[Nix-dev] [haskell NG] Override package

[Daniel Bergey]

On 2015-02-16 at 08:42, YCH <dontdieych at gmail.com> wrote:
> I've read quite many document. Wiki, nix pills, ... . But I'm confused about
> so many different ways doing similar things. And I'm worried about haskell-ng
> specific things. There is already 'haskellngPackages.hdevtools'. So I should
> override using ~/hdevtools. I'll attach my current setup information.

Nix certainly has an overwhelming number of options for similar things.
Here are two options for your situation:

1. Override hdevtools for all your builds, editing
~/.nixpkgs/config.nix, following the explanation in [1].  This is
probably the right thing; I expect you always want the patched
hdevtools.  A complete config.nix:

2. Override hdevtools in your shell.nix, just for this build.  This is
useful when you need a patched version of some Haskell library, rather
than a build tool.  In this case, add to the let definitions (before
pkg)

hdevtools = haskellngPackages.callPackage ~/hdevtools {};

I think you'll need to replace the ~ with your homedir; ISTR Nix doesn't
interpret ~.  Then change your buildTools line to refer to hdevtools,
not haskellngPackages.hdevtools.

cheers,
bergey

Footnotes: 
[1]  http://lists.science.uu.nl/pipermail/nix-dev/2015-January/015601.html