[Nix-dev] GHC pointed at the wrong package

[Peter Simons]

Hi Mateusz,

 > There are problems in package regex-tdfa-1.2.0:
 >   dependency "parsec-3.1.5-ca5ed8f175b69e1a085cfeaf3b95f424" doesn't exist
 > There are problems in package regex-tdfa-rc-1.1.8.3:
 >   dependency "parsec-3.1.5-ca5ed8f175b69e1a085cfeaf3b95f424" doesn't exist

the process that generates those IDs in GHC is non-deterministic. Two
people can compile the same library with the same version of GHC on the
same type of machine yet end up with two distinct IDs. It doesn't happen
often, but it does happen.

I'd recommend running

 $ nix-store --delete /nix/store/*-haskell-parsec-ghc7.8.3-3.1.5-shared

on all your machines. Then the next build will download these packages
from Hydra, and you'll have a consistent build again. Note that you may
have to remove packages from your active profiles to make that deletion
process succeed.

I hope this helps,
Peter