It looked like mkCacheModule was going to do that for me, but it appears that packages in a cabalProject's modules do not override the packages in the haskell.nix Hackage package set. But what if I wanted to use that version of ghcide as a library dependency in my local haskell.nix project? I can't figure out whether there's any existing way to do that. That works great for ghcide because all I need from that package is the ghcide executable. The version pin is trivial to maintain using niv, just like all of my other pinned Nix dependencies. For example, here's what I do to build ghcide from its GitHub repo: With haskell.nix, I can do this very easily for a top-level package. One nice thing about this approach is that the overridden haskellPackages set can then be put in an overlay and shared by multiple downstream Nix projects, and that it can be built and cached by CI just like any other nixpkgs package. Here's an example with nixpkgs.haskellPackges and Agda, where I wanted some features in the latest version (now 2.6.1) that weren't available on Hackage for nearly 9 months: For example, I often need to override a dependency (temporarily) while it's in development and hasn't yet been pushed out to Hackage. With nixpkgs.haskellPackages, it's really easy to override a particular package(s) in the Haskell package set using "pure Nix" that is, without needing to resort to using Cabal or Stackage tooling.
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