what does discontiguous predicate mean?

0
votes

what is the issue with the following script? 

want(grocerystore).
available(grocerystore).
want(X):-
   available(X).
can(grocerystore):-
   want(grocerystore),
   \+work(walmart),
   available(grocerystore).
1
prolog
Not sure what you're asking, as you haven't shown any output. Please edit your question accordingly. Also - questions along the lines of "why isn't my code working" are off-topic. - David Makogon

1 Answers

5
votes

"Discontiguous predicate" means that you defined more than one clause for a given predicate, but defined the clause of a different predicate in between. Prolog wants all of the clauses of a given predicate together.

Here's an example of what properly defined Prolog looks like with multiple clauses for a given predicate or fact:

person(sally).
person(sue).

dog(fido).
dog(fluffy).

You can see all of the person facts are together, and all of the dog facts are together.

Here is an example of a "discontiguous predicate":

person(sally).
dog(fido).

person(sue).
dog(fluffy).

Here, the person facts are "interrupted" by a dog fact (and vice versa).

The same issue applies to predicates as it does to facts. In your case, two different want clauses are "interrupted" by the available fact:

want(grocerystore).
available(grocerystore).   % 'available' clause occurs between two 'want' clauses
want(X) :- ...

Easy fix in this case:

available(grocerystore).

want(grocerystore).
want(X) :- ...

There may be other issues with your program, but I'm only addressing the question you have about "discontiguous predicate".