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Using Unreleased Packages

esy allows to use unreleased versions of packages hosted in their development repositories. This is useful as it allows to try new package versions before they are released.

With esy packages

To use an unreleased version of an esy package specify a dependency resolution in resolutions field alongside the depedency declaration:

"dependencies": {
  "reason": "*"
"resolutions": {
  "reason": "facebook/reason#abcdef"

This will fetch reason package sources from facebook/reason GitHub repository.

Why resolutions?

This is because in case any other package in the project's sandbox depends on reason package then it will probably conflict with facebook/reason declaration (it's most likely others will depend on an already released version of reason instead).

Thus we use resolutions so that constraint solver is forced to use facebook/reason declaration in every place reason package is required.

Other options are:

  • user/repo#<commit> will fetch sources from a GitHub's user/repo repository.

    The <commit> is mandatory and should point to a specific commit (this will be lifted in the future).


    • facebook/reason#7ada18f
  • git://<commit> will fetch sources from a specified git repository.

With opam packages

This corresponds to opam's opam pin workflow.

The same workflow is supported for opam packages.

One can use an unreleased opam packages by specifying its development repository. The only different is that one should also specify an opam package name.


"dependencies": {
  "@opam/lwt": "*",
  "@opam/lwt_ppx": "*"
"resolutions": {
  "@opam/lwt": "ocsigen/lwt:lwt.opam#abc342",

Here we fetch @opam/lwt and @opam/lwt_ppx packages from the single ocsigen/lwt GitHub repository but refer to different opam manifests lwt.opam and lwt_ppx.opam to use corresponding packages.