This script allows you to easily switch between different versions of GHC (the Glasgow Haskell Compiler) that are installed on your system. It's a simple Bash script that manipulates a symlink pointing to a subdirectory with the current preferred version of the GHC binaries.

1. Install the Haskell Platform (HP) for Mac OS X.

   The HP creates a directory /Library/Frameworks/GHC.framework with all the tools needed to use Haskell. In particular, the Versions subdirectory contains the GHC installation. Each future installation of HP adds a new version in a subdirectory of Versions.

   In Versions, there is a symlink Current that points to the subdirectory with the "current" GHC. We use that to determine which GHC we want to use at any one point in time.

2. Update the HP symlinks in /usr/bin to use Current.

   To make the HP tools part of the standard path, the HP installer creates symlinks in /usr/bin that point to the binaries in the current version. By default, they may link to the specific subdirectory in Versions. You should relink them, replacing the specific version with Current.

   You can find the current symlinks with ls -l /usr/bin/ | grep GHC\.framework. For example, I see ghc, ghc-pkg, ghci, hp2ps, hpc, hsc2hs, runghc, and runhaskell, among others. There may also be version-specific ones like ghc-7.4.1 which you may want to delete, since they will not be applicable if you change your current version.

   For each symlink $F from above, recreate it with

    sudo ln -sf /Library/Frameworks/GHC.framework/Versions/Current/usr/bin/$F /usr/bin/$F
   

3. Put ghc-ver in your $PATH.

And you're done!

• Use ghc-ver list to see the versions available.

  It simply looks at the subdirectories in /Library/Frameworks/GHC.framework/Versions and gives you the list of names. Thus, it helps to name the versions appropriately and concisely.

• Use ghc-ver 7.4.1 to select version 7.4.1 if it is available. You will get a notice if it is not, and you will also get a confirmation of the current GHC version, which should be unchanged in the case of failure.

• Install any future versions of the HP as normal. Unless the installer changes, you should just be able to switch to the new version using ghc-ver. (You may need to update the /usr/bin symlinks as described above.)

• Install any non-HP versions of GHC in /Library/Frameworks/GHC.framework/Versions.

  In the case where you are compiling GHC from source, say the version is called HEAD, you can create the directory /Library/Frameworks/GHC.framework/Versions/HEAD and configure GHC as follows:

    ./configure \
    --prefix=/Library/Frameworks/GHC.framework/Versions/HEAD/usr \
    --with-gmp-libraries=/Library/Frameworks/GMP.framework        \
    --with-gmp-includes=/Library/Frameworks/GMP.framework/Headers
  

  Then, sudo make install will do the right thing.

  You may or may not need the extra GMP.framework flags. There have be times in the past where GMP did not work properly when compiling GHC from source.