retract

• Delete the first clause that matches the given clause
• Arity of 1
• If clause does note exist - it fails

likes(pooh,honey).
likes(Person, Anything) :-
 nice(Anything).
likes(fred,fish).

Q: retract((likes(fred,X) :- B)).
X = _567
B = nice(_0032)

On backtracking, this will delete the third clause, binding the variables as follows :

X = fish
Y = true

• retract is non-deterministic, in that a set of clauses can be removed by a single retract clause on backtracking
• Again variations are possible : retractall/1, retractone/1 retractx/2
• retractall : delete ALL clauses that match the given clause head, e.g. retractall(someone). will delete all clauses someone/0

• retractone : deletes the first clause that matches the given clause head, e.g. retractone(someone). will delete the first clause someone/0
• retractx : retract a clause at a specified position, e.g. retractx(someone/2,3). will delete the third clause for the predicate someone/2

Combining retract and assert :

change( Variable, New_value) :- 
 retract( value( Variable, _)),
 assert( value( Variable, New_value)).

and

fast(anne).
slow(tom).
slow(pat).

Q: assert((faster(X,Y) :- fast(X), slow(Y))).
A: yes

Q: faster(A,B).
A: A = anne   B = tom

Q: retract(slow(X)).
A: X = tom; X = pat; no

Q: faster(anne,_)
A: no