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
Related
I have this prolog program.
red(rose).
red(anthurium).
white(rose).
white(gardenia).
white(jasmine).
like(Y,X) :-
red(X),!,
fail
;
white(X).
And below is how it responds to different queries.
?- like(rose,gardenia).
true.
?- like(rose,P).
false.
?- like(Val,anthurium).
false.
?- like(rose,X).
false
The problem I now have is this:
When querying with a variable within the query (Eg: ?- like(rose,X).), Prolog usually responds by returning a value, (something like X=some_val). Why I don't get any value for those variables, but either true or false?
All helpful answers are highly appreciated. Thanks in advance.
Think about what Prolog is doing here:
like(rose,P) succeeds if red(P), so it grabs a possible substitution for P, namely rose or anthurium. Then it traverses the cut and then it fails. But "failing" means that the proof search down that path didn't bring any solution, there are no successful bindings to report (the only fail to get information out of a failing branch is to side-effect to a log file and read check it later). In fact, all bindings will be undone on backtracking. The second branch is white(X), but rose is not white, so we fail here, too.
You can also write:
like(_,X) :- \+ red(X).
like(_,X) :- white(X).
which is a bit more readable. One notices that when calling like(_,X), the goal enclosed by the negation-as-failure operator \+ is nonground. This is bad, and causes a floundering query (in other words, don't do that). I have written this little page on "floundering".
Say I have the following:
person(james).
person(pete).
father(james, pete).
Is there a way to validate that both the arguments to father have been defined i.e to stop a typo such as father(jmes, pete).
There is an easy way of checking these kinds of errors using Prolog: You can call rules at initialization time, i.e. one that checks that your fathers relations is properly defined. This can be useful to catch those typo errors.
:- use_module(library(error)).
check_fathers :-
% Take an element of the father relation
(father(P, _); father(_, P)),
% Proceed, if P is not a person
\+ person(P),
% Throw an error
syntax_error(father_is_no_person(P)).
check_fathers.
:- check_fathers.
person(james).
person(pete).
father(jame, pete). % TYPO! jame instead of james
Then, the program will produce the following output:
ERROR: Syntax error: father_is_no_person(jame)
Warning: <filename>:<line>:
Warning: Goal (directive) failed: user:check_fathers
Your assumptions are wrong in this sense:
father(james, pete). is a fact. It is something that you tell the Prolog processor is uneniably true (similar to a row in a table in a relational database).
So is father(jmes, pete).. Another fact. You are stating that this is so.
There is nothing to check, really.
On the other hand, if father(jmes, pete) appears in a a goal position, the Prolog processor will say "no, I have no indiciation that this is true":
father(james, pete). % that's a fat fact!
?- father(jmes, pete). % that's a query
false. % and the answer is "no, there is no evidence of that"
What you you would like to have is to have something like Java enum types: have the compiler make sure that a keyword comes indeed from a selected set of allowed keywords.
Prolog has has only basic typing, so this cannot be done directly.
But you could do this:
% if X is an unbound variable or a member of the indicated list,
% we are good
allowed_father(X) :-
(var(X);memberchk(X,[james,vader])),!.
% otherwise, we don't just fail, we actually throw an exception
allowed_father(X) :-
throw(error(unknown_father(X))).
% ---
% if X is an unbound variable or a member of the indicated list,
% we are good
allowed_son(X) :-
(var(X);memberchk(X,[pete,luke])),!.
% otherwise, we don't just fail, we actually throw an exception
allowed_father(X) :-
throw(error(unknown_son(X))).
and then you can call the above predicates whenever you need to perform a check:
query_anout(Father) :-
allowed_father(Father),
...
But it's rather awkward. Plus the thrown exception is not ISO-standard and the resulting error message may be confusing (I always rage against ISO standard exceptions which are doubleplusungood awkward)
parent(mel, joan).
parent(jane, betty).
parent(jane, tom).
parent(richard, adam).
parent(richard, rosa).
parent(joan, fran).
For example someone asks me to find all ancestors of a parent. I give him the code:
ancestor(P,C) :- parent(P, C).
ancestor(P,C) :- ancestor(P,P1), parent(P1, C).
But my friend still doesn't know how to use the predicate. Does he call it like
ancestor(richard, C) or ancestor(C, richard) ?
Is there a way to annotate that P is the parameter while C is the return value? And in a complex case, there will be predicates with different names, how should my user know which predicate is the final predicate he wants to use?
To help the human-readable meaning, you could add an extra predicate documenting the parameters as readable name/value pairs:
entry_ancestor_of(ancestor=P, descendent=C) :-
ancestor(P,C).
?- entry_ancestor_of(ancestor=richard, descendent=C).
C = adam .
Above, the suffix *ancestor_of* suggests param 1 is ancestor of param 2, so naming the predicate carefully can make it clearer.
Usually(convention), input parameters are the earlier parameters, and output parameters are later parameters, but where the predicate 'works both ways', ie. either could be input or output, this rule can't hold. This is the case for your predicate:
?- entry_ancestor_of(ancestor=X, descendent=adam).
X = richard .
Either parameter could be input or output, so there is no need to codify/explain them as such, although you might want to comment that it works both ways.
I would usually comment these 'flexible' predicates by putting an example of both of the above usages in a comment next to the predicate.
