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

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<h2 id="sec:control"><a id="sec:4.8"><span class="sec-nr">4.8</span> <span class="sec-title">Control 
Predicates</span></a></h2>

<a id="sec:control"></a>

<p>The predicates of this section implement control structures. Normally 
the constructs in this section, except for <a id="idx:repeat0:581"></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 id="idx:findall3:582"></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_atoms/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 id="fail/0"><strong>fail</strong></a></dt>
<dd class="defbody">
Always fail. The predicate <a id="idx:fail0:583"></a><a class="pred" href="control.html#fail/0">fail/0</a> 
is translated into a single virtual machine instruction.
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="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 id="true/0"><strong>true</strong></a></dt>
<dd class="defbody">
Always succeed. The predicate <a id="idx:true0:584"></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 id="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 id="!/0"><strong>!</strong></a></dt>
<dd class="defbody">
Cut. Discard all choice points created since entering the predicate in 
which the cut appears. In other words, <em>commit</em> to the clause in 
which the cut appears <em>and</em> discard choice points that have been 
created by goals to the left of the cut in the current clause. Meta 
calling is opaque to the cut. This implies that cuts that appear in a 
term that is subject to meta-calling (<a id="idx:call1:585"></a><a class="pred" href="metacall.html#call/1">call/1</a>) 
only affect choice points created by the meta-called term. The following 
control structures are transparent to the cut: <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> 
are opaque to the cut. The table below explains the scope of the cut 
with examples. <i>Prunes</i> here means ``prunes <var>X</var> choice 
point created by <var>X</var>''.

<p><table class="latex frame-void center">
<tr><td><code>t0 :- (a, !, b).</code> </td><td>% prunes a/0 and t0/0 </td></tr>
<tr><td><code>t1 :- (a, !, fail ; b).</code> </td><td>% prunes a/0 and t1/0 </td></tr>
<tr><td><code>t2 :- (a -&gt; b, ! ; c).</code> </td><td>% prunes b/0 and t2/0 </td></tr>
<tr><td><code>t3 :- call((a, !, fail ; b)).</code> </td><td>% prunes a/0 </td></tr>
<tr><td><code>t4 :- <code>\+</code>(a, !, fail).</code> </td><td>% 
prunes a/0 </td></tr>
</table>
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id=",/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 follows (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 id=";/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 id="|/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 id="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 defined as:

<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 id="*->/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 id="idx:clause2:586"></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 id="\+/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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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / control.html
