package config

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Ergonomic, lightweight conditional compilation through attributes


Dune Dependency





Ergonomic, lightweight conditional compilation through attributes inspired by Rust's cfg macro.

Config implements the [@@config] annotation, to conditionally compile modules. You can use a small boolean language to check for conditions including some built-in ones and environment variables.

It can be used to conditionally include/exclude:

  • modules

  • let bindings

  • includes

  • type definitions

  • constructors from variant types and polymorphic variants

  • fields from record types

  • and more

Currently includes/detects the following platforms:

  • Windows 32/64

  • macOS/tvOS/watchOS/iOS

  • FreeBSD/NetBSD

  • Android

  • Linux

And the following architectures:

  • x86-64

  • x86 i386

  • aarch64

  • arm

And the following environment:

  • msvc

  • gnu

  • musl

Getting Started

opam install config

And add it to your dune files:

  (name my_lib)
  (preprocess (pps config.ppx)))

And tag your values with the @@config attribute:

module A = A_unix [@@config any(target_os = "macos", target_os = "linux")]
module A = A_win32 [@@config any(target_os = "windows")]

The Config Language

Config implements a very small boolean language with 4 operations:

  • var = value, that checks if a variable (from the environment or provided by config) is equals to a value. Equality is string or integer equality.

  • all(expr1, expr2, ...), expects all expressions to be true

  • any(expr1, expr2, ...), expects any expression to be true

  • not(expr), negates an expression

To define new variables you can pass them in as environment variables:

; export BAND="rush" dune build

And you'll be able to write (BAND = "rush")


Enabling/Disabling Modules and Includes

module Pro_mode_env = struct
  let name = "pro-mode"
  let coins = 2112
[@@config (project_mode = "pro")]

include Pro_mode_env
[@@config (project_mode = "pro")]

Enabling/Disabling Let definitions

let favorite_band = "rush"
[@@config (is_rush_fan = true)]

let favorite_band = "unknown"
[@@config not (is_rush_fan = true)]

Enabling/Disabling Externals

external dog_bark : unit -> int = "dog_bark"
[@@config (includes = "dog")]

Enabling/Disabling Types

type band = { name: string }
[@@config (use_band = true)]

Enabling/Disabling Variant Constructors

type favorite_bands = 
  | Rush
  | Yes
  | KingCrimson [@config (likes_trumpets = true)]

type has_published_album_recently = 
  [ `nope
  | `yes
  | `maybe [@config (is_indecisive = true)]

Enabling/Disabling Record Fields

type user = {
  name: string;
  pass: string [@config (password_mode = "clear")];
  pass: Password.t [@config (password_mode = "encrypted")];

Enabling/Disabling entire Modules

[@@@config (should_include = "no")]

let do_not_include_me = failwith "oh no"


Currently config relies on C preprocessor definitions to detect different platforms and architectures. They are straightforward to add by extending the file ./config/ In there :

  • To add a new platform, extend the main if-else chain in the caml_config_target_os function to include your OS.

  • To add a new architecture, extend the main if-else chain in the caml_config_target_arch function to include your architecture.


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