Still wrestling with the retrofitting nix to Haskell project

[cdepillabout]

@Skywalker

However, I’m still left puzzling over how many different approaches I’ve read about in various places and their corresponding different examples, and I’d prefer to be following an approach that is considered modern, stable and likely to persist for some reasonable time in the future. I seem to be reading that flakes are the new hotness too, but looking at a few examples of these, it seems they are even more complex and hence an even steeper learning curve.

There are a couple things to unpack here.

First, there are two widely-used approaches to building Haskell projects completely with Nix:

1. The Haskell infrastructure in Nixpkgs
2. haskell.nix

All of the projects that have come up as suggestions so far are all using the APIs provided by the Haskell infrastructure in Nixpkgs (well, other than haskell.nix). So while it seems like there are a bunch of different approaches, the question really boils down to “Do you want to use the Haskell infrastructure from Nixpkgs, or haskell.nix?”

My suggestion for you would be to use the Haskell infrastructure from Nixpkgs, as it seems like you’re trying to do.

The Haskell infrastructure from Nixpkgs provides a few widely-used functions you may have seen, like haskellPackages.mkDerivation, haskellPackages.callPackage, haskellPackages.callCabal2nix (and similar helper functions like haskellPackages.callHackage), haskellPackages.developPackage and haskellPackages.shellFor. I’ve tried to give an overview of these functions here (but this explanation is assuming some level of familiarity with Nix):

Why are these derivations so different?

Let me try to answer your questions, or at least give you some pointers where you can find more information. It appears that one of your derivations is just a normal Haskell derivation, and the other one is using developPackage. Here’s a short overview of the different ways to produce a derivation with the Haskell stuff in Nixpkgs: Most of the important Haskell-related files are in pkgs/development/haskell-modules. And then pkgs/top-level/haskell-packages.nix sort of pulls everything toge…

There are various projects that either show a certain way of working with these functions (like
GitHub - Gabriella439/haskell-nix: Nix and Haskell in production), or build light abstractions on top of these functions (like https://github.com/srid/haskell-template), or build new functionality on top of these functions (like GitHub - cdepillabout/stacklock2nix: Easily build a Haskell project from a stack.yaml.lock file with Nix).

My recommendation for a beginner would be to go with haskellPackages.developPackage to get started, since it is relatively easy to use. But, it sounds like you’re having trouble with it. Would you be able to send a link to the repo of Haskell and Nix code you’re trying to use? For someone looking to help you, it is generally easiest just to get a link to a repo that can be cloned and locally run nix-build to see what the error is.

@Skywalker

I seem to be reading that flakes are the new hotness too, but looking at a few examples of these, it seems they are even more complex and hence an even steeper learning curve.

In a lot of ways, the choice of whether or not to use flakes is orthogonal to the choice of method for building Haskell. Most approaches for building Haskell projects will work with or without flakes. (That said, there are some libraries/templates that are built around flakes and would be hard to use without flakes, like https://github.com/srid/haskell-template)

My personal advice would be to avoid flakes for now, and only come back to it when you’re a little more comfortable with Nix. (Although there are also people that would give the opposite advice to beginners.)