Preprocessing Objective Caml

The compiler

The standard use of Camlp4 is as a preprocessor for Objective Caml with the printer ``pr_dump.cmo''. The Objective Caml compiler ``ocamlc'' recognizes it is a dump, and does not apply another parsing. In case of typing errors, the error is normally showed in the input file.

We remind that the command ``ocaml'' has an option ``-pp'' for calling a preprocessor which automatically deals with intermediate files (see the documentation of Objective Caml). So, the normal use of the combinaison Camlp4-Objective Caml is done by the command:

        ocamlc -pp "camlp4 camlp4-options" ocaml-options input-files

It is often necessary to add the Camlp4 library directory in ocaml's path, using option ``-I''. For information, the Camlp4 command has an option ``-where'' echoing the full name of this directory.

Remark

It is possible to use Camlp4 with ``pr_o.cmo'', instead of ``pr_dump.cmo'' for Objective Caml preprocessing, but it is not recommended, for:

• The parsing would be done twice: one of the user input file by Camlp4, which pretty prints an intermediate file (which takes some time), and one of the result of Camlp4 by the Objective Caml parser.
• Objective Caml may find more syntax errors, for Camlp4 with pa_o.cmo accepts a syntax larger than Objective Caml's (some syntax restriction is done in pr_dump.cmo)
• In case of typing errors, Objective Caml would show the error location in the resulting file of Camlp4, what is not convenient.

However, there is a case where it is interesting to use ``pr_o.cmo'' for objective Caml preprocessing. When the error is located in a quotation (chapter 2) or in a syntax extension (chapter 4), it is not always obvious to understand where the error is exactly located, and there may be no location for the error. Using temporarily camlp4 with ``pr_o.cmo'' allows to see the exact location of the error, which can be understood by the programmer of the quotation expander or the syntax extension.

The toplevel

The Objective Caml toplevel, ``ocaml'' can use Camlp4. To do this, it must be started with the option ``-I Camlp4 library directory'', or, under Unix, ``-I `camlp4 -where`''. Then, one must load:

• Either the file ``camlp4o.cma'' to continue with Objective Caml syntax.
• Or the file ``camlp4r.cma'' to continue with the Righteous syntax.

To change the behavior of Camlp4, just load the needed module with the directives ``#load'' and ``#use'': for example, each load of ``pa_o.cmo'' or ``pa_r.cmo'' would change the current input syntax to be used in the next sentences.

The files included by the directive ``#use'' are treated by the current Camlp4 syntax too. Important remark: a syntax modification takes place after the complete load of a file, not after each sentence inside the file.

It is not possible to load a Camlp4 printing syntax file in the toplevel. The printing results are done by the Objective Caml toplevel in standard Objective Caml syntax.