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Prolog control constructs
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<TR><TD BGCOLOR="#66dbff"><DIV ALIGN=center><TABLE>
<TR><TD><FONT SIZE=4><B><A NAME="htoc60">6.2</A></B></FONT></TD>
<TD WIDTH="100%" ALIGN=center><FONT SIZE=4><B>Prolog control constructs</B></FONT></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE><UL>
<LI><A HREF="manual022.html#toc40"> <TT>true/0</TT>,
<TT>fail/0</TT>,
<TT>!/0</TT></A>
<LI><A HREF="manual022.html#toc41"> <TT>(',')/2</TT> - conjunction,
<TT>(;)/2</TT> - disjunction,
<TT>(->)/2</TT> - if-then</A>
<LI><A HREF="manual022.html#toc42"> <TT>call/1</TT></A>
<LI><A HREF="manual022.html#toc43"> <TT>catch/3</TT>,
<TT>throw/1</TT></A>
</UL>
<BR>
<A NAME="toc40"></A><TABLE CELLPADDING=0 CELLSPACING=0 WIDTH="100%">
<TR><TD BGCOLOR="#98e7ff"><DIV ALIGN=center><TABLE>
<TR><TD><B><A NAME="htoc61">6.2.1</A></B></TD>
<TD WIDTH="100%" ALIGN=center><B><TT>true/0</TT>,
<TT>fail/0</TT>,
<TT>!/0</TT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="true/0"></A>
<BR>
<B>Templates</B>
<DL COMPACT=compact><DT><DD><TT>
true<BR>
fail<BR>
!</TT></DL>
<B>Description</B><BR>
<BR>
<TT>true</TT> always succeeds.<BR>
<BR>
<TT>fail</TT> always fails (enforces backtracking).<BR>
<BR>
<TT>!</TT> always succeeds and the for side-effect of removing all
choice-points created since the invocation of the predicate activating it.<BR>
<BR>
<B>Errors</B><BR>
<BR>
None.<BR>
<BR>
<B>Portability</B><BR>
<BR>
ISO control constructs.<BR>
<BR>
<A NAME="toc41"></A><TABLE CELLPADDING=0 CELLSPACING=0 WIDTH="100%">
<TR><TD BGCOLOR="#98e7ff"><DIV ALIGN=center><TABLE>
<TR><TD><B><A NAME="htoc62">6.2.2</A></B></TD>
<TD WIDTH="100%" ALIGN=center><B><TT>(',')/2</TT> - conjunction,
<TT>(;)/2</TT> - disjunction,
<TT>(->)/2</TT> - if-then</B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<BR>
<B>Templates</B>
<DL COMPACT=compact><DT><DD><TT>
','(+callable_term, +callable_term)<BR>
;(+callable_term, +callable_term)<BR>
->(+callable_term, +callable_term)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>Goal1 , Goal2</TT> executes <TT>Goal1</TT> and, in case of
success, executes <TT>Goal2</TT>.<BR>
<BR>
<TT>Goal1 ; Goal2</TT> first creates a choice-point and executes
<TT>Goal1</TT>. On backtracking <TT>Goal2</TT> is executed.<BR>
<BR>
<TT>Goal1 -> Goal2</TT> first executes <TT>Goal1</TT> and,
in case of success, removes all choice-points created by <TT>Goal1</TT> and
executes <TT>Goal2</TT>. This control construct acts like an if-then
(<TT>Goal1</TT> is the test part and <TT>Goal2</TT> the then part). Note that
if <TT>Goal1</TT> fails <TT>->/2</TT> fails also. <TT>->/2</TT> is
often combined with <TT>;/2</TT> to define an if-then-else as follows:
<TT>Goal1 -> Goal2 ; Goal3</TT>. Note that <TT>Goal1 -> Goal2</TT>
is the first argument of the <TT>(;)/2</TT> and <TT>Goal3</TT> (the else
part) is the second argument. Such an if-then-else control construct first
creates a choice-point for the else-part (intuitively associated to
<TT>;/2</TT>) and then executes <TT>Goal1</TT>. In case of success, all
choice-points created by <TT>Goal1</TT> together with the choice-point for
the else-part are removed and <TT>Goal2</TT> is executed. If <TT>Goal1</TT>
fails then <TT>Goal3</TT> is executed.<BR>
<BR>
<TT>','</TT>, <TT>;</TT> and <TT>-></TT> are predefined infix operators
(section <A HREF="manual037.html#op/3:(Term-input/output)">7.14.10</A>).<BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal1</TT> or <TT>Goal2</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal1</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Goal1)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal2</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Goal2)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Goal1</TT> or
<TT>Goal2</TT> does not correspond to an existing procedure and the value of
the <TT>unknown</TT> Prolog flag is <TT>error</TT> (section <A HREF="manual045.html#set-prolog-flag/2">7.22.1</A>)</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>existence_error(procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO control constructs.<BR>
<BR>
