Question

I’m having an issue with SWI-Prolog’s delete/3 predicate.
The easiest way is just a quick example:

?- delete([(1,1),(1,2),(3,2)], (1,_), List).
List = [(1,2),(3,2)].

I would expect (1,2) to also be deleted, since (1,_) unifies with (1,2). The SWIPL help says:

Delete all members of List1 that simultaneously unify with Elem and unify the result with List2.

Why is this and how can I delete everything that unifies with (1,_)?

Answer 1

” Delete all members of List1 that simultaneously unify with Elem and unify the result with List2.”

(1,X) first unifies with (1,1). therefore, X is unified with 1 and cannot be unified with 2 to delete (1,2).
so the problem is not that it does not delete all of the members; it’s that it doesnt unify simultaneously with (1,2) and (1,1)
(try delete([(1,1),(1,2),(1,1),(3,2)],(1,_),List).

btw, according to the swi-prolog manual:

delete(?List1, ?Elem, ?List2)
Is true when Lis1, with all occurences of Elem deleted results in List2.

also, delete/3 is deprecated:

There are too many ways in which one might want to delete elements from a list to justify the name.
Think of matching (= vs. ==), delete first/all, be deterministic or not.

So the easiest way is to write your own predicate. Something like:

my_delete(Pattern,[Pattern|T],TD):-
   my_delete(Pattern,T,TD).
my_delete(Pattern,[H|T],[H|TD]):-
   my_delete(Pattern,T,TD).

perhaps?

check exclude/3, include/3, partition/4