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I'd like to print a message and stop the evaluation of the predicate. How do I do that?
963
Exception handling in prolog is done with the catch function. We use it below to make sure we can open the requested file. The first parameter of catch is the Goal, or what we want to execute (comparable to a try in C or Java). The second parameter is what exception we want to catch.
They are just terms, a representation of the successor of their argument. So, s(0) is used to represent the successor of 0 (i.e. 1 ), s(s(0)) is used to represent the successor of s(0) (i.e. 2 ), and so on and so forth.
The concept of logical negation in Prolog is problematical, in the sense that the only method that Prolog can use to tell if a proposition is false is to try to prove it (from the facts and rules that it has been told about), and then if this attempt fails, it concludes that the proposition is false.
If you want to exit SWI-Prolog, issue the command halt., or simply type CTRL-d at the SWI-Prolog prompt.
Take a look at this link where the catch/3 and throw/1 mechanisms in Prolog are described.
It is possible to throw an exception or handle an exception using this mechanism.
The example (given on the site) is:
p:- true.
p:- throw(b).
q:- catch(p, B, write('hellop')), r(c).
r(X) :- throw(X).
Then the call:
?- catch(p, X, (write('error from p'), nl)).
will illustrate the ecxeption handling.
133
I played with a few other examples which I found. This might be usefull.
p :- throw(b).
r(X) :- throw(X).
q:- catch(p, B, format('Output1: Error from throwing p')), r(B).
catch(throw(exit(1)), exit(X), format('Output: `w', [X])).
Output: 1
1 is thrown to catcher 'exit(X)' and recovered by format('Output: ~w', [X])),
catch(p, C, format('hellop')).
Output: hellop
p throws 'b' which is recovered by writing 'hellop'
catch(q, C, format('Output2, Recovering q: helloq ')).
Output1: Error from throwing p
Output2, Recovering q: helloq
Ben
21
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