Vabal - The Cabal Companion

What is it?

Have you ever wanted to try the new bleeding-edge GHC release, but also need older GHCs for your projects or programs (XMonad!)?

Have you ever upgraded GHC just to find all your haskell projects broken?

Have you ever dreamt about treating base as all other packages (i.e. change its version without much thought)?

vabal tries to determine a ghc version that complies with the base package constraints found in the .cabal file. Then it uses ghcup to fetch the compiler (if you don't have it yet) and prints to stdout the options to pass to cabal to use the fetched compiler.

By default vabal tries to use compilers already available on the system and downloads them only when it can't do otherwise (or fails if you use the --no-install flag).

This program tries to solve these issues in the easiest possible way. The GHC compiler downloading is managed by ghcup. No need to manually manage different ghc versions by hand!

vabal tries to be as little intrusive as possible, leverages cabal's capabilities of working with different GHC versions, and does not force you in its paradigm. The only change to the global system state vabal does is to tell ghcup to download a compiler, this behavior can be disabled with --no-install flag, so you are always in charge of what's happening!

Requirements

In order to install vabal you need:

• ghc >= 8.4.1,

You can install vabal directly from Hackage by running:

$ cabal v2-install vabal

(Or cabal new-install depending on the version of your cabal).

Or you can install it from sources, by running:

$ git clone https://github.com/Franciman/vabal.git

$ cd vabal

$ cabal v2-install exe:vabal

(Or cabal new-install, depending on the version of your cabal)

Remember to put $HOME/.cabal/bin in your PATH.

vabal needs to run on a POSIX compliant system, because it uses xargs and command utilities, furthermore, these programs are required to be in PATH:

• ghcup,
• cabal >= 2.4.0.0

cabal >= 2.4.0.0 is a requirement of vabal configure.

vabal by itself can be used in combination with older cabal, too. See the How to use vabal: full story paragraph for details.

Quick start

For starters run:

$ vabal update

This will download updated infos about released ghcs and the versions of base they ship, the info is stored here, and will put them in $HOME/.vabal/ghc-metadata.csv. You may want to run this from time to time, when new ghcs get released, so that vabal will know about them.

Then cd into your project directory and run:

$ vabal configure

This will analyze the cabal file in the directory and extract constraints imposed on base, then it will get a compatible version of ghc using ghcup (possibly downloading it) and will finally run cabal v2-configure to configure the project with the obtained version of ghc.

If everything went fine, you now have your project configured to use a ghc compatible with the constraints imposed on base, you can now build your project as you're used to:

$ cabal v2-build

You can also enable and disable flags for your package, like this:

$ vabal configure --flags="flag1 -flagToDisable"

And if you want to pass other flags directly to cabal v2-configure, you can do it after a --, e.g.:

$ vabal configure -- --enable-tests --enable-shared

How to use vabal: full story

vabal tries to leverage the power of composability of shell commands, you can use it in combination with any cabal subcommand. For example, if you don't want vabal to also perform the cabal v2-configure step, but would like to use it in combination with cabal v2-build, you can run (*):

$ vabal --flags="your -flags" | xargs -r cabal v2-build

(If you are on OS X, you don't need to specify the -r option for xargs)

xargs invokes cabal with the arguments and options specified plus options read from stdin. Read the Remark for info about the -r option.

In Haskell jargon, you can see this as partially applying the cabal "function" first, and then providing it the remaining arguments that vabal emitted.

What vabal actually does is analyze the cabal file, obtain a suitable ghc, using ghcup, and then print to stdout options to pass to cabal (already properly escaped to be used with xargs). It follows the Unix philosophy and its power comes from composition with other programs.

In fact, vabal configure is just a shortcut for:

vabal | xargs -r cabal v2-configure

In this mode you can compose vabal with other cabal commands too, also with old-style cabal build, cabal configure, etc..

(*) Remark:

The -r flag you see is only available in the GNU version of xargs. It makes xargs fail if its input is empty, i.e. when vabal fails. This is necessary or cabal will run as well without arguments from vabal. If you use the BSD version of xargs, then this flag is not necessary (and is neither available), because this is the default behavior. Another possibility is to use the -p flag for xargs which is available on all POSIX systems, it will show the command it is about to run and ask the user if he wants to proceed. In this way, if vabal failed, he can stop the execution.

Consult your system's manpage for xargs for further info.

Gotchas

Here are some known gotchas that affect vabal:

• vabal trusts the constraints imposed on base that it finds in the cabal file, therefore it only finds a ghc version that makes it possible to respect the constraints, but it is not guaranteed that the build will be successful. (Generally one should always write correct constraints)

Full list of options

vabal and vabal configure accept these options:

  -g,--with-ghc-version VER
                           Explicitly tell which version of ghc you want to use
                           for the project. (Incompatible with option
                           --with-base-version)
                           
  -b,--with-base-version VER
                           Specify the version of base package you want to use.
                           It is going to be checked against base constraints in
                           the cabal file for validity. (Incompatible with
                           option --with-ghc-version)
                           
  --flags FLAGS            String containing a list of space separated flags to
                           be used to configure the project (You can enable or
                           disable a flag by adding a + or - in front of the
                           flag name. When none is specified, the flag is
                           enabled). Flag assignment determined here is also
                           emitted to stdout as a cabal option
                           (or passed to "cabal v2-configure" in the case of
                           "vabal configure")
                           
  --cabal-file FILE        Explicitly tell which cabal file to use. This option
                           also emitted to stdout as cabal option (or passed to
                           `cabal v2-configure` in the case of "vabal configure")
  
  --no-install             If GHC needs to be downloaded, fail, instead.
  
  --always-newest          Always choose newest GHC possible, don't prefer
                           already installed GHCs

with vabal configure you can also pass arguments directly to cabal, just specify them after --, e.g.

vabal configure -- --cabal-option --enable-tests --haddock-css=PATH

How to uninstall it

If you want to uninstall vabal you can follow the same steps that are necessary to uninstall any executable installed with cabal v2-install (i.e. remove the symlink in "$HOME/.cabal/bin").

Furthermore, you also need to remove the $HOME/.vabal directory containing metadata used by vabal.

Contributing

Pull Requests and suggestions are welcome.

Issues

You can signal issues here

Contributors

Francesco Ariis

Francesco Gazzetta

Francesco Magliocca

Lorenzo Tabacchini

Marco Umbrello