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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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22 Jun 2025
@ners:nixos.devnersBut how is its ancientness encoded in cabal?19:55:00
@ners:nixos.devnersI don't see the ffmpeg version encoded in the cabal file. It's asking for a certain set of pkgconfig libs, I'm giving it those libs.19:55:21
@ners:nixos.devnersI don't see any version checks.19:55:25
@ners:nixos.devners

Here's what the cabal wants:

  pkgconfig-depends:   libavutil, libavformat, libavcodec, libswscale, libavdevice,
                       libswresample

Here's what ffmpeg-full.dev has:

 /nix/store/7v77gvdjk8410qzrpkhymihgijf5s990-ffmpeg-full-7.1.1-dev/lib
└──  pkgconfig
    ├──  libavcodec.pc
    ├──  libavdevice.pc
    ├──  libavfilter.pc
    ├──  libavformat.pc
    ├──  libavutil.pc
    ├──  libpostproc.pc
    ├──  libswresample.pc
    └──  libswscale.pc
19:56:37
@ners:nixos.devners* I don't see any version checks or reasons why a newer version wouldn't work.19:57:03
@emilazy:matrix.orgemily not sure. other than the missing lib prefixes in the error maybe. 19:57:54
@sasha:the-apothecary.clubSashanoraa.gay (she/her, ze/zir) joined the room.20:27:10
@sasha:the-apothecary.clubSashanoraa.gay (she/her, ze/zir)Heya, a while ago I wrote a program that uses the punycode package, which is marked as broken in nixpkgs. I'm trying to figure out why but have gotten nowhere, any suggestions?20:28:32
@sasha:the-apothecary.clubSashanoraa.gay (she/her, ze/zir)* Heya, a while ago I wrote a program that uses the punycode Haskell package, which is marked as broken in nixpkgs. I'm trying to figure out why but have gotten nowhere, any suggestions?20:29:43
@maralorn:maralorn.demaralorn
In reply to @sasha:the-apothecary.club
Heya, a while ago I wrote a program that uses the punycode package, which is marked as broken in nixpkgs. I'm trying to figure out why but have gotten nowhere, any suggestions?
Have you looked for a "broken.yaml" file in nixpkgs and read the line in the corresponding comment? 		20:47:48
@maralorn:maralorn.demaralornAlso have you tried just building it by overriding/ignoring the broken flag?20:48:25
@sasha:the-apothecary.clubSashanoraa.gay (she/her, ze/zir)
In reply to @maralorn:maralorn.de
Have you looked for a "broken.yaml" file in nixpkgs and read the line in the corresponding comment?
I did not because I didn't know that existed. Thx! 		20:49:58
23 Jun 2025
@artem.types:matrix.orgArtem

Hey all! I'm routinely getting strange "missing dependencies" errors when trying to build with cabal on my NixOS machine. Never happens on non-NixOS. I must be missing something? E.g.:

Failed to build unix-2.8.6.0. The failure occurred during the configure step.
Build log (
/home/artem/.cache/cabal/logs/ghc-9.10.1/unix-2.8.6.0-7e70e93a018fa3cd3eeee6c9345819b76e60434f053c8d02acdb7ab743ba3a2a.log
):
Configuring unix-2.8.6.0...
Error: [Cabal-8010]
Encountered missing or private dependencies:
    tasty, tasty-hunit, tasty-quickcheck

happens to various dependencies when building various packages...

17:51:06
@artem.types:matrix.orgArtemI saw reports like that on Discourse but none of the suggestions looked particularly relevant...17:51:35
@alex:tunstall.xyzAlex
In reply to @artem.types:matrix.org

Hey all! I'm routinely getting strange "missing dependencies" errors when trying to build with cabal on my NixOS machine. Never happens on non-NixOS. I must be missing something? E.g.:

Failed to build unix-2.8.6.0. The failure occurred during the configure step.
Build log (
/home/artem/.cache/cabal/logs/ghc-9.10.1/unix-2.8.6.0-7e70e93a018fa3cd3eeee6c9345819b76e60434f053c8d02acdb7ab743ba3a2a.log
):
Configuring unix-2.8.6.0...
Error: [Cabal-8010]
Encountered missing or private dependencies:
    tasty, tasty-hunit, tasty-quickcheck

happens to various dependencies when building various packages...

Are you trying to override the unix package?

Boot libraries have special handling and cannot be overridden in the usual way.
For example:

nix-repl> haskellPackages.unix
null
18:52:14
@artem.types:matrix.orgArtem @alex:tunstall.xyz: I'm building with cabal, not nix, so, I'm not sure I understand your question but no, I'm not doing anything special I just do `cabal build` 19:53:48
@maralorn:maralorn.demaralorn @artem.types:matrix.org 1. Did you run cabal update? 2. Try reseting by removing dist-newstyle. 19:58:09
@alex:tunstall.xyzAlex
In reply to @artem.types:matrix.org
@alex:tunstall.xyz: I'm building with cabal, not nix, so, I'm not sure I understand your question but no, I'm not doing anything special I just do `cabal build`

If you don't understand the question, the answer is "probably not".

Then you might have version constraints in your package that cause Cabal to reject the version installed with the GHC provided by Nix.
Can you try deleting the local copy of the package index? (I can't remember where in ~ cabal puts it.)

19:58:31
@alex:tunstall.xyzAlex Running cabal update produces a local package index that Cabal may try to use to build packages.
When using Nix, you typically don't want the file to exist. 		20:00:14
@maralorn:maralorn.demaralornRedacted or Malformed Event20:18:43
@maralorn:maralorn.demaralornWell with cabal update it should work, even though it is not a very typical workflow.20:20:48
@alex:tunstall.xyzAlex Ah, that's not a workflow I'm familiar with and I vaguely recall it being a problem in the past...
Let's see whether any of our ideas help. 		20:22:41
@maralorn:maralorn.demaralorn Alex: It has the advantage of not needing overrides as long as there is any valid build plan cabal can find on hackage. It has the disadvantage that you get no caching for dependencies and the problem that you get no automatic installation of system dependencies. So traditionally this breaks when you have e.g. zlib in your closure. 20:24:53
@maralorn:maralorn.demaralorn(I guess you know that, but that is probably the problem you remember.)20:25:22
@alex:tunstall.xyzAlex Given that GHC 9.10 is being used, cabal update may be easier than jailbreaking and patching various packages.
I haven't tried upgrading beyond the default GHC lately, but when I do it has always involved Nix overrides... 		20:28:34
@maralorn:maralorn.demaralornI mean for building unix there shouldn’t be any problems either way, tbh.20:36:37
@maralorn:maralorn.demaralornI assume only problem is that test dependencies of boot packages are not boot packages themselves.20:37:10
@alex:tunstall.xyzAlex True, but it's rare for a project to only need boot libraries.
It's hard to say what the easiest approach is without a better understanding of Artem's use case. 		20:38:49
@artem.types:matrix.orgArtem
In reply to @maralorn:maralorn.de
@artem.types:matrix.org 1. Did you run cabal update? 2. Try reseting by removing dist-newstyle.
I tried these, yes. I've been having this issue a while, so I've tried many simple things like that 		20:54:04
@maralorn:maralorn.demaralorn Artem: I see. If you would like more help then maybe you can share the Haskell relevant part of the nix configuration (e.g. the nix- shell or your system config) which installed ghc, etc and also the full log of the failing command? 20:58:38

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