/usr/lib/swi-prolog/doc/Manual/ext-syntax.html

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<h2 id="sec:ext-syntax"><a id="sec:5.3"><span class="sec-nr">5.3</span> <span class="sec-title">Syntax 
changes</span></a></h2>

<a id="sec:ext-syntax"></a>

<p><h3 id="sec:ext-syntax-op"><a id="sec:5.3.1"><span class="sec-nr">5.3.1</span> <span class="sec-title">Operators 
and quoted atoms</span></a></h3>

<a id="sec:ext-syntax-op"></a>

<p>As of SWI-Prolog version&nbsp;7, quoted atoms loose their operator 
property. This means that expressions such as <code>A = 'dynamic'/1</code> 
are valid syntax, regardless of the operator definitions. From questions 
on the mailinglist this is what people expect.<sup class="fn">130<span class="fn-text">We 
believe that most users expect an operator declaration to define a new 
token, which would explain why the operator name is often quoted in the 
declaration, but not while the operator is used. We are afraid that 
allowing for this easily creates ambiguous syntax. Also, many 
development environments are based on tokenization. Having dynamic 
tokenization due to operator declarations would make it hard to support 
Prolog in such editors.</span></sup> To accomodate for real quoted 
operators, a quoted atom that <em>needs</em> quotes can still act as an 
operator.<sup class="fn">131<span class="fn-text">Suggested by Joachim 
Schimpf.</span></sup> A good use-case for this is a unit library<sup class="fn">132<span class="fn-text"><a class="url" href="https://groups.google.com/d/msg/comp.lang.prolog/ozqdzI-gi_g/2G16GYLIS0IJ">https://groups.google.com/d/msg/comp.lang.prolog/ozqdzI-gi_g/2G16GYLIS0IJ</a></span></sup>, 
which allows for expressions such as below.

<pre class="code">
?- Y isu 600kcal - 1h*200'W'.
Y = 1790400.0'J'.
</pre>

<p><h3 id="sec:ext-compound-zero"><a id="sec:5.3.2"><span class="sec-nr">5.3.2</span> <span class="sec-title">Compound 
terms with zero arguments</span></a></h3>

<a id="sec:ext-compound-zero"></a>

<p>As of SWI-Prolog version&nbsp;7, the system supports compound terms 
that have no arguments. This implies that e.g., <code>name()</code> is 
valid syntax. This extension aims at functions on dicts (see <a class="sec" href="dicts.html">section 
5.4</a>) as well as the implementation of domain specific languages 
(DSLs). To minimise the consequences, the classic predicates <a id="idx:functor3:1480"></a><a class="pred" href="manipterm.html#functor/3">functor/3</a> 
and <a class="pred" href="manipterm.html#=../2">=../2</a> have not been 
modified. The predicates <a id="idx:compoundnamearity3:1481"></a><a class="pred" href="manipterm.html#compound_name_arity/3">compound_name_arity/3</a> 
and
<a id="idx:compoundnamearguments3:1482"></a><a class="pred" href="manipterm.html#compound_name_arguments/3">compound_name_arguments/3</a> 
have been added. These predicates operate only on compound terms and 
behave consistently for compounds with zero arguments. Code that <em>generalises</em> 
a term using the sequence below should generally be changed to use <a id="idx:compoundnamearity3:1483"></a><a class="pred" href="manipterm.html#compound_name_arity/3">compound_name_arity/3</a>.

<pre class="code">
    ...,
    functor(Specific, Name, Arity),
    functor(General, Name, Arity),
    ...,
</pre>

<p>Replacement of <a class="pred" href="manipterm.html#=../2">=../2</a> 
by <a id="idx:compoundnamearguments3:1484"></a><a class="pred" href="manipterm.html#compound_name_arguments/3">compound_name_arguments/3</a> 
is typically needed to deal with code that follow the skeleton below.

<pre class="code">
    ...,
    Term0 =.. [Name|Args0],
    maplist(convert, Args0, Args),
    Term =.. [Name|Args],
    ...,
</pre>

<p>For predicates, goals and arithmetic functions (evaluable terms), &lt;<var>name</var>&gt; 
and &lt;<var>name</var>&gt;() are <em>equivalent</em>. Below are some 
examples that illustrate this behaviour.

<pre class="code">
go() :- format('Hello world~n').

?- go().
Hello world

?- go.
Hello world

?- Pi is pi().
Pi = 3.141592653589793.

?- Pi is pi.
Pi = 3.141592653589793.
</pre>

<p>Note that the <em>cannonical</em> representation of predicate heads 
and functions without arguments is an atom. Thus, <code>clause(go(), 
Body)</code> returns the clauses for <a id="idx:go0:1485"></a><span class="pred-ext">go/0</span>, 
but <code>clause(-Head, -Body, +Ref)</code> unifies <var>Head</var> with 
an atom if the clause specified by <var>Ref</var> is part of a predicate 
with zero arguments.

<p><h3 id="sec:ext-blockop"><a id="sec:5.3.3"><span class="sec-nr">5.3.3</span> <span class="sec-title">Block 
operators</span></a></h3>

<a id="sec:ext-blockop"></a>

<p>Introducing curly bracket and array subscripting.<sup class="fn">133<span class="fn-text">Introducing 
block operators was proposed by Jose Morales. It was discussed in the 
Prolog standardization mailing list, but there were too many conflicts 
with existing extensions (ECLiPSe and B-Prolog) and doubt about their 
need to reach an agreement. Increasing need to get to some solution 
resulted in what is documented in this section. These extensions are 
also implemented in recent versions of YAP.</span></sup> The symbols <code>[]</code> 
and
<code>{}</code> may be declared as an operator, which has the following 
effect:

<dl class="latex">
<dt><strong>[&nbsp;]</strong></dt>
<dd class="defbody">
This operator is typically declared as a low-priority <code>yf</code> 
postfix operator, which allows for <code>array[index]</code> notation. 
This syntax produces a term <code>[]([index],array)</code>.</dd>
<dt><strong>{&nbsp;}</strong></dt>
<dd class="defbody">
This operator is typically declared as a low-priority <code>xf</code> 
postfix operator, which allows for <code>head(arg) { body }</code> 
notation. This syntax produces a term <code>{}({body},head(arg))</code>.
</dd>
</dl>

<p>Below is an example that illustrates the representation of a typical 
`curly bracket language' in Prolog.

<pre class="code">
?- op(100, xf, {}).
?- op(100, yf, []).
?- op(1100, yf, ;).

?- displayq(func(arg)
            { a[10] = 5;
              update();
            }).
{}({;(=([]([10],a),5),;(update()))},func(arg))
</pre>

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