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<h2 id="sec:record"><a id="sec:A.29"><span class="sec-nr">A.29</span> <span class="sec-title">library(record):
Access named fields in a term</span></a></h2>
<a id="sec:record"></a>
<a id="sec:lib:record"></a>
<p>The library <code>library(record)</code> provides named access to
fields in a record represented as a compound term such as <code>point(X,
Y)</code>. The Prolog world knows various approaches to solve this
problem, unfortunately with no consensus. The approach taken by this
library is proposed by Richard O'Keefe on the SWI-Prolog mailinglist.
<p>The approach automates a technique commonly described in Prolog
text-books, where access and modification predicates are defined for the
record type. Such predicates are subject to normal import/export as well
as analysis by cross-referencers. Given the simple nature of the access
predicates, an optimizing compiler can easily inline them for optimal
preformance.
<p>A record is defined using the directive <a id="idx:record1:1975"></a><a class="pred" href="record.html#record/1">record/1</a>.
We introduce the library with a short example:
<pre class="code">
:- record point(x:integer=0, y:integer=0).
...,
default_point(Point),
point_x(Point, X),
set_x_of_point(10, Point, Point1),
make_point([y(20)], YPoint),
</pre>
<p>The principal functor and arity of the term used defines the name and
arity of the compound used as records. Each argument is described using
a term of the format below.
<blockquote>
<<var>name</var>>[:<<var>type</var>>][=<<var>default</var>>]
</blockquote>
<p>In this definition, <<var>name</var>> is an atom defining the
name of the argument,
<<var>type</var>> is an optional type specification as defined by <a id="idx:mustbe2:1976"></a><span class="pred-ext">must_be/2</span>
from library <code>library(error)</code>, and <<var>default</var>>
is the default initial value. The
<<var>type</var>> defaults to <code>any</code>. If no default
value is specified the default is an unbound variable.
<p>A record declaration creates a set of predicates through
<em>term-expansion</em>. We describe these predicates below. In this
description, <<var>constructor</var>> refers to the name of the
record (`point' in the example above) and <<var>name</var>> to the
name of an argument (field).
<p>
<ul class="latex">
<li><i>default_<<var>constructor</var>>(-Record)</i><br>
Create a new record where all fields have their default values. This is
the same as make_<<var>constructor</var>>([], Record) .
<p>
<li><i>make_<<var>constructor</var>>(+Fields, -Record)</i><br>
Create a new record where specified fields have the specified values and
remaining fields have their default value. Each field is specified as a
term <<var>name</var>>(<<var>value</var>>). See example in
the introduction.
<p>
<li><i>make_<<var>constructor</var>>(+Fields, -Record,
-RestFields)</i><br>
Same as make_<<var>constructor</var>>/2, but named fields that do
not appear in
<var>Record</var> are returned in <var>RestFields</var>. This predicate
is motivated by option-list processing. See library <code>library(option)</code>.
<p>
<li><i><<var>constructor</var>>_<<var>name</var>>(Record,
Value)</i><br>
Unify <var>Value</var> with argument in <var>Record</var> named <<var>name</var>>.<sup class="fn">163<span class="fn-text">Note
this is not called `get_' as it performs unification and can perfectly
well instantiate the argument.</span></sup>
<p>
<li><i><<var>constructor</var>>_data(?Name, +Record, ?Value)</i><br>
True when <var>Value</var> is the value for the field named <var>Name</var>
in <var>Record</var>. This predicate does not perform type-checking.
<p>
<li><i>set_<<var>name</var>>_of_<<var>constructor</var>>(+Value,
+OldRecord, -NewRecord)</i><br>
Replace the value for <<var>name</var>> in <var>OldRecord</var> by <var>Value</var>
and unify the result with <var>NewRecord</var>.
<p>
<li><i>set_<<var>name</var>>_of_<<var>constructor</var>>(+Value,
!Record)</i><br>
Destructively replace the argument <<var>name</var>> in <var>Record</var>
by
<var>Value</var> based on <a id="idx:setarg3:1977"></a><a class="pred" href="manipterm.html#setarg/3">setarg/3</a>.
Use with care.
<p>
<li><i>nb_set_<<var>name</var>>_of_<<var>constructor</var>>(+Value,
!Record)</i><br>
As above, but using non-backtrackable assignment based on <a id="idx:nbsetarg3:1978"></a><a class="pred" href="manipterm.html#nb_setarg/3">nb_setarg/3</a>.
Use with <em>extreme</em> care.
<p>
<li><i>set_<<var>constructor</var>>_fields(+Fields, +Record0,
-Record)</i><br>
Set multiple fields using the same syntax as make_<<var>constructor</var>>/2,
but starting with <var>Record0</var> rather than the default record.
<p>
<li><i>set_<<var>constructor</var>>_fields(+Fields, +Record0,
-Record, -RestFields)</i><br>
Similar to set_<<var>constructor</var>>_fields/4, but fields not
defined by
<<var>constructor</var>> are returned in <var>RestFields</var>.
<p>
<li><i>set_<<var>constructor</var>>_field(+Field, +Record0,
-Record)</i><br>
Set a single field specified as a term <<var>name</var>>(<<var>value</var>>).
</ul>
<dl class="latex">
<dt class="pubdef"><a id="record/1"><strong>record</strong>(<var>+Spec</var>)</a></dt>
<dd class="defbody">
The construct <code>:- record Spec, ...</code> is used to define access
to named fields in a compound. It is subject to term-expansion (see
<a id="idx:expandterm2:1979"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a>)
and cannot be called as a predicate. See
<a class="sec" href="record.html">section A.29</a> for details.
</dd>
</dl>
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