Nix integration

[fgaz]

Basically just this:

vabal --nix
# runs the command: nix-shell -p cabal2nix --run "cabal2nix --compiler $GHC_VERSION_HERE --shell > shell.nix"

[Franciman]

Hi, with 1c36dce you can achieve this using xargs:

vabal show | xargs -IGHC_VERSION_HERE nix-shell -p cabal2nix --run "cabal2nix --compiler GHC_VERSION_HERE --shell > shell.nix"

Let me know what you think about it.
Note that vabal show, will also download ghc, if the needed version is missing (if you don't provide the --no-install flag). Does it make sense?
Also, Happy new year!

[Franciman]

Ok, I misunderstood the issue. We need to use nix to get ghc, so a new backend powered by nix is needed. Shall work on it.
@fgaz can you provide me a nix script to get the list of cached ghcs ?
This is not a necessary feature, but may be helpful for people who want to avoid installing other ghcs (and thus recompiling dependencies) when a cached one could do the job as well.