Can some explain what does the predicate sort with an example ? I can’t understand what is in the website sort/2

[image] jamesnvc: Annoying, but I’m not sure how best to address this with a purely client-side plugin. Have you asked for help and the Discourse forum ? I do know that using the tag customer with a user name does seem to get better and faster help. Not that most people aren’t getting good an…

[image] Boris: ?- sort([A,B], Sorted), A = 2, B = 1. A = 2, B = 1, Sorted = [2, 1]. can you explain this …

Since you sort before you unify the variables with values, it’s sorted the list of variables, which puts them in standard order (which for variables, means sorted by address), after which you unify those variables with a value, but this happens after they’re sorted. You could use when/2 or freeze/2…

[image] jamesnvc: which for variables, means sorted by address ok. thanks. I find this implementation choice rather odd … since once the variables are bound, then the sort can be incorrect. I would expect default behavior to be as if a when clause was added – or if the variables aren’t boun…

[image] grossdan: I find this implementation choice rather odd … since once the variables are bound, then the sort can be incorrect. My understanding is that the ISO standard specifies the behaviour for sort/2 (and, AFAIK, when/2 isn’t in the standard, so couldn’t be part of it), so we’re som…

sort([A,B,C],[3,2,1]) It succeeds. :roll_eyes: I guess the second argument of sort/2 should not be called Sorted but MaybeSorted. It is not even undetermined whether the argument is sorted (because possibly not all elements are ground and Prolog has a dearth of truth values to communicate such su…

sort/2 has mode (+,-). This means the semantics of this is defined to be equal to this: sort([A,B,C], Tmp), Tmp = [3,2,1]) Tmp is indeed sorted. We just finished a discussion on standard order of terms wrt. variables and unification :slight_smile: Edit note that for (-)-moded arguments the do…

Tmp is indeed sorted. Operationally. But objectively it is not. In fact, after a passage through sort/2, there should be (transitive) constraints between A,B,C so that any future instantiations cannot violate the implied promise of having a successful sort/2 on the proof tree. Same as for dif/2. …

The problem is that a global ordering is defined on any Prolog term, including variables. A variable is smaller than any non-variable. But variables can stop being variables, jumping to any place within the global ordering. So while a “Prolog variable is immutable”, binding a variable does mutate it…

AFAIK, SWI-Prolog is (for a change) completely standard compliant wrt. this behavior of sort/2. That is easy as the ISO makes very minimal promises as we discussed before :slight_smile: . If there is anything worth considering it could be a standard order comparison that raises an exception if t…

What does sort/2 do?

Help! Predicate

EricGT Split this topic May 31, 2020, 7:37am 18

11 posts were split to a new topic: A pure sort/2

Topic		Replies	Views
Order and sorting Algorithm	47	167	April 12, 2026
On sorting non-ground terms and the standard order of terms General	26	1834	January 2, 2020
Sorting predicates in PROLOG Help! how-to	23	3362	August 3, 2022
How to compare/3 without surprises on non-ground terms? Help!	81	2156	March 31, 2023
Ordering residual goals in query answers Help! discussion	10	700	June 28, 2020

What does sort/2 do?

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