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Chapter 8 The camlp4 commands
Camlp4 is a command which you can type from your shell. It has several versions: camlp4, camlp4o, camlp4r. Actually, the basic command is camlp4, the other ones being shortcuts for this command with syntaxes previously loaded (see man page).

Its options are in two parts: the ``load'' options and the ``other'' options. The input file is among the ``other'' options, generally a .mli interface file or .ml implementation file.

8.1 The load options

The ``load'' options *must* come first. They are a list of OCaml object files (cmo) or library file (cma) which are loaded in camlp4 core before any other operation. These files are found in a path. You can use the option -I to add directories in this path. By default, the path contains only the camlp4 library directory (depending on the installation).

Notice that the current directory is *not* by default in the path: to load a file belonging to the current directory, you have to use either the option -I . or to type the object file prefixed by a dot and a slash.

In fact, the command camlp4 alone, without any load option does nothing with the input file and fails. You need to specify at least one parser and one printer. If you don't specify any parser, you get the message ``Failure: no loaded parsing module'', if you don't specify any printer, ``Failure: no printer''. When it runs, camlp4 applies the parser to the input file, which builds a syntax tree and this syntax tree is printed using the printer.

8.2 The other options

The other options follow. If there is a risk of ambiguities, you can use -- to separate the load options and the other ones: indeed, the other options can be extended by the object files loaded in the core: in particular, the provided object file pr_depend.cmo adds -I among the other options.

To see the list of available options, type camlp4 -help or, since the object files can extend the options, type camlp4 <load-options> -help where <load-options> is your load options. Type, for example, and compare the results:
            camlp4 -help
            camlp4 pa_r.cmo pa_extend.cmo -help
            camlp4 pr_depend.cmo -help
8.3 The predefined object files

The predefined object files, available in the camlp4 library directory, are of two kinds: You can see the list of available parsers and printers in the camlp4 library directory or using the man page. The main one are: Note that pr_dump.cmo displays the abstract syntax tree in binary. By default, it is displayed in standard output (except if the option -o is used). Thus, pay attention: it is binary material, if you don't redirect the output to a file, you are likely to perturb your terminal, because it may find escape sequences and interpret them. This printer is used as input to the OCaml compiler.

The other printers are useful to display the result of a parsing, test syntax extensions or quotations expanders. You can print either in normal syntax or in revised syntax. This can be used also to convert from normal to revised syntax and vice-versa.

Printers can also be extended. This is undocumented because the printing facility is not perfected and is a little bit complicated to use. But you can use the predefined printers. They rebuild the text in the initial form. For example pr_extend.cmo rebuilds the EXTEND statements.

An interesting predefined quotation expander is q_phony.cmo which convert all quotations into phony variables. The quotations are therefore printed as they are. Useful to pretty print your program containing quotations without expanding them.

8.4 Commands camlp4o and camlp4r

The commands camlp4o and camlp4r are actually shortcuts for (respectively):
            camlp4 pa_o.cmo pa_op.cmo pr_dump.cmo
            camlp4 pa_r.cmo pa_rp.cmo pr_dump.cmo
You can add parsers and (other) printers to these commands also.

8.5 Toplevel

To use Camlp4 in the toplevel, you have to first load the camlp4 machinery. Just loading pa_r.cmo or pa_o.cmo would not work: the toplevel need a specific object file which contains 1/ a plug-in to tell it to parse using the Camlp4 stuff 2/ the camlp4 library 3/ object files used by the camlp4 parsers (the camlp4 pretty printing does not work in the toplevel).

There are two files which contain all of this: camlp4r.cma starting with the revised syntax and camlp4o.cma starting with the normal syntax. You must then load one of them by typing either:
            #load "camlp4r.cma";;
            #load "camlp4o.cma";;
From now, you are in the Camlp4 machinery. Everything you type is parsed by Camlp4. In particular, if you chose the revised syntax, any other #load statement must end with an alone semicolon, as the revised syntax requires. To use the camlp4 grammar system, load pa_extend.cmo, to use the predefined quotations for the OCaml syntax tree, load q_MLast.cmo.

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