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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.org148 Servers

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21 Mar 2026
@alexfmpe:matrix.orgalexfmpeI also have no idea how the on-demand interpreter will shake things00:35:47
@woobilicious:matrix.orgwoobiliciousLol I got almost no where so far, converting a monomorphic C interface to a typesafe hetrogenous list type is way too complex for my puny brain. 05:07:12
@sternenseemann:systemli.orgsterni (he/him) teo (they/he): do you know whether there is already a GHC issues w.r.t. handling of system gmp/libffi in a cross context? I think a fundamental problem in GHC is that it assumes that the same (system) libffi and gmp flags can be passed during every stage, but when e.g. building a cross compiler, stage0 would need to use a different version of those libraries which has been built for the build platform. 13:59:16
@sternenseemann:systemli.orgsterni (he/him)I assume that the in tree gmp/libffi handle this just fine since it's recompiled for every stage (?)13:59:54
@sternenseemann:systemli.orgsterni (he/him)One option I see would be to build stage0 without libffi and gmp altogether if that's possible14:00:54
@sternenseemann:systemli.orgsterni (he/him)In any case, context is that I'm currently patching the system libffi handling slightly to special-case stage0 and I want to at least reference some issue, so would be good to have something related already, but can also open a new one.14:02:28
@sternenseemann:systemli.orgsterni (he/him)Not sure if the patch would be upstreamable though I suspect that --with-system-* does not work when building a cross compiler with anything but Nix and with Nix it's sometimes broken if the headers are platform specific (i.e. libffi).14:03:17
@sternenseemann:systemli.orgsterni (he/him) * Not sure if the patch would be upstreamable though I suspect that, currently, --with-system-* does not work when building a cross compiler with anything but Nix and with Nix it's sometimes broken if the headers are platform specific (i.e. libffi). So maybe it would be a net positive. 14:03:39
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@sternenseemann:systemli.orgsterni (he/him) alexfmpe: Alex: https://github.com/NixOS/nixpkgs/pull/501983 so this is my idea, just letting cc wrapper deal with stage0 (since we're in stdenv anyways) 16:30:59
@sternenseemann:systemli.orgsterni (he/him)gonna clean that up, patch doesn't seem too terrible to maintain, may even be upstreamable in some shape or form16:31:27
@teoc:matrix.orgTeo (he/him)

I'm not sure if there's an existing issue. Creating one sounds great! https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15231 might cover this but I'm not sure.

Even if it's just nix specific it would be good to have it upstream since then we'd be less likely to break downstream all the time.

16:53:41
24 Mar 2026
@alexfmpe:matrix.orgalexfmpedo we have any idea what causes the rts configure error mentioned in https://github.com/NixOS/nixpkgs/pull/482251#issuecomment-377941325218:25:28
@alexfmpe:matrix.orgalexfmpeI also hit it for s390x18:25:33
@alexfmpe:matrix.orgalexfmpe* affects some powerpc variants, i686, s390x18:25:59
@libjared:matrix.org@libjared:matrix.org left the room.21:26:45
26 Mar 2026
@aiya:catgirl.cloudaiyasay I wanted to fix a C++ lib that is relevant for a haskell package (i.e. fix libtorch to fix libtorch-ffi), would that be done in the haskell-updates branch or another branch?17:18:26
@alexfmpe:matrix.orgalexfmpeUsually haskell-updates is for LTS bump related shenanigans. If your error is present on master and can be fixed there, usually that's what you want 17:27:14
@alexfmpe:matrix.orgalexfmpeUnless it causes mass rebuilds, then it goes on staging17:27:34
@aiya:catgirl.cloudaiyai don't understand, i thought haskell-updates was for all Haskell package changes?17:28:00
@aiya:catgirl.cloudaiya* i don't understand, i thought haskell-updates was for all Haskell package changes? or are you specifically talking about non-haskell packages17:28:09
@sternenseemann:systemli.orgsterni (he/him)it's a development branch for haskell related changes, but that doesn't mean that it needs to be used in every case22:37:26
@sternenseemann:systemli.orgsterni (he/him)we were stricter about this in the past, but due to workflow changes going via haskell-updates means changes don't see master for weeks22:38:17
27 Mar 2026
@mrjtjmn:matrix.orgMrjtjmn changed their display name from Mrjt. to Mrjtjmn.11:11:11
@aiya:catgirl.cloudaiyaokay it's actually a bit involved:18:08:12
@aiya:catgirl.cloudaiya *

okay it's actually a bit involved:

  • C++ library and haskell package are both v1.4 on master
  • C++ library is v1.4 on haskell-updates, haskell package is v1.5

so i think i'm going to update both in haskell-updates otherwise we get dependency hell (it may also be that the haskell package is not following PVP)

18:09:53
28 Mar 2026
@woobilicious:matrix.orgwoobilicious anyone here know how to get pandoc haskell filters to work?
Could not find module ‘Text.Pandoc.JSON’.
09:11:21
@woobilicious:matrix.orgwoobiliciousis this just pandoc packagers forgetting to include pandoc with the interpreter? lol09:12:07
@woobilicious:matrix.orgwoobiliciousoh it just uses the environments runhaskell09:17:24

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