How do I override a dependency's source? (e.g. to build from a fork)

[cumber]

I'm not clear on how I'm supposed to override a package's source using haskell.nix infrastructure. (e.g. at the moment there are a lot of packages that don't build on 9.2.1 that have pull requests floating around to enable them to build; overriding sources to use these pull requests gets my project building in the meantime). Using straight nixpkgs I would have used an overlay.

I've found if I list a source-repository-package in cabal.project I can override it provided the dependency is only a direct dependency. If it's a dependency of a dependency then haskell.nix doesn't provide the source-repository-package version when building the dependency; I can add source-repository-pacage stanzas all the way down the dependency chain, of course, but that's extremely tedious if it's more than a few levels deep.

Is this something I can do with the modules or pkg-def-extras arguments to cabalProject'?

Would be great to have an example in the docs.

[smunix]

Agreed, it would be great to be able to override package dependencies right from project'.

[angerman]

Is this something I can do with the modules or pkg-def-extras arguments to cabalProject'?

yes. You can override the src attribute on packages in a module. E.g.

{ 
   # default is:
   # pkgs.fetchurl {
   #    url = "mirror://hackage/${config.name}.tar.gz";
   #    inherit (config) sha256;
   # };
   package.xyz.src = ... 
}

see: https://input-output-hk.github.io/haskell.nix/reference/modules/#packagessrc

However this will only work if you are modifying the source, for changes to the cabal file, this is a bit trickier, as that would need to go into the hackage.nix index. We don't have support for hackage overlays yet, which would be really nice to have in this case to have a 9.2 compat hackage overlay (as an example head.hackage is a hackage overlay to facilitate working with ghc HEAD).

[cumber]

Aha, good to know. That's still something.

The cabal file is usually part of that tarball, so if I update the src location it could well have cabal file differences. Do I understand right that the problem is none of these differences will be used by haskell.nix's build plan, because it's decided on that by the time it's downloaded the tarball? So e.g. relaxing dependency bounds won't help, right? Can I also work around that with further config? allow-newer in cabal.project, perhaps?

[ilyakooo0]

I made a thing that seems work in all cases (even with cabal changes): https://github.com/ilyakooo0/haskell-nix-extra-hackage

[stale]

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