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OPAM FAQ

This FAQ is for general questions about OPAM and its usage. You may also be interested in the more advanced Tricks for specific use-cases or advanced users.

🐫 OPAM fails, trying to reinstall already installed packages at first upgrade ?

Ubuntu "utopic" currently ships with a broken OPAM package, that shouldn't happen and didn't in any stable OPAM release. See the bug-report on Ubuntu's launchpad for the details.

The best fix is to install OPAM 1.2 using the community packages.

🐫 What is OPAM for ?

Easily installing, upgrading and managing your OCaml compiler(s), tools and libraries. It's made of a tool, the OPAM package manager, and a community-maintained package repository.

🐫 How to get, install and upgrade OPAM ?

See the install guide.

🐫 Where is the manual ?

OPAM has git-like, hierarchical manpages. Try opam --help for a starting point.

Or get started from the Usage guide.

If you want to know more about OPAM packages, see the Packaging Howto.

Last but not least, the reference on the file formats and more is in the Developper Guide.

🐫 Gasp! opam init gives me screens fulls of errors about upgrading !

This is a glitch, at init only, on the older OPAM 1.1. We recommend upgrading to 1.2 -- but 1.1 is still supported through a compatibility layer on the repository, just use the following initialisation command to workaround the errors:

opam init https://opam.ocaml.org/1.1

If your repository is already initialised, opam update should automatically redirect to 1.1 and you should be fine.

🐫 What changes does OPAM do to my filesystem ?

OPAM is designed to be run strictly as user (non-root), and apart for the explicit options provided during opam init, only writes within ~/.opam (and /tmp). This directory -- the default "OPAM root" -- contains configuration, various internal data, a cache of downloaded archives, and your OCaml installations.

🐫 Why does opam init need to add stuff to my init scripts / why is eval $(opam config env) needed ?

You need two things when you install OPAM packages: to have their libraries accessible, and to access binary programs. To provide those, OPAM needs to setup a few ocaml-related environment variables, and to prepend to your PATH variable.

Of course, you may choose not to let OPAM change anything at opam init, and run eval $(opam config env) yourself whenever you will be needing it.

🐫 What is a "switch" ?

An OCaml installation and a set of installed packages within an OPAM installation. This can be used to keep different OCaml versions side-by-side, or different sets of packages. See the related section in the usage manual and opam switch --help. The "prefix" for a given installation is simply ~/.opam/<switch-name>.

A switch is either based on a system-wide OCaml installation, or on a local installation. In the former case, OPAM will need to recompile all packages when your system compiler changes. In the latter case, OCaml will be downloaded and compiled on creation of the switch.

🐫 Can I work on different switches at the same time in different shells ?

Yes. Use one of:

eval $(opam config env --switch <switch>)        # for the current shell
opam config exec --switch <switch> -- <command>  # for one command

This only affects the environment.

🐫 Can I get a new switch with the same packages installed ?

Yes. Use:

opam switch export file.export  # from the previous switch
opam switch <new switch>
opam switch import file.export

OPAM might fail if you had packages installed that are not compatible with the OCaml version in your new switch. In that case, you'll need to remove them from the file.export file by hand (the format is straight-forward, one line per package).

🐫 I installed a package by hand / used ocamlfind remove / fiddled with the installed packages and OPAM is out of sync. Help !

Don't panic. OPAM assumes it's controlling what's below ~/.opam/<switch>, but there are several ways you can recover:

  • opam remove --force will run the uninstall instructions even if the package is not registered as installed. Then retry opam install.
  • opam reinstall will try to remove an installed package, but go on to re-installing even if that fails.
  • If all else fails, you can re-install your current set of packages from scratch using opam switch reinstall.
  • You can force OPAM to register an installation or removal without actually performing anything using opam install|remove --fake. This is not recommended though, as your manual install may not be exactly equivalent to the one expected by other OPAM packages, and OPAM may later on trigger reinstallations or upgrades of the package. Don't complain if you mess up your installation using this! If you want to control how a package is installed or modify it, the right way is opam pin.
  • You shouldn't have to, but if you want to restart from scratch, just delete ~/.opam and get back to opam init

🐫 What are the minimum requirements ?

1GB of memory should be all you need. It was reported that you may run into problems with 512MB of RAM and no swap. Of course, software packages themselves may be more greedy.

🐫 Some package fail during compilation, complaining about missing dependencies ("m4", "libgtk", etc.)

They probably depend on system, non-OCaml libraries: they need to be installed using your system package manager (apt-get, yum, pacman, homebrew, etc.) since they are outside the scope of OPAM. These external dependencies ("depexts") are in the process of being documented in the package repository, and the opam-depext tool should take care of that for you:

opam install depext
opam depext <packages>

If that doesn't work...

  • Check for hints printed by the failing package
  • Dependencies for your specific system may not be known, but check the output of opam list --rec --required-by <package>,<package>... --external: it will list dependencies on all known systems and may get you in the right direction.
  • Lookup the development packages corresponding to the error in your system's package repositories.

