[Nix-dev] Move to GHC 7.8.2

[Peter Simons]

Hi guys,

I would like to get some feedback on how you feel about moving to GHC 7.8.2
as the default compiler of choice?

What are the most significant road blocks right now in your opinion?

Also, I'd like to use the switch as an occasion to remove most of the Haskell
Platform related magic that's currently performed in haskell-defaults.nix,
i.e. I would like to abandon the notion that certain compilers prefer older
versions of HP packages because these versions were "mandated" at the time the
compiler was released. Instead, I'd use the latest version possible of every
package for every compiler.

Here are some ideas in no particular order:

 1) We provide haskellPlatform_X_Y_Z attributes in all-packages.nix that
    install a particular version of HP -- including the corresponding GHC
    --, but the library choices in those packages have no effect on any of
    the haskellPackages_ghcXYZ.foobar packages.

 2) The haskellPackages_ghcXYZ.haskellPlatform attribute becomes a
    collection of packages that corresponds roughly to the set of packages
    that the official HP offers, and you can expect those packages to build
    and interact nicely with each other, but the versions won't necessarily
    conform to any HP standard. For example, that 'haskellPlatform'
    attribute will always contain the *latest version* of Cabal and
    cabal-install, regardless of what the HP standard says. That attribute
    can be thought of as "recommended popular packages in stable versions",
    but it doesn't try to conform to any particular version of the HP. The
    packages from (1) do that. Maybe we shouldn't even call it
    'haskellPlatform'?

 3) I would like to drop a whole lot of obsolete GHC versions. As a rule of
    thumb, I think we should keep the latest version of every major release,
    i.e. 6.10.4, 6.12.3, 7.0.4, 7.2.2, 7.4.2, 7.6.2, and 7.8.2 -- but none
    of the intermediate versions. It's intellectually satisfying to have
    support for GHC 6.12.2 still around, but honestly ... who needs it?

Share your thoughts, please!

Best regards,
Peter