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Version: v11.0.0

asserta/1

Description​

asserta/1 is a predicate that asserts a clause into the database as the first clause of the predicate.

Signature​

asserta(+Clause)

Where:

  • Clause is the clause to assert into the database.

Examples​

Assert a fact into the database​

This scenario demonstrates the process of asserting a new fact into a Prolog database. In Prolog, asserting a fact means adding a new piece of information or knowledge into the database, allowing it to be referenced in subsequent queries. This is particularly useful when you want to dynamically extend the knowledge base with facts or rules based on conditions or interactions during runtime.

Here are the steps of the scenario:

  • Given the program:
assert_fact :- asserta(father(john, pete)).
  • Given the query:
assert_fact, father(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:
  - substitutions:
    - variable: X
      expression: john
    - variable: 'Y'
      expression: pete

Only dynamic predicates can be asserted​

This scenario demonstrates that only dynamic predicates can be asserted. 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:
asserta(parent(john, 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),asserta/1)"

Show that the fact is asserted at the beginning of the database​

This scenario demonstrates that the asserta/1 predicate adds the fact to the beginning of the database. This means that the fact is the first fact to be matched when a query is run.

This is in contrast to the assertz/1 predicate, which adds the fact to the end of the database.

Here are the steps of the scenario:

  • Given the program:
:- dynamic(parent/2).

parent(jane, alice).

assert_fact :- asserta(parent(john, alice)).
  • Given the query:
assert_fact, parent(X, alice).
  • When the query is run (limited to 2 solutions)
  • Then the answer we get is:
height: 42
gas_used: 3977
answer:
  has_more: false
  variables: ["X"]
  results:
  - substitutions:
    - variable: X
      expression: john
  - substitutions:
    - variable: X
      expression: jane

Shows a simple counter example​

This scenario demonstrates a simple counter example using the asserta/1 and retract/1 predicates. In this example, we represent the value of the counter as a dynamic predicate counter/1 that is asserted and retracted to each time the value of the counter is incremented or decremented.

Here are the steps of the scenario:

  • Given the program:
:- dynamic(counter/1).

counter(0).

increment_counter :- retract(counter(X)), Y is X + 1, asserta(counter(Y)).
decrement_counter :- retract(counter(X)), Y is X - 1, asserta(counter(Y)).
  • Given the query:
counter(InitialValue), increment_counter, increment_counter, counter(IncrementedValue), decrement_counter, counter(DecrementedValue).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 3989
answer:
  has_more: false
  variables: ["InitialValue", "IncrementedValue", "DecrementedValue"]
  results:
  - substitutions:
    - variable: InitialValue
      expression: 0
    - variable: IncrementedValue
      expression: 2
    - variable: DecrementedValue
      expression: 1