Just recently I made a transition, I was thinking of for quite some time. What I’m speaking of is my transition to NixOS.
Two days ago I just installed NixOS on my laptop out of the blue. Now I’m trying to get my programming projects to build and work again. On may way to accomplish this, I learned a lot about the system and its configuration.
Overall I must say, that I am quite pleased with the outcome. It is a fairly usable system, which holds some cool advantages for me as a Haskell developer over my old Debian system setup. Te biggest one is, That I can scrap all cabal sandboxes from all my projects and hand the management of the right dependency versions over to Nix.
What I want to share is a small snippet of a nix expression, that lets you
include local dependencies to your Haskell project’s nix shell (like you would
do with cabal sandbox add-source.
First you need to create a nix shell configuration. There is a handy tool called
cabal2nix which you can install simply by invoking
nix-env -iA nixos.cabal2nix. This is the main work horse for converting any
Haskell package to nix and it does its job very well.
To build your Nix shell configuration you call cabal2nix --shell . > shell.nix
from your project directory. This generates the file shell.nix for you. Check
it for any errors. If you want to use a local package, you can add its
definition to the shell.nix file. To get the definition of that package, you
can again use cabal2nix. Simply invoke cabal2nix file://<path to Package> and
copy the resulting block into the let block of your existing shell.nix
like this:
<packageName> = with haskellPackages; callPackage (
<packageDefinition>
) {};Where <packageName> is the variable name for the package inside this Nix
expression. <packagDefinition> is the package definition of the local package
we just obtained from cabal2nix. Then delete the original package name form
the arguments of the f function in the let block of the Nix expression.
Finally align everything to look nicely, and save the file. To see an example of
that, you can look at the
Nix expression of my latest game project.
To test your setup, just invoke nix-shell shell.nix and watch the system build
a temporary environment for your project and give you a shell to it, if
everything went well. Now all dependencies of your project, including your local
ones are installed in this environment, so you only need to invoke
cabal build.
I hope You find this snippet of my Nix experience helpful, it sure took me some time to find out.
