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core-logic 2020-11-05
nitaai 2020-11-05T14:46:21.005200Z
hi. i have ported one prolog example to core.logic:
% P07 (**) Flatten a nested list structure.
my_flatten(X,[X]) :- \+ is_list(X).
my_flatten([],[]).
my_flatten([X|Xs],Zs) :- my_flatten(X,Y), my_flatten(Xs,Ys), append(Y,Ys,Zs).
(defne flatteno [l1 l2]
([x [x]] (pred x (comp not seq?)))
([[] []])
([[x . xs] zs]
(fresh [y ys]
(flatteno x y)
(flatteno xs ys)
(appendo y ys zs))))([[1 [2]]] (1 [2]) (1 2))
and the prolog version only one:
[1 2]
?nitaai 2020-11-06T08:22:13.007Z
(defna flatteno [l1 l2]
([[] []])
([[x . xs] zs]
(fresh [y ys]
(flatteno x y)
(flatteno xs ys)
(appendo y ys zs)))
([x [x]]))predas @hiredman suggested and uses defna. the arrangement of the clauses still matters, but it makes sense.nitaai 2020-11-05T14:55:15.005300Z
I assume some rearrangement of the clauses and use of defna or defnu..
nitaai 2020-11-05T15:00:18.005500Z
yes, my assumption was right:
(defna flatteno [l1 l2]
([[x . xs] zs]
(fresh [y ys]
(flatteno x y)
(flatteno xs ys)
(appendo y ys zs)))
([x [x]] (pred x (comp not seq?)))
([[] []]))defna and defnu do the job and clauses have to be rearranged.
Someone who knows prolog can tell me why this kind of cut is necessary in clojure and not in prolog?
the \+ negation in prolog also does a cut: http://cs.union.edu/~striegnk/learn-prolog-now/html/node90.html#sec.l10.negation.as.failure
I would strongly recommend against using pred, and seq? is likely the wrong predicate there
the re-arrangement of the clauses and using defna or defna are both consequences of seq? being the wrong predicate
user=> (seq? [])
false
user=>but using defna you should be able to get rid of the pred usage altogether
nitaai 2020-11-05T18:59:23.006800Z
Great and interesting hints. Thanks @jkr.sw and @hiredman. Will look into that…