Question

If I have a simple predicate in the database, is there a way that I can request that the results be returned in a particular order (asc/desc) without having to read them all into memory and performing a sort/2 or doing a setof/3?

For instance, consider this database:

animal(dog).
animal(cat).
animal(elephant).
animal(bird).
animal(aardvark).

I would like to somehow declare animal as sorted ASC on its first term so that I can simple query it like:

?- animal(X).
X = aardvark ;
X = bird ;
X = cat ;
X = dog ;
X = elephant ;
No.

Being able to do that would be very convenient for treating my knowledge base more like a conventional database.

Answer 1

No, you cannot sort them if you dont read them.

Ofc you can write the predicates sorted or generate them sorted:

findall(sorted_animal(X),animal(X),Animals),
sort(Animals,SAnimals),
maplist(assertz(X),SAnimals),
compile_predicates([sorted_animal]/1).

or

findall(animal(X),animal(X),Animals),
sort(Animals, SAnimals),
retractall(animal(X)),
maplist(assertz(X),SAnimals),
compile_predicates([sorted_animal]/1).

but you have to declare the predicate animal/1 as dynamic:

:-dynamic(animal/1).
animal(dog).
....

(in the code file)

I used compile_predicates/1 to improve the speed; however, this means that you cannot use assert/1 or retract/1 again on that predicate so if you add/remove animals skip it.

Alternatively you can use ordered sets and pass it as an argument