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Predicate retractall/1
Availability:built-in
[ISO,det]retractall(+Head)
All facts or clauses in the database for which the head unifies with Head are removed. If Head refers to a predicate that is not defined, it is implicitly created as a dynamic predicate. See also dynamic/1.86The ISO standard only allows using dynamic/1 as a directive.

Examples

retractall/1 or abolish/1?

retractall/1 or abolish/1?

The retractall/1 predicate removes all clauses whose head matches the argument from a dynamic predicate. The abolish/1 predicate takes a predicate indicator to remove all clauses and all properties.

:- dynamic p/1.

?- assertz(p(a)).
?- assertz(p(b)).
?- retractall(p(b)).
?- p(a).
true.
?- p(b).
false.
?- retractall(p(_)).
?- p(X).
false.
?- assertz(p(a)).
?- assertz(p(b)).
?- abolish(p/1).
?- p(X).
ERROR: Unknown procedure: p/1 (DWIM could not correct goal)
Dealing with dynamic predicatesJan Wielemaker

Dealing with dynamic predicates

A dynamic predicate is introduced using dynamic/1, after which clauses may be added using assertz/1. Persistent dynamic predicates are realized using library(persistency).

:- dynamic p/1.
?- assertz(p(a)).
?- p(X).
X = a.
?- retractall(p(_)).
?- p(X).
false.
author
- Jan Wielemaker
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  • cleanup
LogicalCaptain said (2020-05-22T12:30:03):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

This predicate never fails.

"If Head refers to a predicate that is not defined, it is implicitly created as a dynamic predicate" means:

?- predicate_property(foo(_,_),dynamic).
false.

?- retractall(foo(a,_)).
true.

?- predicate_property(foo(_,_),dynamic).
true.

Jan Burse writes (code comments added by me):

"[Similarly] there are differences in retract/1 and retractall/1 if the predicate is unknown. For example that SWI-Prolog creates a new dynamic entry during retractall/1, is not required by the standard. The usual bootstrapping(*) would be:

retractall(Head) :-
   retract((Head :- _)),
   fail.                  % failure-driven loop to retract the whole shebang
retractall(_).            % "make this a success" catchall

(*) DRAFT TECHNICAL CORRIGENDUM 2 https://www.complang.tuwien.ac.at/ulrich/iso-prolog/dtc2#retractall