The new opam 2.0 release, currently in beta, introduces several new features. This post gets into some detail on the new
opam build command, its purpose, its use, and some implementation aspects.
opam build is run from the source tree of a project, and does not rely on a pre-existing opam installation. As such, it adds a new option besides the existing workflows based on managing shared OCaml installations in the form of switches.
What does it do ?
Typically, this is used in a fresh git clone of some OCaml project. Like when pinning the package, opam will find and leverage package definitions found in the source, in the form of
- if opam hasn’t been initialised (no
~/.opam), this is taken care of.
- if no switch is otherwise explicitely selected, a local switch is used, and
created if necessary (i.e. in
- the metadata for the current project is registered, and the package installed
after its dependencies, as opam usually does
This is particularly useful for distributing projects to people not used to opam and the OCaml ecosystem: the setup steps are automatically taken care of, and a single
opam build invocation can take care of resolving the dependency chains for your package.
If building the project directly is preferred, adding
--deps-only is a good way to get the dependencies ready for the project:
$ opam build –deps-only
$ eval $(opam config env)
$ ./configure; make; etc.
Note that if you just want to handle project-local opam files,
opam build can also be used in your existing switches: just specify
--switch or make sure the
OPAMSWITCH variable is set. E.g.
opam build --no-autoinit --deps-only is a convenient way to get the dependencies for the local project ready in your current switch.
The installation of the packages happens as usual to the prefix corresponding to the switch used (
<project-root>/_opam/ for a local switch). But it is possible, with
--install-prefix, to further install the package to the system:
opam build –install-prefix ~/local
will install the results of the package found in the current directory below ~/local.
The dependencies of the package won’t be installed, so this is intended for programs, assuming they are relocatable, and not for libraries.
Choosing custom repositories
The user can pre-select the repositories to use on the creation of the local switch with:
opam build –repositories <repos>
<repos> is a comma-separated list of repositories, specified either as
name if already configured on the system.
Multiple packages are commonly found to share a single repository. In this case,
opam build registers and builds all of them, respecting cross-dependencies. The opam files to use can also be explicitely selected on the command-line. In this case, specific opam files must be named
The choice of the compiler, on automatic initialisation, is either explicit, using the
--compiler option, or automatic. In the latter case, the default selection is used (see
opam init --help, section “CONFIGURATION FILE” for details), but a compiler compatible with the local packages found is searched from that. This allows, for example, to choose a system compiler when available and compatible, avoiding a recompilation of OCaml.
--install-prefix, the normal installation is done, then the tracking of package-installed files, introduced in opam 2.0, is used to extract the installed files from the switch and copy them to the prefix.
The packages installed through
opam build are not registered in any repository, and this is not an implicit use of
opam pin: the rationale is that packages installed this way will also be updated by repeating
opam build. This means that when using other commands, e.g.
opam upgrade, opam won’t try to keep the packages to their local, source version, and will either revert them to their repository definition, or remove them, if they need recompilation.
This is still in beta: there are still rough edges, please experiment and give feedback! It is still possible that the command syntax and semantics change significantly before release.
Another use-case that we are striving to improve is sharing of development setups (share sets of pinned packages, depend on specific remotes or git hashes, etc.). We have many ideas to improve on this, but
opam build is not, as of today, a direct solution to this. In particular, installing this way still relies on the default opam repository; a way to define specific options for the switch that is implicitely created on
opam build is in the works.