[Nix-dev] Use GHC 7.10.1 as default Haskell compiler in nixpkgs

[Peter Simons]

Hi guys,

I believe that we should switch our default compiler to GHC 7.10.1
within the next three or four weeks -- assuming that no major bug is
discovered in that compiler. It is true that this update will break some
packages, but GHC versions prior to 7.10.x are a disaster for
distributions like us because they generate non-deterministic library
IDs. This means that two builds running in identical environments can
produce outputs that differ and that break binary compatibility when
linked by other libraries. This is a nightmare for systems like Hydra,
and I've just had to rebuild all our Haskell packages [1] for the
umpteenth time because of that bug. GHC 7.10.1 is supposed to fix that
issue, so I'd say let's go for it.

Those of you who don't want to update can stick to GHC 7.8.4, of course,
it's just that Hydra will no longer provide binaries for that version
after the switch.

Also, I'd like to remove all code related to the old Haskell package set
from Nixpkgs shortly after the switch, to make sure that obsolete code
won't make it into the next release branch. If you haven't migrated to
Haskell NG yet ... now would be a good time to do it. (I'm looking at
you, Agda.)

How do you guys feels about that approach? Any concerns? Other
suggestions?

Best regards,
Peter


[1] https://github.com/NixOS/nixpkgs/commit/945269a48fa8b91024e18a72b485797eeca308ee