[Nix-dev] Haskell NG: Local packages

[Ertugrul Söylemez]

Hello fellow Haskellers,

I have a bunch of local Haskell packages, mostly libraries.  The package
X lives in the directory `<ertes-src/X/main>`.  There are lots of direct
and indirect dependencies between them.  My current solution is ad hoc
and rather ugly.  I'm using the following hand-crafted `shell.nix` file
in every project:

    { pkgs ? import <nixpkgs> {} }:

    let inherit (pkgs.haskellngPackages) callPackage;

        extPkg = path: deps:
            (callPackage (import path) deps).override (args: args // {
                mkDerivation = expr:
                    args.mkDerivation (expr // {
                        src = pkgs.fetchdarcs { url = path; };
                    });
            });

        thisPkg = callPackage (import ./devel.nix) {
            direct-dep1 = extPkg <ertes-src/dep1/main> {
                indirect-dep1 = ...;
                indirect-dep2 = ...;
            };
            direct-dep2 = extPkg <ertes-src/dep2/main> {};
        };

    in thisPkg.env

Do not pay too much attention to the `extPkg` function.  It just works
around cabal2nix' lack of support for Darcs URLs.

My question is:  Is there a nicer and more principled way to solve this?
Ideally I could simply add all of my local packages in some way to my
`~/.nixpkgs/config.nix`.  Then they could just depend on each other
regularly and I wouldn't need any shell.nix hackery.  I'm guessing that
I need to override haskellngPackages in some way or perhaps create my
own using some override.


Greets,
Ertugrul
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