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The cut, in Prolog, is a goal, written as ! , which always succeeds, but cannot be backtracked. Cuts can be used to prevent unwanted backtracking, which could add unwanted solutions and/or space/time overhead to a query. The cut should be used sparingly.
To cut or not to cut - Negation We can query prolog for round(earth) and as expected the answer is false. The negation predicate in prolog is \+ and therefore \+ round(earth) returns true. This limitation of prolog that is a goal cannot be proved then it is false, is called the close world assumption (CWA).
It is used to ensure that the goal succeeds after the database has been searched. It will succeed with only the first line, any call to predicate alldogs will eventually fail.
If you want to exit SWI-Prolog, issue the command halt., or simply type CTRL-d at the SWI-Prolog prompt.
I started learning prolog, and wanted to make the whole cuts thing clearer. I have read that "green cut doesnt change declarative meaning of the program, while red cut does". But, the meaning of the program isnt really pure declarative (just from the fact that prolog actually backtracks for all options).
Here is an example:
p(1).
p(2) :- !.
p(3).
it has been said that this is green cut. But if I run this:
p(X), X =:= 3.
I will get "true" without a cut, and "false" with a cut. so, what do I miss?
Thanks in advance.
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