Prolog: Error Handling and Find Unique

Question

Given:

fruitid('Apple', 'Granny Smith', 1).
fruitid('Apple', 'Cox', 2).
fruitid('Pear', 'Bartlett', 3).

How would I go about finding only unique items for instance:

is_unique(FruitName):-

In the example clauses the answer would be Pear.

I'm also trying to add error handling to my code, so in this instance if an input is:

is_unique(pineapple)

How could I catch this and output an error message?

Thanks,

AS

Answer 1

One solution is as follows, using the Prolog negation:

is_unique(X) :- fruitid(X,T1,I1),
   \+ exists_other_fruit(X,T1,I1).

exists_other_fruit(X,T1,I1) :-
    fruitid(X,T2,I2),
    (T1,I1) \= (T2,I2).

For error handling, you can use the Prolog if/3 operator (defined, e.g., in SICStus Prolog; I don't know about other Prologs) and do the following:

is_unique2(X) :-
      if(fruitid(X,T1,I1),\+ exists_other_fruit(X,T1,I1),
          (print('Unknown Fruit: '), print(X),nl,fail)).

The code works as follows:

| ?- is_unique(X).
X = 'Pear' ? ;
no
| ?- is_unique2(X).  
X = 'Pear' ? ;
no
| ?- is_unique2('Apple').
no
| ?- is_unique2(pineapple).
Unknown Fruit: pineapple