[Nix-dev] (haskell) a question on depending on specific version of packages

[Corey O'Connor]

Hi,
First, I'm a newcomer to Nix, so if this was covered in the
documentation already I must have missed it.

For haskell packages, such as vty, the dependencies are arguments to
the package module. EG:
{cabal, utf8String, terminfo, deepseq, mtl, parallel, parsec, vectorSpace}:

These, as I understand it, may be set to packages of a specific
version in pkgs/top-level/haskell-packages.nix:
  vty = callPackage ../development/libraries/haskell/vty {
    parallel = parallel_2_2_0_1;
  };

Using parallel 2.2.0.1 is fine for GHC 6.12. However, when using GHC
7, I prefer using vty with parallel 3.1.0.1. Course, depending on the
situation, somebody might want to use vty with parallel 3.1.0.1 under
GHC 6.12. Which leads me to wonder:

1. How to change this dependency for the case where GHC 7 is selected
by the user?
2. How to enable the user to select either parallel 2.2.0.1 or 3.1.0.1
with GHC 6.12?

-Corey O'Connor
coreyoconnor at gmail.com
http://inebriatedrobot.com/


* I don't recall, off the top of my head, if vty will work with
parallel 2.2.0.1 with GHC 7. I suspect not.