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Version: Next

retract/1

Description​

retract/1 is a predicate that retracts a clause from the database.

Signature​

retract(+Clause)

Where:

  • Clause is the clause to retract from the database.

Examples​

Retract a fact from the database​

This scenario demonstrates the process of retracting a fact from a Prolog database. In Prolog, retracting a fact means removing a piece of information or knowledge from the database, making it unavailable for subsequent queries. This is particularly useful when you want to dynamically remove facts or rules based on conditions or interactions during runtime.

Here are the steps of the scenario:

  • Given the query:
assertz(parent(john, alice)), retract(parent(john, alice)), parent(X, Y).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3977
answer:
  has_more: false
  variables: ["X","Y"]
  results:

Only dynamic predicates can be retracted​

This scenario demonstrates that only dynamic predicates can be retracted. In Prolog, dynamic predicates are those that can be modified during runtime. This is in contrast to static predicates, which are fixed and cannot be modified.

Here are the steps of the scenario:

  • Given the program:
parent(jane, alice).
  • Given the query:
retract(parent(jane, alice)).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3975
answer:
  has_more: false
  results:
  - error: "error(permission_error(modify,static_procedure,parent/2),retract/1)"