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Haskell in Nixpkgs/NixOS

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For discussions and questions about Haskell with Nix, cabal2nix and haskellPackages in nixpkgs | Current Docs: https://haskell4nix.readthedocs.io/ | More Nix: #community:nixos.org | More Haskell: #haskell-space:matrix.org141 Servers

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6 Jul 2025
@alex:tunstall.xyzAlex
In reply to @alexfmpe:matrix.org
might need withHoogle = true, don't remember if default in shellFor
Not default, unless it has changed recently.
00:10:58
@alexfmpe:matrix.orgalexfmpeindeed00:12:02
@alexfmpe:matrix.orgalexfmpehttps://github.com/NixOS/nixpkgs/blob/a697a033bd07502703b151797085e5c7b64c6f31/pkgs/development/haskell-modules/make-package-set.nix#L45300:12:03
@alex:tunstall.xyzAlex
In reply to @collinarnett:matrix.org
Does the hoogle server serve the haddocs as well? I'm not really sure what I'm looking for so I would like to browse the docs like I would on hackage.
Note that how Hoogle serves Haddocks is affected by the --local flag.
(For most users, running with --local works better.)
See https://github.com/ndmitchell/hoogle/blob/e83646ee1b62eb724def6c838a34017b60fc519a/docs/Install.md#server
00:14:08
@woobilicious:matrix.orgwoobiliciousIs there anyway to "export" a build plan for haskell apps based on the nixpkgs version? trying to think how to compile my code for windows, and cabal obvious isn't going to pull in the exact same versions without some hints. I guess I could look at cross compiling using mingw64 version of ghc in nixpkgs, but that might just be a huge headache.05:56:37
@alex:tunstall.xyzAlex
In reply to @woobilicious:matrix.org
Is there anyway to "export" a build plan for haskell apps based on the nixpkgs version? trying to think how to compile my code for windows, and cabal obvious isn't going to pull in the exact same versions without some hints. I guess I could look at cross compiling using mingw64 version of ghc in nixpkgs, but that might just be a huge headache.
Have you tried cabal freeze?
06:23:48
@woobilicious:matrix.orgwoobiliciousoh thanks, I didn't realise that existed...kinda half forgot cabal added some modern features, as I used to use stack on windows and nixpkgs now.06:44:58
8 Jul 2025
@dmjio:matrix.org@dmjio:matrix.org

is there a function to parse a flake lockfile into a set?

let sources = parseFlakeLock (builtins.readFile ./flake.lock);

something like this

16:07:35
@ners:nixos.devnersYes, in fact flake-compat is based on this principle. See examples here: https://github.com/nix-community/flake-compat16:13:58
@ners:nixos.devners *

Yes, in fact flake-compat is based on this principle. See examples here: https://github.com/nix-community/flake-compat

Basically: builtins.fromJSON (builtins.readFile ./flake.lock);

16:14:48
@dmjio:matrix.org@dmjio:matrix.org

so I'm just doing this rn

  # fetch from flake.lock
  with (builtins.fromJSON (builtins.readFile ../flake.lock));
  fetchFromFlakeLock = args:
    fetchFromGitHub {
      inherit (args.locked) owner repo rev;
      hash = args.locked.narHash;
    };
  servant = fetchFromFlake (nodes.servant);
19:50:58
@dmjio:matrix.org@dmjio:matrix.org does fetchFromFlakeLock already exist somewhere 19:51:18
@dmjio:matrix.org@dmjio:matrix.orgokay will check it out, we'll see19:54:50
@dmjio:matrix.org@dmjio:matrix.org Also, is there a way to generate a flake.nix on the fly, similar to how callCabal2nix works 19:55:07
@dmjio:matrix.org@dmjio:matrix.orgnot all my repos have flakes and I'd have to add it to each one basically19:55:29
@dmjio:matrix.org@dmjio:matrix.org but I'm going to just override the nixpkgs with nixpkgs.follows anyways 19:55:39
@dmjio:matrix.org@dmjio:matrix.orgotherwise the flakes will be mutually recursive20:02:12
@dmjio:matrix.org@dmjio:matrix.org* otherwise the flakes will be mutually recursive (some of them)20:02:23
@dmjio:matrix.org@dmjio:matrix.org Also 21:19:51
@dmjio:matrix.org@dmjio:matrix.org Who is the CEO of Flakeroku 21:19:57
@dmjio:matrix.org@dmjio:matrix.org What is the flake heroku equivalent 21:20:15
@sellout:matrix.orgselloutI’m having a hell of a time caching this one project, I think it must be related to Haskell / IFD. I follow the Cachix instructions for caching build & runtime deps (https://docs.cachix.org/pushing#pushing-build-and-runtime-dependencies), I do that on all three supported systems, and then I expect in this CI run (https://github.com/sellout/unison/actions/runs/16144540250), that it won’t have to build anything. But that’s never what happens. You can see the two macOS runs are 2-3 hours, and the linux one gets “canceled” for some reason I can’t deduce (but not before trying to build a ton of stuff). It’s on this branch (https://github.com/sellout/unison/tree/update-nix), but I feel like I run into this every time I update Nix at all. When I run it on a different machine locally, I just see “evaluating derivation '…#all'” for ages before anything else happens, which is what makes me think it’s IFD.22:58:47
@vigress9:matrix.org@vigress9:matrix.org left the room.23:21:02
9 Jul 2025
@jonhermansen:matrix.orgjonhermansen joined the room.01:18:54
@prsteele:matrix.orgPatrick Steele Does anyone have an example of how to structure a flake to allow builds to run with profiling enabled? If I add -prof to my cabal file, I get Failed to load dynamic interface file for Prelude errors, even in a simple project with no dependencies. 02:18:05
@prsteele:matrix.orgPatrick SteeleMy flake is https://pastebin.com/hUxz89uA right now. I've tried various incantations from https://github.com/NixOS/nixpkgs/blob/master/doc/languages-frameworks/haskell.section.md to try to coerce profiling to work.02:20:01
@prsteele:matrix.orgPatrick Steele I'm wondering whether my use of pkgs.haskellPackages, as opposed to e.g. pkgs.haskell.packages.ghc984, is somehow an issue 02:21:34
@eldritchcookie:matrix.orgeldritchcookiefor those that use the nixpkgs haskell infrastructure on ghc 9.12.2 do you have rename capability with hls? i am trying to narrow down where the issue is and while it seems that version 2.11.0.0 has full support i was unable to find if the version 2.10.0.0 has the hls-rename-plugin 11:35:17
@b:chreekat.netchreekati've thought about this12:33:53
@maralorn:maralorn.demaralornI am pretty sure unison uses haskell.nix yes, that is a ton of IFD. However of course building the exact derivation twice should work.14:04:09

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