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<title>SWI-Prolog 7.3.6 Reference Manual: Section 5.1</title><link rel="home" href="index.html">
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<h2 id="sec:ext-lists"><a id="sec:5.1"><span class="sec-nr">5.1</span> <span class="sec-title">Lists
are special</span></a></h2>
<a id="sec:ext-lists"></a>
<p>As of version 7, SWI-Prolog lists can be distinguished
unambiguously at runtime from <a class="function" href="arith.html#f-./2">./2</a>
terms and the atom <code>'[]'</code>. The constant <code>[]</code> is
special constant that is not an atom. It has the following properties:
<pre class="code">
?- atom([]).
false.
?- atomic([]).
true.
?- [] == '[]'.
false.
?- [] == [].
true.
</pre>
<p>The `cons' operator for creating list cells has changed from the
pretty atom <code>'.'</code> to the ugly atom <code>'[|]'</code>, so we
can use the
<code>'.'</code> for other purposes. See <a class="sec" href="dicts.html">section
5.4.1</a>.
<p>This modification has minimal impact on typical Prolog code. It does
affect foreign code (see <a class="sec" href="foreign.html">section 10</a>)
that uses the normal atom and compound term interface for manipulation
lists. In most cases this can be avoided by using the dedicated list
functions. For convenience, the macros <code>ATOM_nil</code> and <code>ATOM_dot</code>
are provided by
<code>SWI-Prolog.h</code>.
<p>Another place that is affected is <a id="idx:writecanonical1:1444"></a><a class="pred" href="termrw.html#write_canonical/1">write_canonical/1</a>.
Impact is minimized by using the list syntax for lists. The predicates <a id="idx:readterm2:1445"></a><a class="pred" href="termrw.html#read_term/2">read_term/2</a>
and
<a id="idx:writeterm2:1446"></a><a class="pred" href="termrw.html#write_term/2">write_term/2</a>
support the option <code>dotlists(true)</code>, which causes
<a id="idx:readterm2:1447"></a><a class="pred" href="termrw.html#read_term/2">read_term/2</a>
to read <code>.(a,[])</code> as <code>[a]</code> and <a id="idx:writeterm2:1448"></a><a class="pred" href="termrw.html#write_term/2">write_term/2</a>
to write <code>[a]</code> as <code>.(a,[])</code>.
<p><h3 id="sec:ext-list-motivation"><a id="sec:5.1.1"><span class="sec-nr">5.1.1</span> <span class="sec-title">Motivating
' ' and[]for lists</span></a></h3>
<a id="sec:ext-list-motivation"></a>
<p>Representing lists the conventional way using <a class="function" href="arith.html#f-./2">./2</a>
as cons-cell and '[]' as list terminator both (independently) poses
conflicts, while these conflicts are easily avoided.
<p>
<ul class="latex">
<li>Using <a class="function" href="arith.html#f-./2">./2</a> prevents
using this commonly used symbol as an operator because <code>a.B</code>
cannot be distinguished from <code>[a|B]</code>. Freeing <a class="function" href="arith.html#f-./2">./2</a>
provides us with a unique term that we can use for functional notation
on dicts as described in
<a class="sec" href="dicts.html">section 5.4.1</a>.
<p>
<li>Using <code>'[]'</code> as list terminator prevents dynamic
distinction between atoms and lists. As a result, we cannot use type
polymorphism that involve both atoms and lists. For example, we cannot
use
<em>multi lists</em> (arbitrary deeply nested lists) of atoms. Multi
lists of atoms are a in some situations a good representation of a flat
list that is assembled from sub sequences. The alternative, using
difference lists or DCGs is often less natural and sometimes demands for
`opening' proper lists (i.e., copying the list while replacing the
terminating empty list with a variable) that have to be added to the
sequence. The ambiguity of atom and list is particularly painful when
mapping external data representations that do not suffer from this
ambiguity.
<p>At the same time, avoiding <code>'[]'</code> as a list terminator
makes the various text representations unambiguous, which allows us to
write predicates that require a textual argument to accept both atoms,
strings, and lists of character codes or one-character atoms.
Traditionally, the empty list can be interpreted both as the string "[]"
and "".
</ul>
<p></body></html>
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