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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://nixos.org/manual/nixpkgs/unstable/#haskell | Current PR: https://github.com/nixos/nixpkgs/pulls?q=is%3Apr+is%3Aopen+head%3Ahaskell-updates | Maintainer Docs: https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/HACKING.md | More Nix: #community:nixos.org | More Haskell: #haskell-space:matrix.org | Merger Schedule: https://cloud.maralorn.de/apps/calendar/p/H6migHmKX7xHoTFa/dayGridMonth/now | Join #haskell.nix:libera.chat for question about the alternative haskell.nix infrastructure142 Servers

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31 May 2026
@whispers:catgirl.cloudwhispers [& it/fae]seems to work, thank you both!19:37:51
1 Jun 2026
@bglgwyng:matrix.orgbglgwyng Is is possible to use wasm compiler with pkgs.haskellPackages now? It doesn't seem so. 13:33:25
@alexfmpe:matrix.orgalexfmpeTL;DR: not yet https://github.com/NixOS/nixpkgs/pull/22500013:59:22
@alexfmpe:matrix.orgalexfmpeUse ghc-wasm-meta for now13:59:34
3 Jun 2026
@philipwilk:matrix.orgJade w changed their display name from Philip W to Jade w.15:01:19
4 Jun 2026
@callmeecho:matrix.orgEcho changed their profile picture.04:23:34
@sternenseemann:systemli.orgsterniI think downgrading is no longer an option. https://github.com/NixOS/nixpkgs/pull/52796212:53:48
@artem.types:matrix.orgArtem sterni: 4.17 is good enough: it doesn't break the PVP... 12:54:51
@artem.types:matrix.orgArtem also, sterni , regarding https://github.com/NixOS/nixpkgs/pull/521260#issuecomment-4622508007 let me know what you think. I think https://github.com/NixOS/nixpkgs/pull/520243 looks much better than the last time you looked at it. Even it's LLM-assisted, I think it's a good step forward. 14:02:50
@alexfmpe:matrix.orgalexfmpeon our stackage nightly bump, HLS is broken on 9.10 due to apply-refact what I don't understand is how we made it work on master as that dep was always there on cabal but was missing from hackage-packages.nix14:52:34
@alexfmpe:matrix.orgalexfmpe* on our stackage nightly bump, HLS is broken on 9.10 due to its dep, apply-refact, being broken on 9.10 period what I don't understand is how we made it work on master as that dep was always there on cabal but was missing from hackage-packages.nix14:52:47
@alexfmpe:matrix.orgalexfmpe huuuuh technically the dependency is on HLS's cabal file, but on the hls-hlint-plugin sublibrary 14:53:54
@alexfmpe:matrix.orgalexfmpewe've been adding apply-refact manually https://github.com/NixOS/nixpkgs/blob/0a130039b0ecb0f9690581660080baca073fdd5a/pkgs/development/haskell-modules/configuration-common.nix#L237 which makes sense given the dep wasn't on hackage-packages.nix (presumably due to it being behind a cabal conditional) but then why does the dep show up now14:59:46
@sternenseemann:systemli.orgsternibecause the conditionals are resolved with impl being ghc 9.1217:44:09
@alexfmpe:matrix.orgalexfmpeAaah17:44:22
6 Jun 2026
@hellwolf:matrix.orghellwolf

trying to fixing my part in haskell-updates. however, facing this optics package issue Is it being dealt:

error: Cannot build '/nix/store/6fj48f1agb8y7wyvjxqv8lmc6jabaaxq-optics-0.4.2.1.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/2gvmjpzcgqa8xhq47mz9xrd5w2hm86pw-optics-0.4.2.1
         /nix/store/xkdgyjjf6wvb5lp51m940ffrg81lwywf-optics-0.4.2.1-doc

11:38:19
@hellwolf:matrix.orghellwolf *

trying to fixing my part in haskell-updates. however, facing this optics package issue Is it being dealt:

error: Cannot build '/nix/store/6fj48f1agb8y7wyvjxqv8lmc6jabaaxq-optics-0.4.2.1.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/2gvmjpzcgqa8xhq47mz9xrd5w2hm86pw-optics-0.4.2.1
         /nix/store/xkdgyjjf6wvb5lp51m940ffrg81lwywf-optics-0.4.2.1-doc
       Last 25 log lines:
       >         +++ OK, passed 100 tests.
       >       set (lens f g) = g:                                                            OK
       >         +++ OK, passed 100 tests.
       >     Prism
       >       review (prism f g) = f:                                                        OK
       >         +++ OK, passed 100 tests.
       >       matching (prism f g) = g:                                                      OK
       >         +++ OK, passed 100 tests.
       >     AffineTraversal
       >       matching (atraversal f g) = g:                                                 OK
       >         +++ OK, passed 100 tests.
       >       set (atraversal f g) ~= flip g:                                                OK
       >         +++ OK, failed as expected. Falsified (after 5 tests and 20 shrinks):
       >         {2->Left 1, _->Right 1}
       >         {_->2}
       >         2
       >         1
       >         1 /= 2
       >       isRight (f s) ==> set (atraversal f g) = flip g:                               OK
       >         +++ OK, passed 100 tests; 84 discarded.
       >
       > 3 out of 127 tests failed (0.01s)
       > Test suite optics-tests: FAIL
       > Test suite logged to: dist/test/optics-0.4.2.1-optics-tests.log
       > 0 of 1 test suites (0 of 1 test cases) passed.
       For full logs, run:
         nix log /nix/store/6fj48f1agb8y7wyvjxqv8lmc6jabaaxq-optics-0.4.2.1.drv
11:38:39
@alexfmpe:matrix.orgalexfmpeThat looks like a legitimate test failure? Maybe file an issue in the library? 13:00:45
@hellwolf:matrix.orghellwolf
  • I checked out the latest https://github.com/well-typed/optics
  • ran cabal test optics
    • with ghc910, passes
    • with ghc912, 1 failed.
14:01:08
@alexfmpe:matrix.orgalexfmpeHuh same seed? Maybe it's some ghc bug or some sketchy optimizations thing14:03:11
@alexfmpe:matrix.orgalexfmpeDefinitely file issue14:03:24
@hellwolf:matrix.orghellwolfsame seed. reported. Fwiw, the haskell-updates build produces 2 more errors. 14:09:45
7 Jun 2026
@sternenseemann:systemli.orgsterniLet's hope this goes away with GHC 9.12.5-rc1 :)19:47:21
@worldsgonemad:matrix.orgWorldsGoneMad joined the room.20:34:18
@mangoiv.:matrix.orgMangoIV I’ll try tomorrow. 21:18:06
@mangoiv.:matrix.orgMangoIV Do you have a link to the actual errors (this is missing the errors I think)? Otherwise I can just reproduce tomorrow myself 21:19:11
8 Jun 2026
@hellwolf:matrix.orghellwolfRedacted or Malformed Event09:39:22
@hellwolf:matrix.orghellwolfhttps://github.com/well-typed/optics/issues/54509:41:08
@hellwolf:matrix.orghellwolf MangoIV: I am still in zurihac, fwiw. In the work room near aula auditorium. 09:41:33
@hellwolf:matrix.orghellwolfis it a ghc bug?10:17:26

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