esy-installer

OPAM packages follow a convention of emitting .install files so that package managers can install the built artifacts. opam-installer is one such example. Thus, esy is responsible for reading .install files and installing the built artifacts correctly. esy-installer is an internal command, ie available only while creating esy recipes, that can read .install files and install them in the sandbox.

By default, if a package creates one .install file, esy automatically runs esy-install files internall. However, in some customised setups, eg, reason-native, it's necessary to specify how the built artifacts are installed.

Here's another example.

While overriding @opam/unison with the following,

    "@opam/unison": {
      "version": "opam:2.53.4",
      "override": {
        "buildsInSource": "_build",
        "buildEnv": {
          "HOME": "#{$cur__install}",
          "OSTYPE": "#{os == 'windows' ? 'cygwin': ''}",
          "OSARCH": "#{os == 'windows' ? 'win32': ''}",
          "CC": "#{os == 'windows' ? 'x86_64-w64-mingw32-gcc.exe': 'gcc'}"
        },
        "build": "dune build -p unison",
        "install": "esy-installer ./unison.install",
        "dependencies": {
          "@opam/dune": "*",
          "ocaml": "*"
        }
      }
    }

it was necesary to specify esy-installer ... command since unison package contained multiple .install files after the build.

esy-installer is kind of like dune install command, except that it was written with esy's sandbox in mind.