optcomp - optional compilation with cpp-like directives

Optcomp is a syntax extension which handles #if, #else, ... directives in ocaml source files.

For example, to switch between two pieces of code according to the ocaml compiler version, one can write:

#if ocaml_version < (3, 10)
let x = 1
#else
let x = 2
#endif

What the difference between cpp and optcomp ?

Optcomp is more OCaml-friendly than cpp:

it does not interpret //, /*, and */ as comment delimiters
it does not complains about missing '
it is easier to integrate in the build process when using other camlp4 syntax extensions

By the way optcomp does not do macro expansion while cpp does.

What the difference between pa_macro and optcomp ?

Optcomp does not require code that will be dropped to be valid caml code. So for example this code will be rejected by camlp4(<3.13)+pa_macro:

IFDEF HAVE_GADTS THEN
type 'a t =
  | Int : int t
  | String : string t
ENDIF

But this one will be accepted by camlp4+optcomp:

#if HAVE_GADTS
type 'a t =
  | Int : int t
  | String : string t
#endif

Installation

To build and install optcomp:

$ ./configure
$ make
$ make install

Documentation (optional)

To build the documentation:

$ make doc

It will then be installed by make install.

Tests (optionnal)

To build and execute tests:

$ ./configure --enable-tests
$ make test

Usage

As a syntax extension

You can use optcomp as a regular syntax extension with camlp4. If you have ocamlfind installed, you can use:

$ ocamlfind ocamlc -syntax camlp4o -pakcage camlp4,optcomp file.ml

You can also embed pa_optcomp.ml in your project sources.

As a preprocessor

Optcomp can be used as a preprocessor, for that there is the two executable optcomp-o and optcomp-r:

optcomp-o is for when directives are written using original syntax
optcomp-r is for when directives are written using revised syntax

To use them:

$ ocamlc -pp optcomp-o <file.ml>
$ ocamlc -pp optcomp-r <file.ml>

The preprocessor version is especially usefull for .mli because it does not modify the layout of your code, which is important for ocamldoc.

Hacking

To add support to more expressions, you can modify the eval function of pa_optcomp.ml. It takes a camlp4 expression ast and must return something of type value.

xulingfeng / optcomp