/usr/lib/swi-prolog/doc/Manual/control.html

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<H2><A NAME="sec:4.7"><SPAN class="sec-nr">4.7</SPAN> <SPAN class="sec-title">Control 
Predicates</SPAN></A></H2>

<A NAME="sec:control"></A>

<P>The predicates of this section implement control structures. Normally 
the constructs in this section, except for <A NAME="idx:repeat0:520"></A><A class="pred" href="control.html#repeat/0">repeat/0</A>, 
are translated by the compiler. Please note that complex goals passed as 
arguments to meta-predicates such as <A NAME="idx:findall3:521"></A><A class="pred" href="allsolutions.html#findall/3">findall/3</A> 
below cause the goal to be compiled to a temporary location before 
execution. It is faster to define a sub-predicate (i.e. one_character_atom/1 
in the example below) and make a call to this simple predicate.

<PRE class="code">
one_character_atoms(As) :-
        findall(A, (current_atom(A), atom_length(A, 1)), As).
</PRE>

<DL class="latex">
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="fail/0"><STRONG>fail</STRONG></A></DT>
<DD class="defbody">
Always fail. The predicate <A NAME="idx:fail0:522"></A><A class="pred" href="control.html#fail/0">fail/0</A> 
is translated into a single virtual machine instruction.
</DD>
<DT class="pubdef"><A NAME="false/0"><STRONG>false</STRONG></A></DT>
<DD class="defbody">
Same as fail, but the name has a more declarative connotation.
</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="true/0"><STRONG>true</STRONG></A></DT>
<DD class="defbody">
Always succeed. The predicate <A NAME="idx:true0:523"></A><A class="pred" href="control.html#true/0">true/0</A> 
is translated into a single virtual machine instruction.
</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="repeat/0"><STRONG>repeat</STRONG></A></DT>
<DD class="defbody">
Always succeed, provide an infinite number of choice points.
</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="!/0"><STRONG>!</STRONG></A></DT>
<DD class="defbody">
Cut. Discard choice points of parent frame and frames created after the 
parent frame. As of SWI-Prolog 3.3, the semantics of the cut are 
compliant with the ISO standard. This implies that the cut is 
transparent to <A class="pred" href="control.html#;/2">;/2</A>, <A class="pred" href="control.html#send_arrow/2">-&gt;/2</A> 
and <A class="pred" href="control.html#*->/2">*-&gt;/2</A>. Cuts 
appearing in the <EM>condition</EM> part of <A class="pred" href="control.html#send_arrow/2">-&gt;/2</A> 
and
<A class="pred" href="control.html#*->/2">*-&gt;/2</A> as well as in <A class="pred" href="control.html#\+/1">\+/1</A> 
are local to the condition.<SUP class="fn">32<SPAN class="fn-text">Up to 
version 4.0.6, the sequence X=!, X acted as a true cut. This feature has 
been deleted for ISO compliance.</SPAN></SUP>

<P>
<CENTER>
<TABLE BORDER=0 FRAME=void RULES=groups>
<TR VALIGN=top><TD><CODE>t1 :- (a, !, fail ; b).</CODE> </TD><TD>% cuts a/0 
and t1/0 </TD></TR>
<TR VALIGN=top><TD><CODE>t2 :- (a -&gt; b, ! ; c).</CODE> </TD><TD>% 
cuts b/0 and t2/0 </TD></TR>
<TR VALIGN=top><TD><CODE>t3 :- call((a, !, fail ; b)).</CODE> </TD><TD>% 
cuts a/0 </TD></TR>
<TR VALIGN=top><TD><CODE>t4 :- <CODE>\+</CODE>(a, !, fail ; b).</CODE> </TD><TD>% 
cuts a/0 </TD></TR>
</TABLE>

</CENTER>
</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME=",/2"><VAR>:Goal1</VAR> <STRONG>,</STRONG> <VAR>:Goal2</VAR></A></DT>
<DD class="defbody">
Conjunction. True if both `Goal1' and `Goal2' can be proved. It is 
defined as (this definition does not lead to a loop as the second comma 
is handled by the compiler):

