Skip to content
/ pesy Public

Builds simple esy native packages with minimal configuration.


Notifications You must be signed in to change notification settings


Repository files navigation

pesy: Native Reason Project from Json.

Use package.json to automatically configure libraries and executables built with Dune.



  • pesy : Creates a new project in the current directory.
  • esy : Builds the current project (just like every other esy project).
  • esy pesy : Updates your build config from package.json (run this any time you change package.json).

pesy (Create New Project)

pesy global command creates esy projects instantly inside of any directory.

npm install -g pesy

cd my-project
pesy      # Hit enter to accept default name

This creates:

  • package.json with useful dependencies/compilers.
  • .gitignore and with instructions for new contributors.
  • .circleci continuous integration with cache configured for ultra-fast pull requests.
  • library/, executable/ and test/ directory with starter modules.

esy (Build The Project)

Just like with any esy project, running esy from the project directory will build it and fetch/install any dependencies you might need.


Your project's field is set to pesy, which will run pesy to verify that all your build config is up to date before invoking the Dune build. It will let you know if you need to run esy pesy to update your build config from new changes to package.json.

esy pesy: (Update Build Config Based On package.json)

esy pesy

If you change your buildDirs config in package.json, run this command to update build configuration based on your latest package.json. If you forget to run this command and try to build (by running esy) without first running esy pesy, the build will remind you.


Configure your package.json's buildDirs field for multiple libraries and executables. buildDirs.DirectoryName means that a library or executable will be located at ./DirectoryName. The field determines the public name of the library or executable. a name ending in .exe is automatically configured as an executable, and a name of the form packageName.anything is automatically configured to be a library with the public name of packageName.anything.

"buildDirs": {
  "MyLibrary": {
    "name": "packageNameMyLibrary",
    "namespace": "MyLibrary",
    "require": ["console.lib"]
  "Tests": {
    "name": "Tests.exe",
    "description": "Runs all the tests natively",
    "flags": ["-linkall"],
    "require": ["console.lib", "packageNameMyLibrary""]

Supported Config

Not all config is supported. This is just a proof of concept. If you'd like to add support for more config fields, PRs are welcomed.


Field Type Description
name string The name of the binary that must end with .exe.
main string The name of the module that serves as the main entrypoint of the binary.
modes list(string) Advanced linking modes. Each string should be of the form "(<compilation-mode> <binary-kind>)" where <compilation-mode> is one byte, native or best and <binary-kind> is one of c, exe, object, shared_object.


Field Type Description
name string The name of the library
modes list("byte"|"native"|"best") Mode which should be built by default. Useful for disabling native compilation for some libraries.
cNames list(string) List of strings to use as C stubs (filenames without the .c extension).
virtualModules list(string) List of modules within the library that will have interfaces but no implementation, causing this library to be considered "virtual". Another library can then claim to "implement" this library by including "implements": "yourLibName". See Virtual Libraries
implements list(string) List of virtual library names that this library implements.
wrapped `true false`

Both Libraries And Binaries

Field Type Description
require list(string) Public library names you want to be able to use.
flags list(string) List of strings to pass to both native and bytecode compilers.
ocamlcFlags list(string) List of flags to pass to ocamlc
ocamloptFlags list(string) List of flags to pass to ocamlopt
jsooFlags list(string) List of flags passed to jsoo
preprocess list(string) List of preprocess options to enable. Primarily used to enable PPX
ignoredSubdirs list(string) Subdirectory names to ignore (This feature is soon to be deprecated).
includeSubdirs "no"|"unqualified" Default is "no", and changing to "unqualified" will compile modules at deeper directories than the place where the dune file is generated. See Dune docs
rawBuildConfig list(string) Raw build config to be injected into the build config for this target.
rawBuildConfigFooter list(string) Raw build config to be injected into the footer of the build config.

Consuming New Package And Library Dependencies:

  • Add dependencies to dependencies in package.json.

  • Add the name of that new dependencies library to package.json's buildDirs section that you want to use the library within. For example, if your project builds a library in the exampleLib/ directory, and you want it to depend on a library named from an opam package named bos, change the package.json to look like this:

      "name": "my-package",
      "dependencies": {
        "@opam/bos": "*"
      "buildDirs": {
        "exampleLib": {
          "namespace": "Examples",
          "name": "my-package.example-lib",
          "require": [ "" ]
  • Then run:

    esy install  # Fetch dependency sources
    esy pesy     # Configure the build based on package.json
    esy build    # Do the build

Note: After adding/building a new dependency you can use esy ls-libs to see which named libraries become available to you by adding the package dependency.


esy-pesy is good for rapidly making new small executables/libraries. Once they grow, you'll want to "eject out" of esy-pesy and begin customizing using a more advanced build system.

Adding pesy to an existing project.

You probably don't need pesy if you have an existing project that is working well, but to add pesy to an existing project, follow these steps:

1. Add a dependency on pesy, and configure buildDirs:

  "name": "my-package",
  "dependencies": {
    "pesy": "*"
  "buildDirs": {
    "exampleLib": {
      "namespace": "Examples",
      "name": "my-package.example-lib",
      "require": [ "" ]
    "bin": {
      "name": "my-package.exe",
      "require": [

2.Install and Build:

esy install
esy pesy  # Generate the project build config from json
esy build

Future Development:

The next major version of pesy is getting even simpler and better, and has undergone a full native rewrite.

Follow the work in its new repo:


version 0.4.3 (6/20/2019) Moved pesy to a devDependency of all newly created projects. Also did the same for refmterr. This causes fewer package conflicts.

version 0.4.2 (6/16/2019)

Make new projects pin to ocaml 4.7.1004 so that it compiles with Reason, since we're still waiting on Reason to work with 4.8.

version 0.4.0 (12/21/2018)

  • Allow buildDirs to contain deeper directories such as "path/to/my-lib": {...}".
  • Added support for wrapped property on libraries.
  • Added support for virtualModules and implements - properties for Dune virtual libraries. (This will only be supported if you mark your project as Dune 1.7 - not yet released).
  • Stopped using ignore_subdirs in new projects, instead using (dirs (:standard \ _esy)) which only works in Dune 1.6.0+, so made new projects have a lower bound of Dune 1.6.0.
  • Support new properties rawBuildConfig which will be inserted at the bottom of the target being configured (library/executable).
    • It expects an array of strings, each string being a separate line in the generated config.
  • Support new properties rawBuildConfigFooter which will be inserted at the bottom of the entire Dune file for the target being configured.
    • It expects an array of strings, each string being a separate line in the generated config.
  • Support new properties modes for binaries and libraries list(string).


Builds simple esy native packages with minimal configuration.







No releases published


No packages published