<A NAME="toc42"></A><TABLE CELLPADDING=0 CELLSPACING=0 WIDTH="100%">
<TR><TD BGCOLOR="#98e7ff"><DIV ALIGN=center><TABLE>
<TR><TD><B><A NAME="htoc63">6.2.3</A></B></TD>
<TD WIDTH="100%" ALIGN=center><TT><B>call/1</B></TT></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="call/1"></A>
<BR>
<B>Templates</B>
<DL COMPACT=compact><DT><DD><TT>
call(+callable_term)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>call(Goal)</TT> executes <TT>Goal</TT>. <TT>call/1</TT> succeeds if
<TT>Goal</TT> represents a goal which is true. When <TT>Goal</TT> contains a
cut symbol <TT>!</TT> (section <A HREF="#true/0">6.2.1</A>) as a subgoal, the effect of
<TT>!</TT> does not extend outside <TT>Goal</TT>. <BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Goal)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Goal</TT> does not
correspond to an existing procedure and the value of the <TT>unknown</TT>
Prolog flag is <TT>error</TT> (section <A HREF="manual045.html#set-prolog-flag/2">7.22.1</A>)</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>existence_error(procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO control construct.<BR>
<BR>
<A NAME="toc43"></A><TABLE CELLPADDING=0 CELLSPACING=0 WIDTH="100%">
<TR><TD BGCOLOR="#98e7ff"><DIV ALIGN=center><TABLE>
<TR><TD><B><A NAME="htoc64">6.2.4</A></B></TD>
<TD WIDTH="100%" ALIGN=center><B><TT>catch/3</TT>,
<TT>throw/1</TT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="catch/3"></A>
<BR>
<B>Templates</B>
<DL COMPACT=compact><DT><DD><TT>
catch(?callable_term, ?term, ?term)<BR>
throw(+nonvar)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>catch(Goal, Catcher, Recovery)</TT> is similar to <TT>call(Goal)</TT>
(section <A HREF="#call/1">6.2.3</A>). If this succeeds or fails, so does the call to
<TT>catch/3</TT>. If however, during the execution of <TT>Goal</TT>, there
is a call to <TT>throw(Ball)</TT>, the current flow of control is
interrupted, and control returns to a call of <TT>catch/3</TT> that is being
executed. This can happen in one of two ways:
<UL><LI>implicitly, when an error condition for a built-in predicate is
satisfied.<BR>
<BR>
<LI>explicitly, when the program executes a call of <TT>throw/1</TT>
because the program wishes to abandon the current processing, and instead to
take an alternative action.</UL>
<TT>throw(Ball)</TT> causes the normal flow of control to be transferred
back to an existing call of <TT>catch/3</TT>. When a call to
<TT>throw(Ball)</TT> happens, <TT>Ball</TT> is copied and the stack is
unwound back to the call to <TT>catch/3</TT>, whereupon the copy of
<TT>Ball</TT> is unified with <TT>Catcher</TT>. If this unification
succeeds, then <TT>catch/3</TT> executes the goal <TT>Recovery</TT> using
<TT>call/1</TT> (section <A HREF="#call/1">6.2.3</A>) in order to determine the success or
failure of <TT>catch/3</TT>. Otherwise, in case the unification fails,
the stack keeps unwinding, looking for an earlier invocation of
<TT>catch/3</TT>. <TT>Ball</TT> may be any non-variable term.<BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Goal</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Goal)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Goal</TT> does not
correspond to an existing procedure and the value of the <TT>unknown</TT>
Prolog flag is <TT>error</TT> (section <A HREF="manual045.html#set-prolog-flag/2">7.22.1</A>)</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>existence_error(procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Ball</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
If <TT>Ball</TT> does not unify with the <TT>Catcher</TT> argument of
any call of <TT>catch/3</TT>, a system error message is displayed and
<TT>throw/1</TT> fails.<BR>
<BR>
When <TT>catch/3</TT> calls <TT>Recovery</TT> it uses <TT>call/1</TT>
(section <A HREF="#call/1">6.2.3</A>), an <TT>instantiation_error</TT>, a <TT>type_error</TT>
or an <TT>existence_error</TT> can then occur depending on
<TT>Recovery</TT>. <BR>
<BR>
<B>Portability</B><BR>
<BR>
ISO control constructs.<BR>
<BR>
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Copyright (C) 1999-2002 Daniel Diaz
<BR>
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