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<H1><A NAME="SEC160" HREF="ciao_toc.html#TOC160">Exception handling</A></H1>
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<STRONG>Author(s):</STRONG> The CLIP Group.
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<STRONG>Version:</STRONG> 1.10#7 (2006/4/26, 19:22:13 CEST)
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<STRONG>Version of last change:</STRONG> 1.9#344 (2004/4/29, 12:56:34 CEST)
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This module includes predicates related to exceptions, which alter the normal flow of Prolog.
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<LI><A HREF="ciao_26.html#SEC161">Usage and interface (exceptions)</A>
<LI><A HREF="ciao_26.html#SEC162">Documentation on exports (exceptions)</A>
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<H2><A NAME="SEC161" HREF="ciao_toc.html#TOC161">Usage and interface (<CODE>exceptions</CODE>)</A></H2>
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<LI><STRONG>Library usage:</STRONG>
These predicates are builtin in Ciao, so nothing special has to be done to use them.
<LI><STRONG>Exports:</STRONG>
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<LI><EM>Predicates:</EM>
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<CODE>catch/3</CODE>,
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<CODE>intercept/3</CODE>,
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<CODE>throw/1</CODE>,
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<CODE>halt/0</CODE>,
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<CODE>halt/1</CODE>,
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<CODE>abort/0</CODE>.
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<H2><A NAME="SEC162" HREF="ciao_toc.html#TOC162">Documentation on exports (<CODE>exceptions</CODE>)</A></H2>
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<DT><span class="define">PREDICATE:</span> <B>catch/3:</B>
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<CODE>catch(Goal, Error, Handler)</CODE>
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Executes <CODE>Goal</CODE>. If an exception is raised during its execution, <CODE>Error</CODE> is unified with the exception, and if the unification succeeds, the entire execution derived from <CODE>Goal</CODE> is aborted, and <CODE>Handler</CODE> is executed. The execution resumes with the continuation of the catch/3 call. For example, given the code
<PRE>
p(X) :- throw(error), display('---').
p(X) :- display(X).
</PRE>
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the execution of "<CODE>catch(p(0), E, display(E)), display(.), fail.</CODE>" results in the output "<CODE>error.</CODE>".
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<EM>Meta-predicate</EM> with arguments: <CODE>catch(goal,?,goal)</CODE>.
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<STRONG>Usage:</STRONG> <CODE>catch(+callable, ?term, +callable)</CODE>
<KBD> * ISO * </KBD>
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<LI><EM>The following properties hold globally:</EM>
This predicate is understood natively by CiaoPP.
(<CODE>basic_props:native/1</CODE>)
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<DT><span class="define">PREDICATE:</span> <B>intercept/3:</B>
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<CODE>intercept(Goal, Error, Handler)</CODE>
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Executes <CODE>Goal</CODE>. If an exception is raised during its execution, <CODE>Error</CODE> is unified with the exception, and if the unification succeeds, <CODE>Handler</CODE> is executed and then the execution resumes after the predicate which produced the exception. Note the difference with builtin
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<CODE>catch/3</CODE>, given the same code defined there, the execution of "<CODE>intercept(p(0), E, display(E)), display(.), fail.</CODE>" results in the output "<CODE>error---.0.</CODE>".
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<EM>Meta-predicate</EM> with arguments: <CODE>intercept(goal,?,goal)</CODE>.
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<DT><span class="define">PREDICATE:</span> <B>throw/1:</B>
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<CODE>throw(Ball)</CODE>
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Raises an error, throwing the exception <CODE>Ball</CODE>, to be caught by an ancestor
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<CODE>catch/3</CODE> or
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<CODE>intercept/3</CODE>. The closest matching ancestor is chosen. Exceptions are also thrown by other builtins in case of error.
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<STRONG>Usage:</STRONG>
<KBD> * ISO * </KBD>
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<LI><EM>Calls should, and exit will be compatible with:</EM>
<CODE>Ball</CODE> is currently a term which is not a free variable.
(<CODE>term_typing:nonvar/1</CODE>)
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<DT><span class="define">PREDICATE:</span> <B>halt/0:</B>
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<CODE>halt(halt</CODE>
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Halt the system, exiting to the invoking shell.
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<STRONG>Usage:</STRONG>
<KBD> * ISO * </KBD>
<UL>
<LI><EM>The following properties hold globally:</EM>
This predicate is understood natively by CiaoPP.
(<CODE>basic_props:native/1</CODE>)
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</DL>
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<DT><span class="define">PREDICATE:</span> <B>halt/1:</B>
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<CODE>halt(Code)</CODE>
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Halt the system, exiting to the invoking shell, returning exit code <CODE>Code</CODE>.
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<DT><span class="define">PREDICATE:</span> <B>abort/0:</B>
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<CODE>abort(abort</CODE>
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Abort the current execution.
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