a certain meaning, namely that for father(X,Y) X is the father of Y. A father/3 predicate is a different thing altogether. What do you want to achieve with that third argument? Normally, you use additional rules, which derive things from the father/2 predicate, or even resolve it to a father/3 argument.
The main question remains: what's the purpose of the third argument? If you want your resolution to work for certain specific 3rd arguments based on the existance of a corresponding father/2 predicate for example, you could do father(X, Y, 'something') :- father(X,Y) which will succeed if you have a corresponding father/2 fact.
PS: Do learn your terminology. In Prolog we don't speak of procedures and we don't write functions. Instead we have predicates, facts, rules, ...
PPS: I am not sure which Prolog implementation you are using, but you might want to use 'something' instead of "something". The latter usually creates a list of character codes, not a string:
?- X = 'some'.
X = some.
?- X = "some".
X = [115, 111, 109, 101].
Simply writing
father(nic, adam).
As a predicate already defines it. It is like stating a fact: you declare that father(nic, adam) is true, then you can execute the following with these expected results :
?- father(nic, adam).
Yes
?- father(nic, X).
X = adam