Haskell in Nixpkgs/NixOS - Public Room Timeline

	Haskell in Nixpkgs/NixOS	724 Members
	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.org	145 Servers

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Sender	Message	Time
21 Jul 2025
Alex	In reply to @magic_rb:matrix.redalder.org im benchmarking a thing i wrote and im making a nix package out of a cabal file using `cabal2nix`, however if i change the source cabal file and remove `-fllvm`, `-O2`, or `-fexpose-all-unfoldings` the performance measurements dont seem to change even though im positive they should. Is nixpkgs overwriting my flags somehow? If you're using a profiling build, beware of automatic cost centres (IIRC enabled by default in cabal).	10:37:54
magic_rb	In reply to @alex:tunstall.xyz If you're using a profiling build, beware of automatic cost centres (IIRC enabled by default in cabal). Nope, i know about those	10:49:29
22 Jul 2025
maralorn	Does anyone here grok/has used the useIntermediates stuff? I have one question: Can it reuse the intermediates of a previous build even when that build had other dependency hashes on the nix side? The argument was that cabal-install does not always have to recompile your project when you modify your nix shell. And the reusing is probably happening based on some cabal level hashing?	15:02:25
sterni (he/him)	Well development shell is out anyways since that's Setup.hs which cabal-install won't be able to reuse in any shape or form I think	18:57:16
maralorn	Are you sure? Maybe the components are similar enough? I mean they have been compiled with the same ghc. Isn’t it basically a dir with .hie and object files in both cases?	20:55:26
24 Jul 2025
	Keeton Brewster joined the room.	00:12:02
Keeton Brewster	Hi everyone, is there a Haskell specific Nix beginner guide out there?	00:14:32
keypusher	I think it's a bit fragmented depending on your needs / approach as can be seen in this thread https://www.reddit.com/r/haskell/comments/1cqfboq/latest_guidance_on_using_haskell_with_nix/ . Some years ago my searches often ended up here https://haskell4nix.readthedocs.io/	00:45:35
keypusher	I would suggest you start wo using these evolved approaches and try to have as few layers on top of "vanilla"/"flakes" nix	01:09:48
Keeton Brewster	Thank you!	01:51:31
Keeton Brewster	There seems to be a thousand ways to do it which is confusing for a newbie	01:56:06
	bglgwyng joined the room.	05:41:12
bglgwyng	Hello! I'm trying to build a `nixpkgs.haskellPackages` from an arbitrary stackage snapshot. I just found `regenerate-hackage-packages.sh` in nixpkgs and also `hackage2nix` in `cabal2nix`. Is `hackage2nix` enough for my purpose? I don't fully understand how two tools differ.	05:54:08
maralorn	@keetonbrewster:matrix.org haskell4nix.readthedocs.io is outdated. It's uptodate version is the nixpkgs manual Haskell section.	06:07:02
maralorn	In reply to @bglgwyng:matrix.org Hello! I'm trying to build a `nixpkgs.haskellPackages` from an arbitrary stackage snapshot. I just found `regenerate-hackage-packages.sh` in nixpkgs and also `hackage2nix` in `cabal2nix`. Is `hackage2nix` enough for my purpose? I don't fully understand how two tools differ. Huh, I don't think we expect downstream users of hackage2nix. I fear that you won't get very far. But if you really want do this the easiest way would probably be to look at the update stackage script in nixpkgs modify it to not use the newest but your desired snapshot, run it and then run the regenerate script. You can think of the regenerate script as running hackage2nix with all correct configuration for nixpkgs in place. Running hackage2nix without it would probably require duplicating parts of it.	06:15:22
bglgwyng	What if I want to avoid making my own version of nixpkgs? I prefer more local apporach such as generating .nix file that can be used to replace `nixpkgs.haskellPackage`. Can I do the same thing using 'regenerate-hackage-packages'?	06:45:03
maralorn	It's not meant to be used that way, no. But maybe you can take it as an inspiration. Maybe you want to have a look at stacklock2nix it might be closer to your needs.	06:47:25
bglgwyng	Do you mean https://github.com/cdepillabout/stacklock2nix this repo?	06:59:05
maralorn	In reply to @keetonbrewster:matrix.org Hi everyone, is there a Haskell specific Nix beginner guide out there? That reminds me. I actually did a talk about that at zurihac. It might be helpful: https://m.youtube.com/watch?v=xOjKAatxD_g&list=PLOvRW_utVPVlFEXuyaIVILO1t_48e4Ai2&index=9&pp=iAQB	06:59:34
bglgwyng	Indeed, I tried an approach using 'plan.json', cabal dry run generates plan.json and nix fetches the source code, however it caused too frequent rebuild of existing library. So I'm looking for a way to pre-bake package set.	07:01:47
maralorn	In reply to @maralorn:maralorn.de That reminds me. I actually did a talk about that at zurihac. It might be helpful: https://m.youtube.com/watch?v=xOjKAatxD_g&list=PLOvRW_utVPVlFEXuyaIVILO1t_48e4Ai2&index=9&pp=iAQB It's sadly not super structured or dense, but I actually think we covered a lot of useful information.	07:04:59
MangoIV	Hey guys. what is the current status of static linking with ghc? Is ghc 9.4.8 still the only supported compiler?	08:51:07
sterni (he/him)	all GHCs work, but TemplateHaskell isn't supported for >= 9.6	10:02:35
Teo (he/him)	out of interest, is there a ticket that tracks this?	10:38:26
Teo (he/him)	found it: https://github.com/NixOS/nixpkgs/issues/275304	10:38:45
MangoIV	teo (they/he): I will try out with your patch and report back. thank you!	12:44:14
25 Jul 2025
iqubic (she/her)	Hmm... I think I might be doing something wrong. So I have this bare-bones starter project of mine: https://github.com/IQubic/mandelbrot	00:10:59
iqubic (she/her)	And entering the `nix-shell` works. But then attempting to run `cabal repl` fails with this: `Resolving dependencies... Error: [Cabal-7107] Could not resolve dependencies: [__0] next goal: mandelbrot (user goal) [__0] rejecting: mandelbrot-0.1.0.0 (conflict: requires GHC2024 which is not supported) [__0] fail (backjumping, conflict set: mandelbrot) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: mandelbrot`	00:12:02
iqubic (she/her)	Does `cabal` not support GHC2024 yet?	00:12:13
@le:4d2.org	In reply to @mequbic:matrix.org Does `cabal` not support GHC2024 yet? `GHC2024` is only supported if you have at least version 9.10 of ghc, according to a warning when initialising projects with cabal. nix installs 9.8.4 by default though. When I explicitly install a newer version using `nix-shell -p cabal-install haskell.compiler.ghc9122`, which installs ghc version 9.12.2, it works. (Well "works": I get a different error, which could be resolved with changing `base ^>= 4.19.2.0` to `base >= 4.19.2.0` in mandelbrot.cabal, but that's a different topic)	05:49:07

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