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title: "opam 2.0 preview release!" authors: "Louis Gesbert" {"mailto:louis.gesbert(à)ocamlpro.com"} date: "2016-09-20" --BODY--

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We are pleased to announce a preview release for opam 2.0, with over 700 patches since 1.2.2. Version 2.0~alpha4 has just been released, and is ready to be more widely tested.

This version brings many new features and changes, the most notable one being that OCaml compiler packages are no longer special entities, and are replaced by standard package definition files. This in turn means that opam users have more flexibility in how switches are managed, including for managing non-OCaml environments such as Coq using the same familiar tools.

A few highlights

This is just a sample, see the full changelog for more:

  • Sandboxed builds: Command wrappers can be configured to, for example, restrict permissions of the build and install processes using Linux namespaces, or run the builds within Docker containers.

  • Compilers as packages: This brings many advantages for opam workflows, such as being able to upgrade the compiler in a given switch, better tooling for local compilers, and the possibility to define coq as a compiler or even use opam as a generic shell scripting engine with dependency tracking.

  • Local switches: Create switches within your projects for easier management. Simply run opam switch create <directory> <compiler> to get started.

  • Inplace build: Use opam to build directly from your source directory. Ensure the package is pinned locally then run opam install --inplace-build.

  • Automatic file tracking:: opam now tracks the files installed by packages and is able to cleanly remove them when no existing files were modified. The remove: field is now optional as a result.

  • Configuration file: This can be used to direct choices at opam init automatically (e.g. specific repositories, wrappers, variables, fetch commands, or the external solver). This can be used to override all of opam's OCaml-related settings.

  • Simpler library: the OCaml API is completely rewritten and should make it much easier to write external tools and plugins. Existing tools will need to be ported.

  • Better error mitigation: Through clever ordering of the shell actions and separation of build and install, most build failures can keep your current installation intact, not resulting in removed packages anymore.

Roll out

You are very welcome to try out the alpha, and report any issues. The repository at opam.ocaml.org will remain in 1.2 format (with a 2.0 mirror at opam.ocaml.org/2.0~dev in sync) until after the release is out, which means the extensions can not be used there yet, but you are welcome to test on local or custom repositories, or package pinnings. The reverse translation (2.0 to 1.2) is planned, to keep supporting 1.2 installations after that date.

The documentation for the new version is available at http://opam.ocaml.org/doc/2.0/. This is still work in progress, so please do ask if anything is unclear.

Interface changes

Commands opam switch and opam list have been rehauled for more consistency and flexibility: the former won't implicitly create new switches unless called with the create subcommand, and opam list now allows to combine filters and finely specify the output format. They may not be fully backwards compatible, so please check your scripts.

Most other commands have also seen fixes or improvements. For example, opam doesn't forget about your set of installed packages on the first error, and the new opam install --restore can be used to reinstall your selection after a failed upgrade.

Repository changes

While users of opam 1.2 should feel at home with the changes, the 2.0 repository and package formats are not compatible. Indeed, the move of the compilers to standard packages implies some conversions, and updates to the relationships between packages and their compiler. For example, package constraints like

available: [ ocaml-version >= "4.02" ]

are now written as normal package dependencies:

depends: [ "ocaml" {>= "4.02"} ]

To make the transition easier,

  • upgrade of a custom repository is simply a matter of running opam-admin upgrade-format at its root;
  • the official repository at opam.ocaml.org already has a 2.0 mirror, to which you will be automatically redirected;
  • packages definition are automatically converted when you pin a package.

Note that the ocaml package on the official repository is actually a wrapper that depends on one of ocaml-base-compiler, ocaml-system or ocaml-variants, which contain the different flavours of the actual compiler. It is expected that it may only get picked up when requested by package dependencies.

Package format changes

The opam package definition format is very similar to before, but there are quite a few extensions and some changes:

  • it is now mandatory to separate the build: and install: steps (this allows tracking of installed files, better error recovery, and some optional security features);
  • the url and description can now optionally be included in the opam file using the section url {} and fields synopsis: and description:;
  • it is now possible to have dependencies toggled by globally-defined opam variables (e.g. for a dependency needed on some OS only), or even rely on the package information (e.g. have a dependency at the same version);
  • the new setenv: field allows packages to export updates to environment variables;
  • custom fields x-foo: can be used for extensions and external tools;
  • allow """ delimiters around unescaped strings
  • & is now parsed with higher priority than |
  • field ocaml-version: can no longer be used
  • the remove: field should not be used anymore for simple cases (just removing files)

Let's go then -- how to try it ?

First, be aware that you'll be prompted to update your ~/.opam to 2.0 format before anything else, so if you value it, make a backup. Or just export OPAMROOT to test the alpha on a temporary opam root.

Packages for opam 2.0 are already in the opam repository, so if you have a working opam installation of opam (at least 1.2.1), you can bootstrap as easily as:

opam install opam-devel

This doesn't install the new opam to your PATH within the current opam root for obvious reasons, so you can manually install it as e.g. "opam2" using:

sudo cp $(opam config var "opam-devel:lib")/opam /usr/local/bin/opam2

You can otherwise install as usual:

  • Using pre-built binaries (available for OSX and Linux x86, x86_64, armhf) and our install script:

    wget https://raw.github.com/ocaml/opam/2.0-alpha4-devel/shell/opam_installer.sh -O - | sh -s /usr/local/bin

    Equivalently, pick your version and download it to your PATH;

  • Building from our inclusive source tarball: download here and build using ./configure && make lib-ext && make && make install if you have OCaml >= 4.01 already available, make cold && make install otherwise;

  • Or from source, following the included instructions from the README. Some files have been moved around, so if your build fails after you updated an existing git clone, try to clean it up (git clean -fdx).