Why does this core.logic program never terminate if you ask for too many results?

[Laurens Van Houtven]

Hi,

I'm trying to divide a list into 3 lists such that (= my-list (concat a b c)). I guess you could say I'm writing concato :-)

(l/run n

  [a b c]
  (l/fresh [A B] ;; uppercase are internal accumulators
    (l/appendo a b A)
    (l/appendo A c B)

    (l/== B '(p q r s))))

... running this program with small n (let's say 5) works fine. Ask for more answers than there are (e.g. 20) and it ostensibly runs forever.

1. Why?

2. How do I fix that? (I've tried reordering the goals.)

thanks

lvh

[Marc Limotte]

At first glance, I'd guess that the empty list is causing your problem.  You can always come up with one more path (which may not create a unique solution, but a path to explore) that has one more empty list appended.

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[Satyam Ramawat]

On a very simple way, you can restrict by using if statement by crossing checking the count/length of the list, 
https://clojuredocs.org/clojure.core/if

[Laurens Van Houtven]

Satyam: you're suggesting limiting the answers asked from run, or something else?

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[Satyam Ramawat]

I mean you can limit the answer according to need, by running into loop until end users value satisfy, will can be either 5,20 or N. so it will throw output according to run of a loop.

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[Laurens Van Houtven]

I always need all of them. Figuring out how many that are combinatorially is surprisingly tricky!

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[Satyam Ramawat]

Laurens,

Have you tried checking up to what extent it goes?

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[Laurens Van Houtven]

Sure: I'm saying figuring out the formula from a handful of examples is tricky. But I can do that, and I already know Clojure has conditionals. I'm trying to learn something about how the logic engine works.

lvh

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[Satyam Ramawat]

Grand!!

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