[Nix-dev] Trying to nix-shell a Haskell package tries to build its older version.

[Peter Simons]

Hi Mateusz,

 > What's happening is that HTTP_4000_2_12 is defined to be used in
 > haskellPlatformArgs_future and ghcHEAD block inherits that without
 > overwriting.

right, when Haskell Platform packages are updated, we also have to
update "haskellPlatformArgs_future" to make ghc-head see the new
versions. The overrides in haskell-defaults.nix are typically used only
if we deliberately break Haskell Platform compliance for some reason,
which isn't the case for ghc-head, because there's no Haskell Platform
for it to begin with.

The whole situation is a bit messy. I'm working on cleaning it up, but
it's a slow process.

Take care,
Peter