<PRE class="code">
Goal1, Goal2 :- Goal1, Goal2.
</PRE>

</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME=";/2"><VAR>:Goal1</VAR> <STRONG>;</STRONG> <VAR>:Goal2</VAR></A></DT>
<DD class="defbody">
The `or' predicate is defined as:

<PRE class="code">
Goal1 ; _Goal2 :- Goal1.
_Goal1 ; Goal2 :- Goal2.
</PRE>

</DD>
<DT class="pubdef"><A NAME="|/2"><VAR>:Goal1</VAR> <STRONG>|</STRONG> <VAR>:Goal2</VAR></A></DT>
<DD class="defbody">
Equivalent to <A class="pred" href="control.html#;/2">;/2</A>. Retained 
for compatibility only. New code should use <A class="pred" href="control.html#;/2">;/2</A>.</DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="send_arrow/2"><VAR>:Condition</VAR> <STRONG>-&gt;</STRONG> <VAR>:Action</VAR></A></DT>
<DD class="defbody">
If-then and If-Then-Else. The <A class="pred" href="control.html#send_arrow/2">-&gt;/2</A> 
construct commits to the choices made at its left-hand side, destroying 
choice-points created inside the clause (by <A class="pred" href="control.html#;/2">;/2</A>), 
or by goals called by this clause. Unlike <A class="pred" href="control.html#!/0">!/0</A>, 
the choice-point of the predicate as a whole (due to multiple clauses) 
is <STRONG>not</STRONG> destroyed. The combination <A class="pred" href="control.html#;/2">;/2</A> 
and <A class="pred" href="control.html#send_arrow/2">-&gt;/2</A> acts as 
if defines by:

<PRE class="code">
If -&gt; Then; _Else :- If, !, Then.
If -&gt; _Then; Else :- !, Else.
If -&gt; Then :- If, !, Then.
</PRE>

<P>Please note that (If <CODE>-&gt;</CODE> Then) acts as (If <CODE>-&gt;</CODE> 
Then ;
<B>fail</B>), making the construct <EM>fail</EM> if the condition fails. 
This unusual semantics is part of the ISO and all de-facto Prolog 
standards.</DD>
<DT class="pubdef"><A NAME="*->/2"><VAR>:Condition</VAR> <STRONG>*-&gt;</STRONG> <VAR>:Action 
; :Else</VAR></A></DT>
<DD class="defbody">
This construct implements the so-called `soft-cut'. The control is 
defined as follows: If <VAR>Condition</VAR> succeeds at least once, the 
semantics is the same as (<VAR>Condition</VAR>, <VAR>Action</VAR>). If
<VAR>Condition</VAR> does not succeed, the semantics is that of (<CODE>\+</CODE> <VAR>Condition</VAR>, <VAR>Else</VAR>). 
In other words, If
<VAR>Condition</VAR> succeeds at least once, simply behave as the 
conjunction of <VAR>Condition</VAR> and <VAR>Action</VAR>, otherwise 
execute <VAR>Else</VAR>.

<P>The construct <VAR>A</VAR> <CODE>*-&gt;</CODE> <VAR>B</VAR>, i.e. without 
an
<VAR>Else</VAR> branch, is translated as the normal conjunction <VAR>A</VAR>,
<VAR>B</VAR>.<SUP class="fn">bug<SPAN class="fn-text">The decompiler 
implemented by <A NAME="idx:clause2:524"></A><A class="pred" href="examineprog.html#clause/2">clause/2</A> 
returns this construct as a normal conjunction too.</SPAN></SUP></DD>
<DT class="pubdef"><span class="pred-tag">[ISO]</span><A NAME="\+/1"><STRONG>\+</STRONG> <VAR>:Goal</VAR></A></DT>
<DD class="defbody">
True if `Goal' cannot be proven (mnemonic: <CODE><CODE>+</CODE></CODE> 
refers to <EM>provable</EM> and the backslash (<CODE><CODE>\</CODE></CODE>) 
is normally used to indicate negation in Prolog).
</DD>
</DL>

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