Using cabal with multiple GHC versions

Question

I got both ghc6 and ghc7 on my desktop. To install new packages (for the specific ghc version), I use cabal with the flag --with-compiler=<

Answer 1

This answer serves to complement the other answers (that are already enlightening).

First of all, know that there are significant differences between minor versions of GHC. For example, the change from GHC 7.8 to GHC 7.10 (cf. burning bridges proposal). So, it might be better to name your GHC binaries also including minor version numbers, like: ghc7.8 and ghc7.10.

Supposing you have several GHCs installed with the following names:

/usr/bin/ghc
/usr/bin/ghc-pkg
/usr/bin/haddock
...
/usr/bin/ghc-7.8
/usr/bin/ghc-pkg-7.8
/usr/bin/haddock-ghc-7.8
...
/usr/bin/ghc-7.6
/usr/bin/ghc-pkg-7.6
/usr/bin/haddock-ghc-7.6
...
(and so on)

For GHC 7.8, you can create a file called ~/.cabal-ghc-7.8/config with the following contents (that point to the locations described above):

remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive
remote-repo-cache: /home//.cabal-ghc-7.8/packages
world-file:        /home//.cabal-ghc-7.8/world
compiler: ghc
extra-prog-path:   /home//.cabal-ghc-7.8/bin
build-summary:     /home//.cabal-ghc-7.8/logs/build.log
remote-build-reporting: anonymous
jobs: $ncpus

install-dirs user
  prefix:          /home//.cabal-ghc-7.8

program-locations
  ghc-location:     /usr/bin/ghc-7.8
  ghc-pkg-location: /usr/bin/ghc-pkg-7.8
  haddock-location: /usr/bin/haddock-ghc-7.8
  hpc-location:     /usr/bin/hpc-ghc-7.8
  hsc2hs-location:  /usr/bin/hsc2hs-ghc-7.8

You can create an executable possibly called cabal-ghc-7.8 in your PATH (it uses the --config-file option described in n.m.'s answer):

#!/bin/bash
exec cabal --config-file=$HOME/.cabal-ghc-7.8/config "$@"

Now, in your cabalized source dir, you can simply run cabal-ghc-7.8 build to build your source files using GHC 7.8. (or cabal-ghc-7.8 test or anything else)

You can repeat the process for all the GHCs you have installed. Of course, you should not worry about the standard named GHC. By default, Cabal searches for a GHC named ghc.

This answer assumes a UNIX/Linux system (like the use of bash), but can be adapted to other systems with small changes.