title: "new opam features: "opam build"" authors: "Louis Gesbert" {"mailto:louis.gesbert(à)ocamlpro.com"} date: "2017-03-16" --BODY--
UPDATE: after discussions following this post, this feature was abandoned with the interface presented below. See this post for the details and the new interface!
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.
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 opam files.
- 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
./_opam/) - 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 --no-autoinit, --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.
The user can pre-select the repositories to use on the creation of the local switch with:
opam build --repositories <repos>
where <repos> is a comma-separated list of repositories, specified either as
name=URL, or 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 <package-name>.opam.
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.
When using --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.
NOTE: this article is cross-posted on opam.ocaml.org and ocamlpro.com. Please head to the latter for the comments!