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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.org147 Servers

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13 Jan 2026
@sjanssen:matrix.orgSpencer Janssen changed their profile picture.18:00:46
@sjanssen:matrix.orgSpencer Janssen changed their profile picture.18:02:01
16 Jan 2026
@perigord:matrix.orgPerigord changed their profile picture.12:37:05
19 Jan 2026
@ashinnv:matrix.orgm changed their display name from Dr Mayhem, esq to Grand Warden Mayhem, Punisher of Deer.00:08:13
20 Jan 2026
@mangoiv.:matrix.orgMangoIVhls 2.13.0.0 adds support for ghc 9.14 which would be nice to have because of the haskell debugger. https://github.com/haskell/haskell-language-server/releases/tag/2.13.0.0 is there anything I can do to accelerate having that land? 08:45:08
@magic_rb:matrix.redalder.orgmagic_rbthe haskell debugger, forgot about that one, cant wait to try it12:50:58
@innocentzer0:matrix.orginnocentzer0 joined the room.21:25:05
21 Jan 2026
@teoc:matrix.orgTeo (he/him)Would it be worthwhile to add a nix expression that allows one to use ghc nightly bindists with nix? It would basically be the same as the current binary ones but targeting HEAD. I wanted to get your thoughts on this before implementing anything. I basically need to maintain this for head.hackage anyway14:01:16
@teoc:matrix.orgTeo (he/him)* Would it be worthwhile to add a nix expression that allows one to use ghc nightly bindists with nix? It would basically be the same as the current binary ones but targeting HEAD. I wanted to get your thoughts on this before opening a PR. I basically need to maintain this for head.hackage anyway14:01:41
@josh:quadrivium.ukdutchie is there an easy way to get a git version of a cabal library? I would like to use optics with GHC 9.12 but it needs this commit to pass the tests. 14:04:26
@morj:morj.menMorj Something like this? https://github.com/maurges/re/blob/5f92f51cd437664e961b45cb2e7908e3c1bdadca/default.nix#L6-L13 14:17:13
@josh:quadrivium.ukdutchie that looks plausible, does it work in haskellPackages.mkDerivation as well? 14:26:07
@maralorn:maralorn.demaralorn Huh, I have also stumbled over the optics problem. I resolved to use 9.10 instead, but that sucks because of missing hlint support in hls on that ghc. 14:29:47
@morj:morj.menMorj I like how developPackage is only documented on the wiki, and mkDerivation is only documented in the nixpkgs manual 14:33:01
@morj:morj.menMorjWith mkDerivation can't you just supply the fetchTarball to libraryDepends?14:34:31
@josh:quadrivium.ukdutchie

I tried this:

optics = pkgs.haskell.lib.compose.overrideSrc {
  src = pkgs.fetchFromGitHub {
    owner = "well-typed";
    repo = "optics";
    rev = "9ef04b58176c77b0ef93076196705d21b6bd6824";
    hash = "sha256-TOINPPBBmUg1gbqwOX+FrqyFuch2LRX6i5NdEHUtbEY=";
  };
} hpkgs.optics;

but it fails:

Running phase: setupCompilerEnvironmentPhase
Build with /nix/store/h9k9g4l9sq4y59jh5hnizhww0ing5a0a-ghc-9.12.2.
Running phase: unpackPhase
unpacking source archive /nix/store/pnx7r3pv632p5gq7dhpsi4pkpx4g59hi-source
source root is source
Running phase: patchPhase
sed: no input files
14:34:34
@josh:quadrivium.ukdutchieI guess I need to point it at a subdir or something14:35:08
@josh:quadrivium.ukdutchie hmm I can just overrideCabal { doCheck = false; } 14:44:02
@josh:quadrivium.ukdutchie or dontCheck actually 14:53:40
22 Jan 2026
@ashinnv:matrix.orgm changed their display name from Grand Warden Mayhem, Punisher of Deer to Supreme Allied Commander Mayhem, Lord of the Nine Stars.01:27:17
@lordmzte:mzte.deLordMZTE joined the room.16:12:54
@lordmzte:mzte.deLordMZTE

It seems like GHCJS isn't in nixpkgs. Does anyone know a good way to set it up in a dev shell?

16:14:23
@acidbong:envs.netAcid Bongtwas removed: https://github.com/NixOS/nixpkgs/pull/42234216:15:05
@acidbong:envs.netAcid Bongyou can still save that recipe and use it locally tho16:16:12
@lordmzte:mzte.deLordMZTE

I see. Surely someone must have taken over maintaining this out of tree, right?

16:17:10
@emilazy:matrix.orgemily AIUI the newer GHCJS is a native GHC backend and is accessible with pkgsCross.ghcjs or something 16:22:15
@emilazy:matrix.orgemilyit was the old GHCJS that was removed16:22:19
@acidbong:envs.netAcid Bongoh16:22:26
@acidbong:envs.netAcid Bongi can't find ghcjs in the search16:22:38
@emilazy:matrix.orgemilyit's a platform, not a package16:31:38

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