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

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<h2 id="sec:solutionsequences"><a id="sec:A.32"><span class="sec-nr">A.32</span> <span class="sec-title">library(solution_sequences): 
Modify solution sequences</span></a></h2>

<p><a id="sec:solutionsequences"></a>

<dl class="tags">
<dt class="mtag">See also</dt>
<dd>
- all solution predicates <a class="pred" href="allsolutions.html#findall/3">findall/3</a>, <a class="pred" href="allsolutions.html#bagof/3">bagof/3</a> 
and <a class="pred" href="allsolutions.html#setof/3">setof/3</a>. <br>
- <code>library(aggregate)</code>
</dd>
</dl>

<p>The meta predicates of this library modify the sequence of solutions 
of a goal. The modifications and the predicate names are based on the 
classical database operations DISTINCT, LIMIT, OFFSET, ORDER BY and 
GROUP BY.

<p>These predicates were introduced in the context of the
<a class="url" href="http://swish.swi-prolog.org">SWISH</a> Prolog 
browser-based shell, which can represent the solutions to a predicate as 
a table. Notably wrapping a goal in <a class="pred" href="solutionsequences.html#distinct/1">distinct/1</a> 
avoids duplicates in the result table and using
<a class="pred" href="solutionsequences.html#order_by/2">order_by/2</a> 
produces a nicely ordered table.

<p>However, the predicates from this library can also be used to stay 
longer within the clean paradigm where non-deterministic predicates are 
composed from simpler non-deterministic predicates by means of 
conjunction and disjunction. While evaluating a conjunction, we might 
want to eliminate duplicates of the first part of the conjunction. Below 
we give both the classical solution for solving variations of (<code>a(X)</code>,
<code>b(X)</code>) and the ones using this library side-by-side.

<dl class="latex">
<dt><b>Avoid duplicates of earlier steps</b></dt>
<dd>

<pre class="code">
  setof(X, a(X), Xs),		distinct(a(X)),
  member(X, Xs),			b(X)
  b(X).
</pre>

<p>Note that the <a class="pred" href="solutionsequences.html#distinct/1">distinct/1</a> 
based solution returns the first result of <code>distinct(a(X))</code> 
immediately after <span class="pred-ext">a/1</span> produces a result, 
while the <a class="pred" href="allsolutions.html#setof/3">setof/3</a> 
based solution will first compute all results of <span class="pred-ext">a/1</span>.
</dd>
<dt><b>Only try <code>b(X)</code> only for the top-10 <code>a(X)</code></b></dt>
<dd>

<pre class="code">
  setof(X, a(X), Xs),		limit(10, order_by([desc(X)], a(X))),
  reverse(Xs, Desc),		b(X)
  first_max_n(10, Desc, Limit),
  member(X, Limit),
  b(X)
</pre>

<p>Here we see power of composing primitives from this library and 
staying within the paradigm of pure non-deterministic relational 
predicates.
</dd>
</dl>

<dl class="latex">
<dt class="pubdef"><a id="distinct/1"><strong>distinct</strong>(<var>:Goal</var>)</a></dt>
<dt class="pubdef"><a id="distinct/2"><strong>distinct</strong>(<var>?Witness, 
:Goal</var>)</a></dt>
<dd class="defbody">
True if <var>Goal</var> is true and no previous solution of <var>Goal</var> 
bound
<var>Witness</var> to the same value. The variant <a class="pred" href="solutionsequences.html#distinct/1">distinct/1</a> 
is equivalent to <code>distinct(Goal,Goal)</code>. Semantically, <a class="pred" href="solutionsequences.html#distinct/1">distinct/1</a> 
is the same as the code below, but answers are returned as soon as they 
become available rather than first computing the complete answer set.

<pre class="code">
distinct(Goal) :-
    findall(Goal, Goal, List),
    list_to_set(List, Set),
    member(Goal, Set).
</pre>

</dd>
<dt class="pubdef"><a id="limit/2"><strong>limit</strong>(<var>+Count, 
:Goal</var>)</a></dt>
<dd class="defbody">
Limit the number of solutions. True if <var>Goal</var> is true, 
returning at most <var>Count</var> solutions. Solutions are returned as 
soon as they become available.</dd>
<dt class="pubdef"><a id="offset/2"><strong>offset</strong>(<var>+Count, 
:Goal</var>)</a></dt>
<dd class="defbody">
Ignore the first <var>Count</var> solutions. True if <var>Goal</var> is 
true and produces more than <var>Count</var> solutions. This predicate 
computes and ignores the first <var>Count</var> solutions.</dd>
<dt class="pubdef"><a id="order_by/2"><strong>order_by</strong>(<var>Spec, 
Goal</var>)</a></dt>
<dd class="defbody">
Order solutions according to <var>Spec</var>. <var>Spec</var> is a list 
of terms, where each element is one of. The ordering of solutions of <var>Goal</var> 
that only differ in variables that are <i>not</i> shared with <var>Spec</var> 
is not changed.

<dl class="latex">
<dt><strong>asc</strong>(<var>Term</var>)</dt>
<dd class="defbody">
Order solution according to ascending <var>Term</var>
</dd>
<dt><strong>desc</strong>(<var>Term</var>)</dt>
<dd class="defbody">
Order solution according to descending <var>Term</var>
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="group_by/4"><strong>group_by</strong>(<var>+By, 
+Template, :Goal, -Bag</var>)</a></dt>
<dd class="defbody">
Group bindings of <var>Template</var> that have the same value for <var>By</var>. 
This predicate is almost the same as <a class="pred" href="allsolutions.html#bagof/3">bagof/3</a>, 
but instead of specifying the existential variables we specify the free 
variables. It is provided for consistency and complete coverage of the 
common database vocabulary.
</dd>
</dl>

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swi-prolog-nox 7.2.3-2 / usr / lib / swi-prolog / doc / Manual / solutionsequences.html
