package opam-0install

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Opam solver using 0install backend


Dune Dependency






Opam's default solver is designed to maintain a set of packages over time, minimising disruption when installing new programs and finding a compromise solution across all packages (e.g. avoiding upgrading some library to prevent uninstalling another program).

In many situations (e.g. a CI system building in a clean environment, a project-local opam root, or a duniverse build) this is not necessary, and we can get a solution much faster by using a different algorithm.

This package does that by using 0install's (pure OCaml) solver with opam packages.


Run the opam-0install binary with the packages you want to install:

$ dune exec -- opam-0install utop
[NOTE] Opam library initialised in 0.16 s
base-bigarray.base base-bytes.base base-threads.base base-unix.base camomile.1.0.2 charInfo_width.1.1.0 conf-m4.1 cppo.1.6.6 dune.2.1.3 dune-configurator.2.1.3 dune-private-libs.2.1.3 lambda-term.2.0.3 lwt.5.1.1 lwt_log.1.1.1 lwt_react.1.1.3 mmap.1.1.0 ocaml.4.09.0 ocaml-base-compiler.4.09.0 ocaml-config.1 ocamlbuild.0.14.0 ocamlfind.1.8.1 ocplib-endian.1.0 react.1.2.1 result.1.4 seq.base topkg.1.0.1 utop.2.4.3 zed.2.0.4
[NOTE] Solve took 0.25 s

Note: the first run may be slow, as the opam library it uses may decide to rebuild its index first.

opam-0install outputs the set of packages that should be installed (but doesn't install them itself). The output is in a format suitable for use as input to opam. e.g.

opam install $(opam-0install utop)

Note that it does not look at the current switch's OCaml version and may therefore choose a newer (or older) one. You can pass the version explicitly to constrain it. e.g.

opam-0install utop ocaml.4.08.1


opam-0install utop 'ocaml<4.09'

You can also pass other packages and constraints here too, as with opam itself. opam-0install will optimise the packages in order, so opam-0install foo bar will always pick the newest possible version of foo, even if that means choosing an older version of bar (but it will choose an older version of foo if there is no other way to get bar at all).


Running make test will run various tests (some fixed and some random) using both opam-0install and opam's solver and compare the results.

When testing changes to the code, you may want to do:

dune exec -- ./test/dump.exe --jobs=NN baseline.csv
[ make changes ]
dune exec -- ./test/dump.exe --jobs=NN new.csv

dump.exe takes each package name in opam-repository and solves for it individually, generating a CSV file with the solutions. NN is the number of processes to use to speed it up; use the number of cores your machine has. Depending on the speed of your computer, this is likely to take several minutes.

You may want to use --root to use a separate opam root directory, e.g.

opam init --no-setup --root ./opam-root /path/to/opam-repository

This means you can upgrade your default opam root without changing the test results.

To compare the results, use:

dune exec -- ./test/diff.exe baseline.csv new.csv


The library provides these sub-modules under Opam_0install:

  • Solver is used to create a solver, given an S.CONTEXT (source of opam packages).

  • Switch_context gets packages from the user's opam switch, including any pinned packages.

  • Dir_context reads the packages directly from a checkout of opam-repository.

  • Model is used internally by Solver. It provides the interface needed by 0install.


let env =

let context =
  Opam_0install.Dir_context.create "/tmp/opam-repository/packages"

module Solver = Opam_0install.Solver.Make(Opam_0install.Dir_context)

let () =
  let result = Solver.solve context [OpamPackage.Name.of_string "utop"] in
  match result with
  | Error e -> print_endline (Solver.diagnostics e)
  | Ok selections ->
    Solver.packages_of_result selections
    |> List.iter (fun pkg -> Printf.printf "- %s\n" (OpamPackage.to_string pkg))


The core 0install solver does not depend on the rest of 0install and just provides a functor that can be instantiated with whatever package system you like (see Simplifying the Solver With Functors). Zeroinstall_solver.S describes the interface required by the 0install-solver package.

opam-0install provides an implementation of this interface using opam package metadata. It's a little complicated because 0install doesn't support alternatives in dependencies (e.g. ocaml-config depends on "ocaml-base-compiler" | "ocaml-variants" | "ocaml-system"). The mapping introduces a "virtual" package in these cases (so ocaml-config depends on a virtual package that has three available versions, with dependencies on the real packages).

A virtual package is also created if you specify multiple packages on the command-line.


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