Redefining OCaml syntax

Redefining OCaml syntax

Better than just syntax extensions, it is possible to redefine the whole syntax of the language. For example, to:

  • define your own syntax, like the revised syntax does,
  • have a version whose keywords are translated in your native language,
  • restrict the OCaml language,
  • interpret XML (or other languages) as OCaml source,
  • and so on…

Starting with an example

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.

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):

To test, you can compile it with the command:

ocamlc -pp camlp5r -I $(camlp5 -where) -c mysyntax.ml

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”:

ocamlc -pp 'camlp5 ./mysyntax.cmo pr_dump.cmo' -c foo.ml

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).

Reading the way Camlp5 extensible grammars and syntax tree work (both used in “pa_o.ml” and “pa_r.ml”), you can make more complicated changes or change everything, if you want.

A file for an OCaml syntax

This is what you can find in the files “pa_o.ml” and “pa_r.ml”.

An OCaml syntax files uses the Camlp5 library module Pcaml. 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.

If using the same lexer (provided in Camlp5 library module Plexer), it is done by:

Grammar.Unsafe.gram_reinit Pcaml.gram (Plexer.gmake ())

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:

Grammar.Unsafe.clear_entry Pcaml.interf;
Grammar.Unsafe.clear_entry Pcaml.implem;
Grammar.Unsafe.clear_entry Pcaml.expr;
Grammar.Unsafe.clear_entry Pcaml.patt;
...

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:

Pcaml.parse_interf.val := Pcaml.Grammar.Entry.parse interf;
Pcaml.parse_implem.val := Pcaml.Grammar.Entry.parse implem;

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.