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hsenv - Virtual Haskell Environment

What is it?

hsenv is a tool (inspired by Python's virtualenv) to create isolated Haskell environments.

What does it do?

It creates a sandboxed environment in a .hsenv/ subdirectory of your project, which, when activated, allows you to use regular Haskell tools (ghc, ghci, ghc-pkg, cabal) to manage your Haskell code and environment. It's possible to create an environment, that uses a different GHC version than your currently installed system GHC version. Very simple emacs integration mode is included.

Basic usage

First, choose a directory where you want to keep your sandboxed Haskell environment, usually a good choice is a directory containing your cabalized project (if you want to work on a few projects (perhaps an app and its dependent library), just choose any of them, it doesn't really matter). Enter that directory:

cd ~/projects/foo

Next, create your new isolated Haskell environment (this is a one-time-only (per environment) step):

hsenv

Now, every time you want to use this environment, you have to activate it:

source .hsenv/bin/activate

That's it! Now it's possible to use all regular Haskell tools like usual, but it won't affect your global/system Haskell environment, and also your per-user environment (from ~/.cabal and ~/.ghc) will stay the same. All cabal-installed packages will be private to this environment, and the external environments (global and user) will not affect it (this environment will only inherit very basic packages, mostly ghc and Cabal and their deps).

When you're done working with this environment, enter command deactivate_hsenv, or just close the current shell (with exit).

deactivate_hsenv

Named vs Unnamed Environments

By default, hsenv creates an "unnamed" environment, but sometimes for particular use cases you might want to create different environments under the same directory. This is achievable creating a "named" environment. To do that, just pass the flag --name=<ENVIRONMENT_NAME> to hsenv:

hsenv --name=<ENVIRONMENT_NAME>

This will make hsenv generate a folder of the form .hsenv_<ENVIRONMENT_NAME>.

Customization

If you want to customize activation and deactivation, create one or more of the following files in ~/.hsenv/bin/: pre-activate, post-activate, pre-deactivate, post-deactivate. These shell scripts will be sourced automatically by the main activation script.

Advanced usage

Here's the most advanced usage of hsenv. Let's say you want to:

• Hack on a json library
• Do so comfortably
• Use your own version of the parsec library
• And do all this using the nightly version of GHC

First, download the binary distribution of GHC for your platform (e.g. ghc-7.3.20111105-i386-unknown-linux.tar.bz2).

Create a directory for you environment:

mkdir /tmp/test
cd /tmp/test

Then, create a new environment using that GHC:

hsenv --name=test --ghc=/path/to/ghc-7.3.20111105-i386-unknown-linux.tar.bz2

Activate it:

source .hsenv_test/bin/activate

Download a copy of json library and your private version of parsec:

darcs get http://patch-tag.com/r/Paczesiowa/parsec
cabal unpack json

Install parsec:

cd parsec2
cabal install

Install the rest of the json deps:

cd ../json-0.5
cabal install --only-dependencies

Now, let's say you want to hack on Parsec module of json library. Open it in emacs:

emacsclient Text/JSON/Parsec.hs

Activate the virtual environment (hsenv must be required earlier):

M-x hsenv-activate <RET> /tmp/test/ <RET>

Edit some code and load it in ghci using 'C-c C-l'. If it type checks, you can play around with the code using nightly version of ghci running in your virtual environment. When you're happy with the code, exit emacs and install your edited json library:

cabal install

And that's it.

Fetching and downloading a remote version of GHC

Recent versions of hsenv include the possibility to automatically download and install a GHC version directly from the GHC main repository. To do that, all you need to do is to pass the desired version of GHC you want to install:

hsenv --ghc=7.4.1

Or a valid URL pointing to a compressed GHC archive:

hsenv --ghc=http://www.haskell.org/ghc/dist/7.6.3/ghc-7.6.3-x86_64-apple-darwin.tar.bz2

Misc

hsenv has been tested on Linux, Mac OS X, and FreeBSD systems, but it should work on any POSIX platform. The external (from tarball) GHC feature requires a binary GHC distribution compiled for your platform which that can be extracted with tar and installed with "./configure --prefix=PATH; make install".

FAQ

Q: Can I use it together with tools like cabal-dev or capri?

A: No. All these tools work more or less the same (wrapping cabal command, setting GHC_PACKAGE_PATH env variable), so something will probably break.

Q: Using GHC from tarball fails with a bunch of make tool gibberish on FreeBSD. What do I do?

A: Try the '--make-cmd=gmake' switch.

Q: Can I use hsenv inside hsenv?

A: No. It may be supported in future versions.

Q: Does it work on x64 systems?

A: Yes.

Q: Will it work on Mac?

A: Yes.

Q: Will it work on Windows?

A: I really doubt it would even compile. I don't have access to any Windows machines, so you're on your own, but patches/ideas/questions are welcome. Maybe it would work on Cygwin.

Q: Does it require Bash?

A: No, it should work with any POSIX-compliant shell. It's been tested with bash, bash --posix, dash, zsh and ksh.

Q: Can I use it with a different haskell package repository than hackage?

A: Yes, just adjust the url in .hsenv/cabal/config file.

Q: How do I remove the whole virtual environment?

A: If it's activated - 'deactivate_hsenv' it. Then, delete the .hsenv/ directory.

Q: Is every environment completely separate from other environments and the system environment?

A: Yes. The only (minor) exception is ghci history - there's only one per user history file. Also, if you alter your system's GHC, then virtual environments using system's GHC copy will probably break. Virtual environments using GHC from a tarball should continue to work.

Q: Can I share one cabalized project directory among multiple environments (e.g. build a cabalized project in the same directory using different environments)?

A: Yes. hsenv also overrides cabal with a wrapper, that will force using different build directories, so there shouldn't be even any recompilation between different environments.

Q: Is it possible to activate an environment upon entering its directory?

A: Yes, if you really know what you're doing. Here's a snippet for bash, which will activate both named and unnamed environments:

    function precmd() {
        if [[ -z $HSENV ]]; then
            NUMBER_OF_ENVS=$(find . -maxdepth 1 -type d -name ".hsenv*" | wc -l)
            case ${NUMBER_OF_ENVS} in
                "0") ;;
                "1") source .hsenv*/bin/activate;;
                *) echo multiple environments, manual activaton required;;
            esac
        fi
    }
    export PROMPT_COMMAND=precmd

Q: Can I use hsenv on a machine, that doesn't have cabal binary (from cabal-install package) installed?

A: Yes. Just use the '--bootstrap-cabal' switch. In fact, you can use hsenv on a machine that doesn't have anything Haskell-related installed - just combine '--ghc=' and '--bootstrap-cabal' options.