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<h2 id="sec:quasiquotations"><a id="sec:A.23"><span class="sec-nr">A.23</span> <span class="sec-title">library(quasi_quotations): 
Define Quasi Quotation syntax</span></a></h2>

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

<dl class="tags">
<dt class="tag">author</dt>
<dd>
Jan Wielemaker. Introduction of Quasi Quotation was suggested by Michael 
Hendricks.
</dd>
<dt class="tag">See also</dt>
<dd>
<a class="url" href="http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/mainland07quasiquoting.pdf"></a>
</dd>
</dl>

<p>Inspired by
<a class="url" href="http://www.haskell.org/haskellwiki/Quasiquotation">Haskell</a>, 
SWI-Prolog support <i>quasi quotation</i>. Quasi quotation allows for 
embedding (long) strings using the syntax of an external language (e.g., 
HTML, SQL) in Prolog text and syntax-aware embedding of Prolog variables 
in this syntax. At the same time, quasi quotation provides an 
alternative to represent long strings and atoms in Prolog.

<p>The basic form of a quasi quotation is defined below. Here, <var>Syntax</var> 
is an arbitrary Prolog term that must parse into a <i>callable</i> (atom 
or compound) term and Quotation is an arbitrary sequence of characters, 
not including the sequence <code>|}</code>. If this sequence needs to be 
embedded, it must be escaped according to the rules of the target 
language or the `quoter' must provide an escaping mechanism.

<pre class="code">
{|Syntax||Quotation|}
</pre>

<p>While reading a Prolog term, and if the Prolog flag <code>quasi_quotes</code> 
is set to <code>true</code> (which is the case if this library is 
loaded), the parser collects quasi quotations. After reading the final 
full stop, the parser makes the call below. Here, <var>SyntaxName</var> 
is the functor name of <var>Syntax</var> above and <var>SyntaxArgs</var> 
is a list holding the arguments, i.e., <code>Syntax =.. [SyntaxName|SyntaxArgs]</code>. 
Splitting the syntax into its name and arguments is done to make the 
quasi quotation parser a predicate with a consistent arity 4, regardless 
of the number of additional arguments.

<pre class="code">
call(+SyntaxName, +Content, +SyntaxArgs, +VariableNames, -Result)
</pre>

<p>The arguments are defined as

<p>
<ul class="latex">
<li><var>SyntaxName</var> is the principal functor of the quasi 
quotation syntax. This must be declared using <a class="pred" href="quasiquotations.html#quasi_quotation_syntax/1">quasi_quotation_syntax/1</a> 
and there must be a predicate SyntaxName/4.
<li><var>Content</var> is an opaque term that carries the content of the 
quasi quoted material and position information about the source code. It 
is passed to <span class="pred-ext">with_quasi_quote_input/3</span>.
<li><var>SyntaxArgs</var> carries the additional arguments of the <var>Syntax</var>. 
These are commonly used to make the parameter passing between the clause 
and the quasi quotation explicit. For example:

<pre class="code">
    ...,
    {|html(Name, Address)||
     &lt;tr&gt;&lt;td&gt;Name&lt;td&gt;Address&lt;/tr&gt;
     |}
</pre>

<p>
<li><var>VariableNames</var> is the complete variable dictionary of the 
clause as it is made available throug <a class="pred" href="termrw.html#read_term/3">read_term/3</a> 
with the option
<code>variable_names</code>. It is a list of terms <code>Name = Var</code>.
<li><var>Result</var> is a variable that must be unified to resulting 
term. Typically, this term is structured Prolog tree that carries a 
(partial) representation of the abstract syntax tree with embedded 
variables that pass the Prolog parameters. This term is normally either 
passed to a predicate that serializes the abstract syntax tree, or a 
predicate that processes the result in Prolog. For example, HTML is 
commonly embedded for writing HTML documents (see
<code>library(http/html_write)</code>). Examples of languages that may 
be embedded for processing in Prolog are SPARQL, RuleML or regular 
expressions.
</ul>

<p>The file <code>library(http/html_quasiquotations)</code> provides 
the, suprisingly simple, quasi quotation parser for HTML.

<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="with_quasi_quotation_input/3"><strong>with_quasi_quotation_input</strong>(<var>+Content, 
-Stream, :Goal</var>)</a></dt>
<dd class="defbody">
Process the quasi-quoted <var>Content</var> using <var>Stream</var> 
parsed by <var>Goal</var>.
<var>Stream</var> is a temporary stream with the following properties:

<p>
<ul class="latex">
<li>Its initial <i>position</i> represents the position of the start of 
the quoted material.
<li>It is a text stream, using <code>utf8</code> <i>encoding</i>.
<li>It allows for repositioning
<li>It will be closed after <var>Goal</var> completes.
</ul>
<table class="arglist">
<tr><td><var>Goal</var> </td><td>is executed as <code>once(Goal)</code>. <var>Goal</var> 
must succeed. Failure or exceptions from <var>Goal</var> are interpreted 
as syntax errors. </td></tr>
</table>

<dl class="tags">
<dt class="tag">See also</dt>
<dd>
<a class="pred" href="quasiquotations.html#phrase_from_quasi_quotation/2">phrase_from_quasi_quotation/2</a> 
can be used to process a quotation using a grammar.
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="phrase_from_quasi_quotation/2"><strong>phrase_from_quasi_quotation</strong>(<var>:Grammar, 
+Content</var>)</a></dt>
<dd class="defbody">
Process the quasi quotation using the DCG <var>Grammar</var>. Failure of 
the grammer is interpreted as a syntax error.

<dl class="tags">
<dt class="tag">See also</dt>
<dd>
<a class="pred" href="quasiquotations.html#with_quasi_quotation_input/3">with_quasi_quotation_input/3</a> 
for processing quotations from stream.
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="quasi_quotation_syntax/1"><strong>quasi_quotation_syntax</strong>(<var>:SyntaxName</var>)</a></dt>
<dd class="defbody">
Declare the predicate <var>SyntaxName</var>/4 to implement the the quasi 
quote syntax <var>SyntaxName</var>. Normally used as a directive.</dd>
<dt class="pubdef"><a id="quasi_quotation_syntax_error/1"><strong>quasi_quotation_syntax_error</strong>(<var>+Error</var>)</a></dt>
<dd class="defbody">
Report <code>syntax_error(Error)</code> using the current location in 
the quasi quoted input parser.

<dl class="tags">
<dt class="tag">throws</dt>
<dd>
<code>error(syntax_error(Error), Position)</code>
</dd>
</dl>

</dd>
</dl>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / quasiquotations.html
