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

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<h2 id="sec:syntaxext"><a id="sec:B.3"><span class="sec-nr">B.3</span> <span class="sec-title">Syntax 
extensions</span></a></h2>

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

<p>Prolog is commonly used to define <em>domain specific languages</em> 
(DSL) as well as to interact with external languages that have a 
concrete syntax, such as HTML, JavaScript or SQL. Standard Prolog 
provides operators (see <a class="sec" href="operators.html">section 
4.25</a>) for extending its syntax. Unfortunately, Prolog's syntax is 
rather peculiar and operators do not allow for commonly seen syntactical 
patterns such as array subscripting, expressing attributes, scope or a 
body using curly brackets, distinguishing identifiers or strings from 
`functions', etc.

<p>The syntactic extensions described in <a class="sec" href="syntaxext.html">section 
B.3.1</a> provide additional constructs to extend the syntax. These 
extensions allow for coping with a large part of the <em>curly bracket 
languages</em>, which allows for defining DSLs that are more natural to 
use, in particular for people that are less familiar with Prolog.

<p>For some external languages it can be sufficient to support the 
simple data model using a completely different Prolog concrete syntax. 
This is for example the case for HTML, as implemented by the library
<code>library(http/html_write)</code>. With the extensions of <a class="sec" href="syntaxext.html">section 
B.3.1</a>, this also becomes true for the statistics language <b>R</b>, 
which was one of the motivations for these extensions. These extensions 
are motivated in <cite><a class="cite" href="Bibliography.html#WLPE/Wielemaker/2012">Wielemaker &amp; 
Angelopoulos,</a></cite>.

<p>Other languages, such as full JavaScript, are too different from 
Prolog for dealing with them using (extended) Prolog operator. While 
most of the JavaScript syntax can be covered with the extended notion of 
operators, the produced Prolog term does not unambiguishly describe the 
JavaScript abstract syntax. For example, both <code>++a</code> and <code>a++</code> 
are, with the appropriate operator declarations, valid Prolog syntax. 
However, they both map to the term <code>++(a)</code> and thus a Prolog 
JavaScript serialization does not know which these two forms the 
generate.<sup class="fn">126<span class="fn-text">This example comes 
from Richard O'Keefe.</span></sup> More classical, <code>"string"</code> 
produces the same Prolog term as
<code>[115,116,114,105,110,103]</code>.

<p>An alternative to syntax extension using (extended) operators are
<em>quasi quotations</em> <cite><a class="cite" href="Bibliography.html#DBLP:conf/haskell/Mainland07">Mainland, 
2007</a></cite>. Quasi quotations embed external languages in a Prolog 
term using their native syntax. The language is specified in the 
quotation. Parsing such a term causes Prolog to call the associated 
parser which creates an abstract syntax tree that unambiguosly 
represents the code fragment and which can be processed in Prolog or 
serialized for external processing. Quasi quotations are realised by 
library <code>library(quasi_quotations)</code>, which is documented in <a class="sec" href="quasiquotations.html">section 
A.23</a>.

<p><h3 id="sec:blockop"><a id="sec:B.3.1"><span class="sec-nr">B.3.1</span> <span class="sec-title">Block 
operators</span></a></h3>

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

<p>Introducing curly bracket, array subscripting and empty argument 
lists is achieved using <em>block operators</em>.<sup class="fn">127<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 their need to 
reach an agreement. Increasing need to get to some solution resulted in 
what is documented in this section.</span></sup> The atoms <code>[]</code>, <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>
<dt><strong>(&nbsp;)</strong></dt>
<dd class="defbody">
This operator can only meaningfully be declared as a low-priority
<code>xf</code> postfix operator, which allows for <code>name()</code> 
notation. This syntax produces a term <code>'()'(name)</code>. This 
transformation only applies if the opening bracket immediately follows 
the functor name, i.e., following the same rules as for constructing a 
compound term.
</dd>
</dl>

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

<pre class="code">
?- op(1, xf, '()').
?- 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 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / syntaxext.html