For entrypoint labelling, just do one or more of the following:
explicitly name the predicate as an entrypoint, as above
document using comments in the code which are the entrypoints
arrange the entrypoints in the same physical section with a comment
block saying that the predicates below are entrypoints.
Edit: Extra things re: coding guidelines / other answers.
In Coding guidelines for Prolog, section 3.8, it says 'For example, mother_of(A, B) is ambiguous;', so I gave bad advice on that.. perhaps acapelli's suggestion would be more useful on that.
In that document, also have a look at:
3.5 Choose sensible names for auxiliary predicates
3.8 Choose predicate names to help show the argument order
3.13 Decide whether predicate names should carry the types on which they operate
4.1 Begin every predicate (except perhaps auxiliary predicates) with an introductory comment in a well-defined format
The '?' system for identifying parameter types that will ness mentioned is on page 21.
a useful convention, sponsored for instance by Markus Triska, builds a predicate functor by joining the parameters 'names' - in a wide, applicable sense. Your example could be
parent_child(mel, joan).
...
ancestor_descendant(P, C) :- parent_child(P, C).
ancestor_descendant(A, D) :- ancestor_descendant(A, I), parent_child(I, D).
Also ISO-Prolog, and - for instance - SWI-Prolog library, attempt to follow this strategy.
For instance
atom_codes(Atom, Codes) :- ...
WRT to declare the type and status of arguments, some Prolog provide declarations - for instance Turbo Prolog, ECLiPSe, others... Sometime such declarations are required - usually to check correctness, often to speed up the computation.
SWI-Prolog offers 'just' structured comments, that IDE process automatically, and there has been a contribution aiming to exploit such declarations with runtime check.
Yes, with comments, and/or meaningful argument names,
% ancestor( ?Ancestor, ?Descendent).
ancestor(P,C) :- parent(P, C).
ancestor(P,C) :- ancestor(P,P1), parent(P1, C).
? means the argument can be used both as input (already set when the call is made), or for output (not yet set when the call is made).
The convention promoted in The Art of Prolog (I think) is that you place the name of the predicate after its first argument, to get at the intended argument ordering: P "is" ancestor C. Presumably "ancestor_of". But if you use that name, someone unfamiliar with that convention might read ancestor_of(P,C) as "ancestor of P is C", so it's a double-edged sword.
Is there any way that I can get a binding for a variable in Prolog even if the goal fails. I have a predicate where I am binding a variable with some value and after this I am failing the goal explicitly, but when I run the query it simply results in a fail without providing any biding for the variable. Something similar:
a(X) :-
X = 'something',
fail.
#Will Ness is correct (+1), assert can be used to capture bindings of variables as shown.
However, if you strictly need to retrieve bindings for variables in predicates like a and you know which parts could fail (and you don't care about them), then you could use combinations of cut (!) and true to permit a to proceed regardless. For example, consider:
a(X) :-
goalA(X), % a goal for which we definitely want a binding
(goalB, ! ; true). % an 'optional' goal which may fail
goalA('something').
goalB :- fail.
Executing this gives a('something'), even though goalB failed. Note that this isn't a commonly used way to program in Prolog, but if you know exactly what you're doing...
yes, this is how it is supposed to happen in Prolog. fail means the rejection of bindings made so far, because it says that these bindings are invalid, do not satisfy the goal.
but you can save some binding that will be undone on backtracking, with e.g. asserta predicate:
a(X) :-
X = 'something',
asserta(saved_x(X)),
fail.
Then, if you query saved_x(Z) afterwards, you will recover that value. Of course this is the part of Prolog that is extra-logical, i.e. outside of the logical programming paradigm.
I'm working on a college assignment where I must verify if a certain clause (as a fact or as a rule) exists in the current clause database.
The idea is to use a rule whose head is verify(+name, +arguments). This rule should be true if in the database exists another rule whose head is name(arguments)
Any help would be greatly appreciated...
Using call/1 is not a good idea because call/1 actually calls the goal, but you just want to find out if the fact/rule exists, and you don't want to wait after a long calculation that the call might trigger, and you don't want to have something printed on the screen if the called rule in turn calls e.g. writeln/1. In addition, you would want verify/2 to succeed even if the call failed (but the fact/rule is otherwise there).
As a solution, SWI-Prolog offers callable/1
callable(+Term)
True if Term is bound to an atom or a compound term,
so it can be handed without type-error to call/1, functor/3 and =../2.
Here are two version of verify/2, one using call/1 and the other using callable/1.
verify1(Name, Arguments) :-
Term =.. [Name | Arguments],
call(Term).
verify2(Name, Arguments) :-
Term =.. [Name | Arguments],
callable(Term).
father(abraham, isaac) :-
writeln('hello').
father(abraham, adam) :-
fail.
Are you familiar with the concept of unification? What you have to do is: just call a predicate that looks like the one you're trying to find.
So, say in your database is:
father(abraham,isaac).
Now you want to call something like:
verify(father,[abraham,isaac]).
Your predicate body will then have to contain a mechanism of calling father(abraham,isaac). which should then return true. Calling father(abraham,adam) should fail.
You will need two predicates for this: =../2 and call/2. If you are using SWI-Prolog, call help(=..). and help(call) from the interpreter's command line to access the documentation.
I hope I didn't spoil the assignment for you. You still have to find out what to do with partially instantiated predicates (so, say something like verify(father,[abraham,X]). on your own, but it shouldn't be hard from here.
Good luck.