How to detect duplicate facts in the knowledge base?

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I’m not a fan of cuts, so I’d write it:

assert_insect(Insect) :-
    assertion(ground(Insect)),
    (  insect(Insect)
    -> true
    ;  assertz(insect(Insect))
    ).

The assertion/1 is because this code could have unexpected results if Insect is a term that contains a variable.

Another way of doing this is to add the table/1 directive:

?- dynamic insect/1.
true.

?- table insect/1.
true.

?- assertz(insect(fly)).
true.

?- assertz(insect(flea)).
true.

?- assertz(insect(fly)).
true.

?- bagof(Insect, insect(Insect), Insects).
Insects = [flea, fly].

If the table/1 directive is removed, the bagof/3 query gets Insects = [fly, flea, fly].

PS: assert/1 is deprecated and assertz/1 is recommended instead.

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