Optional compilation for OCaml

ppx_optcomp stands for Optional Compilation. It is a tool used to
handle optional compilations of pieces of code depending of the word
size, the version of the compiler, ...

The syntax is based on OCaml item extension nodes, with keywords similar to cpp.

[%%if ocaml_version < (4, 02, 0)]
let x = 1
let y = 2


ppx_optcomp is implemented using ppx_driver and operates on ocaml AST.
This means that whole file needs to be grammatically correct ocaml.

The general syntax is:

[%%keyword expression]

Most of the statements are only supported on the toplevel. See
grammar description
for detailed information where [%% ] directives may be placed.

Note in particular that the item extensions cannot be placed inside an
expression, and this would result in syntax error.

(* SYNTAX ERROR: let x = [%%if defined(abc) ] 1 [%%else] 2 [%%endif] *)

Additional syntax is provided for optional type variant declaration, as in

type t =
| BAR [@if ocaml_version < (4, 02, 0)]


Defining variables

  • [%%define identifier expression]

  • [%%undef identifier]

We also allow: [%%define identifier]. This will define
identifier to (). The undefined identifiers are not valid in
subsequent expressions, but for expression defined(identifier),
which evaluates to false.

The scope of identifiers follows the same scoping rules as OCaml
variables. For instance:

(* [x] is undefined *)
[%%define x 0]
(* [x] is bound to [0] *)
module A = struct
  (* [x] is bound to [0] *)
  [%%define x 42]
  (* [x] is bound to [42] *)
(* [x] is bound to [0] *)


The following directives are available for conditional compilations:

  • [%%if expression]

  • [%%elif expression]

  • [%%else]

  • [%%endif]

  • [@if expression]

In all cases expression must be an expression that evaluates to a
boolean value. Ppx_optcomp will fail if it is not the case.

Pseudo-function defined(identifier) may be then used in
expressions to check whether a given identifier has been defined.
Note that identifiers that were not defined or undefined beforehand
are assumed to be a typo, and therefore are rejected, with a notable
exception of

[%%ifndef FOO]
[%%define FOO]

which is allowed even if FOO was not seen before.

The last form may be used only in type-variant definitions and pattern
matching, following constructors which are to be optional. If you
need a few constructors under the same condition, you need to copy the
directive multiple times, sorry.

type t =
| A of int
| B of int * int [@if ocaml >= 4.04]

match (v: t) with
| A x -> something x
| B (y,z) [@if ocaml >= 4.04] -> something' y z

Warnings and errors

[%%warning _string_] will cause the pre-processor to print a
message on stderr.

[%%error _string_] will cause the pre-processor to fail with the
following error message.

Note that in both cases expression can be an arbitrary expression.


Ppx_optcomp allows one to import another file using:

[%%import filename]

where filename is a string constant. Filenames to import are
resolved as follow:

  • if filename is relative, i.e. doesn't start with /, it is
    considered as relative to the directory of the file being parsed

  • if filename is absolute, i.e. starts with /, it is used as is

Only optcomp directives are allowed in the imported files. The
intended use is including some configuration variables at the
beginning of a file:

[%%import "config.mlh"]

If imported file's extension is .h, an alternate C-like syntax is
expected in the file. This is to allow importing both from C and
OCaml single configuration file like:

#ifndef CONFIG_H
#define CONFIG_H

#define FOO
#undef BAR
#define BAZ 3*3 + 3


Expressions and patterns

ppx_optcomp supports a subset of OCaml expressions and patterns:

  • literals: integers, characters and strings

  • tuples

  • true and false

  • let-bindings

  • pattern matching

And it provides the following functions:

  • comparison operators: =, <, ...

  • boolean operators: ||, &&, not, ...

  • arithmetic operators: +, -, *, /

  • min and max

  • fst and snd

  • conversion functions: to_int, to_string, to_char, to_bool

  • defined, not_defined: check whether a variable is defined

  • show: act as identity, but pretty-print a value to stderr

31 May 2020
>= "0.11.0" & < "0.18.0"
>= "2.0.0"
>= "v0.14" & < "v0.15"
>= "v0.14" & < "v0.15"
>= "4.04.2"
Reverse Dependencies
>= "v0.14.0" & < "v0.15.0"
>= "0.12.0" & < "1.0.0"
>= "0.2.0"
= "v0.14.0"
>= "4.2"
= "v0.14.0"
= "v0.14.0"
= "v0.14.0"