In any of those case, that's useful information that was missing from the OPAM repository: we would really appreciate that you take a minute to save others the trouble of looking by filling an issue in the opam-repository tracker, with your system details, the output of opam depext --flags, and the solution, if you found it. Thanks!

🐫 I have weird checksum errors: where do they come from ?

First of all, you should make sure your repositories are up-to-date:

opam update

If this isn't enough, or if you get the checksum errors while running opam init, this could be caused by a badly configured proxy cache that is serving stale files. To clear your proxy cache, you can use wget --no-cache <remote-file> and retry.

As a last resort, you can bypass the checksum checks using --no-checksums.

🐫 OPAM is prompting me to install or upgrade packages that I am not interested in, or doesn't install the latest version by default. Why ? What can I do ?

  • You can be more explicit in your request (opam upgrade PACKAGES, opam install 'PACKAGE>=VERSION' PACKAGE..., etc.)
  • Action resolution in a package set is known to be a NP-complete problem; OPAM uses state-of-the-art algorithms through an external, dedicated solver: make sure you have an external solver installed
  • Another benefit of the external solvers is that they allow to be quite expressive on your expectations.

🐫 When trying to install a new package, OPAM wants to remove or downgrade packages that I have installed. How to know why ?

There is likely a conflict between them or their dependencies and what you are requesting, here is how to find out. We'll suppose you were trying to install foo and bar got removed:

  • opam install foo bar, if not possible, will tell you why.
  • The above may find a solution by using older version of the packages, in that case try and force the latest versions thusly: opam install foo.2.0 bar.1.1 (you can also use constraints like 'foo>=2.0').
  • Like in previous question, make sure you have aspcud installed, the proposed solutions may not be as accurate without it.

🐫 Where do I report Bugs, Issues and Feature Requests?

The standard way of doing this is to use ocamlfind, which is orthogonal to OPAM: ocamlfind query <lib>.

Your libraries are installed to the directory returned by opam config var lib, which is by default ~/.opam/<switch>/lib. Note that using ocamlc's option -I +dir will make dir relative to lib/ocaml, and will only work for the libraries that ship with the compiler. Also, remember to add the dependency when you package your project !

🐫 How does OPAM tell which version of a package is newer ?

We use the basics of the version ordering from Debian:

  • version strings are sliced into alternate, possibly empty non-digit / digit sequences, always starting with a non-digit sequence.
  • those are sorted lexicographically, using resp. ASCII (with symbol > letter) and number order. For example a gives ["a"], and 1 gives ["";1], so a is latest ("" < "a").
  • the ~ character is special as it sorts even before the end of sequence. It's most convenient for pre-releases, allowing 1.0~beta to be before 1.0.

Here is an example of an ordered sequence: ~~, ~, ~beta2, ~beta10, 0.1, 1.0~beta, 1.0, 1.0-test, 1.0.1, 1.0.10, dev, trunk

🐫 What does the --jobs option do ? It doesn't seem to enable parallel builds.

It does, but at the package granularity: it will only be noticeable if you build independent packages in the same command. Each package has its own build commands, and it's up to them to enable parallelism.

If you are a packager, you may use the jobs opam variable, e.g. make "-j%{jobs}%".

🐫 What is "mixed mode VC pin" ?

When you pin a package to a local directory that is under version control, we used to have OPAM use the version control mechanism to synchronise the package. This turned to be often confusing, because it wouldn't "see" your latest changes until you committed them. Pinning the directory as a raw path isn't perfect either, because it makes OPAM register all files, including temporary files or build artifacts.

The idea of "mixed mode", which is the default for version-control pins in OPAM 1.2.1, is to take the best of both worlds: OPAM will synchronise only files under version control, but at their current state on the filesystem. You may just need to remember to register them if you added new files (e.g. git add).

If for some reason you want the old behaviour, use one of:

  • --kind path for raw filesystem pinning
  • an address of the form vc-controlled-dir#branch, typically #master
  • on Git you can use #HEAD to always get to the currently checked out branch (which used to be the default, and still is for remote gits)

🐫 OPAM wants to do reinstallations after update. Can I skip them ?

OPAM registers the need to recompile packages when they had meaningful changes in the repository, to guarantee consistency and allow to propagate fixes to already installed packages ; the official repository maintainers generally don't abuse this. There is no built-in command to reset them, in purpose, but removing the file ~/.opam/<switch>/reinstall is enough to make OPAM forget about them. It's quite safe, but don't report issues if you did and your system is not up to date, obviously.

🐫 Some package installation fails with "ocamlfind: already installed"

Probably the package was either installed from outside of OPAM, or uncleanly removed. You should try:

opam remove <package> --force
opam install <package>

This will process the uninstall instructions, even if OPAM has no knowledge of the package being installed. You may also try to uninstall directly with ocamlfind, or just remove the problematic files.

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