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I define an operator as follows:
:- op(500, xfx, =>).
When I try something like:
assert(a => b).
Prolog raises an error that says 'No permission to modify static_procedure (=>)/2'.
Any solution?
700
SWI-Prolog -- assert/1. Assert a clause (fact or rule) into the database. The predicate asserta/1 asserts the clause as first clause of the predicate while assertz/1 assert the clause as last clause. The deprecated assert/1 is equivalent to assertz/1.
Prolog provides a predicate that performs this function. It is called the cut, represented by an exclamation point (!). The cut effectively tells Prolog to freeze all the decisions made so far in this predicate. That is, if required to backtrack, it will automatically fail without trying other alternatives.
process_and_fail(end_of_file) :- !. process_and_fail(X) :- process(X), fail. The cut in process_file/1 is another example of terminating a generate-and-test loop. In general, a cut should always be placed after a repeat/0 so that the backtracking loop is clearly terminated.
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.
As a security, you have to warn SWI that you are going to modify a predicate at runtime:
:- dynamic (=>)/2.
put at the top of the file should do it.
54
You must have meant another symbol in place of (=>)/2. Probably (->)/2 which is a control construct that cannot be modified.
Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 6.1.3-116-gf1c7e06) ... ?- asserta((a -> b)). ERROR: asserta/1: No permission to modify static procedure `(->)/2' ERROR: Defined at /opt/gupu/pl-devel/lib/swipl-6.1.3/boot/init.pl:194 ?- op(500, xfx, =>). true. ?- asserta(a => b). true.
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