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<title>Redefining OCaml syntax</title>
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<h1 class="top">Redefining OCaml syntax</h1>
<p>Better than just <a href="syntext.html">syntax extensions</a>, it is
possible to redefine the whole syntax of the language. For example,
to:</p>
<ul>
<li>define your own syntax, like the <a href="revsynt.html">revised
syntax</a> does,</li>
<li>have a version whose keywords are translated in your native
language,</li>
<li>restrict the OCaml language,</li>
<li>interpret XML (or other languages) as OCaml source,</li>
<li>and so on...</li>
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<h2>Starting with an example</h2>
<p>A way to start doing this is to take, in Camlp5 sources, one of the
files "etc/pa_o.ml" or file "meta/pa_r.ml". The first one defines
the OCaml standard syntax and the second one the revised syntax.</p>
<p>Let's say you want to take the normal syntax and make some
readjustments. You first make a copy of "etc/pa_o.ml" naming it,
e.g., "mysyntax.ml" (the example below works similarly if you
take "meta/pa_r.ml" instead):</p>
<p>To test, you can compile it with the command:</p>
<pre>
ocamlc -pp camlp5r -I $(camlp5 -where) -c mysyntax.ml
</pre>
<p>This produces the file "mysyntax.cmo". Now you can compile one of
your files, e.g. "foo.ml", if written in this syntax, i.e. the
normal OCaml syntax if you made no changes in "mysyntax.ml":</p>
<pre>
ocamlc -pp 'camlp5 ./mysyntax.cmo pr_dump.cmo' -c foo.ml
</pre>
<p>If there si no changes in "mysyntax.ml" from "pa_o.ml", this is
just a compilation with the normal OCaml syntax. To make changes,
you can edit the file "mysyntax.ml" and recompile it. As an
exercice, try to translate some keywords in your native language
(or another language if it is not English).</p>
<p>Reading the way Camlp5 <a href="grammars.html">extensible
grammars</a> and <a href="ast_strict.html">syntax tree</a> work
(both used in "pa_o.ml" and "pa_r.ml"), you can make more
complicated changes or change everything, if you want.</p>
<h2>A file for an OCaml syntax</h2>
<p>This is what you can find in the files "pa_o.ml" and "pa_r.ml".</p>
<p>An OCaml syntax files uses the Camlp5 library
module <a href="pcaml.html">Pcaml</a>. All grammar entries are
defined there. The first thing is the reinitialization of the
grammar (which clear all tokens and define a lexer) and all grammar
entries, to be sure that no possible previous loaded grammars
remain.</p>
<p>If using the same lexer (provided in Camlp5 library
module <a href="library.html#a:Plexer-module">Plexer</a>), it is
done by:</p>
<pre>
Grammar.Unsafe.gram_reinit Pcaml.gram (Plexer.gmake ())
</pre>
<p>The cleanup of all grammar entries are done by calls to the function
"Grammar.Unsafe.clear_entry". The main entries are Pcaml.interf, for
compiling an interface (a ".mli" file) and Pcaml.implem, for compiling
an implementation (a ".ml" file). And all other grammars entries you
want to use must be cleared:</p>
<pre>
Grammar.Unsafe.clear_entry Pcaml.interf;
Grammar.Unsafe.clear_entry Pcaml.implem;
Grammar.Unsafe.clear_entry Pcaml.expr;
Grammar.Unsafe.clear_entry Pcaml.patt;
...
</pre>
<p>Actually, the camlp5 command can compile the input file with other
ways than using the Camlp5 grammars. The variables "Pcaml.parse_interf"
and "Pcaml.parse_implem" are references to the functions called by
camlp5. By default, it is the Camlp5 grammar syntax, but to be sure
it goes on using it (if a previous load changed that), the following
statement are added:</p>
<pre>
Pcaml.parse_interf.val := Pcaml.Grammar.Entry.parse interf;
Pcaml.parse_implem.val := Pcaml.Grammar.Entry.parse implem;
</pre>
<p>In the files "pa_o.ml" and "pa_r.ml", some local functions follow,
which are themselves followed by a call to the big statement
"EXTEND", the main statement of the Camlp5 extensible grammars
system.</p>
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