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Portable Reason/OCaml applications

While the compiler itself is available on a wide range of operating systems and CPU architectures, the packages on opam aren't necessarily so. This could be due to a lot of reasons - often, many packages are just bindings to libraries written in C which may not be written in portability in mind. This noticed especially on Windows.

Some packages are platform-specific - eio_main depends on platform-specific packages (eg, eio-posix, eio-windows etc). Opam files support available field which takes expressions, like os != "win32" for example.

However, this can make things a bit difficult - a given project could result in different lock files on different platforms. Therefore, esy supports computing lock files for multiple platforms and persisting them (in one lock file).

By default, esy considers the target set of platforms to be

  1. Macos Arm64 and x86_64
  2. Linux x86_64
  3. Windows x86_64

This can be configured in package.json/esy.json.

Possible values for os: darwin | linux | cygwin | unix | windows Possible values for cpu: x86_32 | x86_64 | ppc32 | ppc64 | arm32 | arm64

Syntax:

{
  "esy": { ... }
  "available": Array<[<os>, <cpu>]>
}

Example:

```json
{
  "esy": { ... }
  "available": [["darwin", "arm64"], ["linux", "x86_64"]]
}

Note that, this feature is a way of saying, "this project is expected to build correctly on the available set of platforms. On the other platforms, we don't know. It may build, or it may not.". This is because, platform specific packages are made available in the default solution, and it's the default solution that gets installed on unlisted platforms.