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<h1 class="top">Syntax tree quotations in user syntax</h1>
<p>The quotation kit "<tt>q_ast.cmo</tt>" allows to use syntax tree
quotations in user syntax. It fully works only in "strict" mode. In
"transitional" mode, there is no way to use antiquotations, which
restricts its utility.</p>
<p>If this kit is loaded, when a quotation of syntax tree is found,
the current OCaml language parser is called. Then, the resulting
tree is reified (except the antiquotations) to represent <em>the
syntax tree of the syntax tree</em>.</p>
<h2>Antiquotations</h2>
<p>The OCaml langage parser used, and its possible extensions, must
have been built to allow the places of the antiquotations. The
symbols enclosed by the meta-symbol "<tt>V</tt>" (see the chapter
about <a href="grammars.html">extensible grammars</a>, section
"symbols"), define where antiquotations can take place.</p>
<p>There is no need to specify antiquotations for the main types
defined in the AST tree module ("<tt>MLast</tt>"): "<tt>expr</tt>",
"<tt>patt</tt>", "<tt>expr</tt>", "<tt>str_item</tt>",
"<tt>sig_item</tt>", and so on. All syntax parts of these types are
automatically antiquotable.</p>
<p>For example, in Camlp5 sources, the grammar rule defining the
"let..in" statement is:</p>
<pre>
"let"; r = V (FLAG "rec") "flag" "opt";
l = V (LIST1 let_binding SEP "and"); "in"; x = expr
</pre>
<p>All symbols of these rules, except the keywords, are antiquotable:</p>
<ul>
<li>The "rec" flag, because enclosed by the "V" meta symbol. The two
strings which follow it gives the possible antiquotation kinds:
"flag" (the normal antiquotation kind) and "opt" (kept by backward
compatibility, but not recommended). It is therefore possible to
antiquote it as: "<tt>$flag:...$</tt>" or "<tt>$opt:...$</tt>"
where the "<tt>...</tt>" is an expression or a pattern depending
on the position of the enclosing quotation</li>
<li>The binding list is also antiquotable, since it is also enclosed
by the "V" meta symbol. Its antiquotation kind is "list" (the
default when the meta symbol parameter is a list). It is therefore
possible to write "<tt>$list:...$</tt>" at the place of the
binding list.</li>
<li>The expression after the "in" is also antiquotable, because it
belongs to the main types defined in the module "<tt>MLast</tt>".</li>
</ul>
<p>In that example, the variable "<tt>r</tt>" is of type "<tt>Ploc.vala
bool</tt>", the variable "<tt>r</tt>" of type "<tt>Ploc.vala (list
(patt * expr))</tt>" and the variable "<tt>x</tt>" of type
"<tt>MLast.expr</tt>".</p>
<p>... to be completed ...</p>
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