iterative deepening and stoping when solution is found

[mvolkmann]

I wrote a simple program that searches a tree in order to learn about iterative deepening.
It finds a path from one tree node to another.
It works correctly, but once a solution is found, it evaluates the rest of the nodes in the tree unnecessarily.
See https://github.com/mvolkmann/prolog-examples/blob/main/search.pl#L71.
Is there an easy way to make it stop once a solution is found?

[triska]

On the toplevel, you can easily get this by pressing RET after the first solution is found:

?- length(Path, L),
   from_to_path(a, g, Path).
   Path = "acg", L = 3
;  ... .

Note that using the initialization/1 directive to run specific goals and then halt precludes trying the defined predicates in various ways. If you absolutely need a goal that only produces output, consider calling it run/0, and then invoke Scryer Prolog with the -g flag to run the goal, such as:

$ scryer-prolog -g run,halt your_file.pl

It is unusual for Prolog code to invoke halt/0 explicitly, because usually, after loading a program, one wants to actually interact with it and post queries.

[mvolkmann]

Thanks! I didn't know about the -g option.
I pushed changes to that file to remove :- initialization((...)) and add a run rule.
But that didn't fix the main issue which is that searching continues after a solution is found.

Here is a sketch of the tree the code searches:

I'm trying to show that a benefit of using iterative deepening is that I can avoid searching the entire tree. In this case I'm searching for a path from a to g. Since that is found 2 levels deep in the tree, I expect to avoid considering any edges that go deeper than that. It doesn't go deeper before finding the solution, but once the solution is found it searches the remainder of the tree. I understand it does that because there is a possibility of finding more g nodes. But in this case I know all the nodes are unique and it would be acceptable to stop searching. It seems that if I was okay with searching the entire tree then I just wouldn't bother using iterative deepening.

Is there a way I can stop the search once the first solution is found?

[david-sitsky]

One benefit of iterative deepening is you are changing your search strategy to find solutions with shorter lengths first. However it is the nature of Prolog to keep searching for alternative solutions assuming you requested it to do so from the top-level. I have seen previous code use the once(Goal) predicate to force only a single solution. I'm not sure if that is considered "best practice" or if there are alternate methods.

[mvolkmann]

I have tried using once and cut at multiple places in the code and haven’t found anything that works. It seems to me that there are problems where you know that there is only one solution OR you definitely only need one solution. In those cases I think it would be desirable to be able to stop searching after the first solution is found. Could it be the case that there is no way to do this in Prolog?