haskell: cabal unable to find alex and happy in a shell produced with shellFor

[d-xo]

Describe the bug

I have alex and happy listed in the build-tool-depends section of my cabal file (here), but when I enter a nix shell created with haskellPackages.shellFor cabal is unable to find these dependencies and gives the following error:

Warning: The package list for 'hackage.haskell.org' does not exist. Run 'cabal
update' to download it.RemoteRepo {remoteRepoName = RepoName
"hackage.haskell.org", remoteRepoURI = http://hackage.haskell.org/,
remoteRepoSecure = Just True, remoteRepoRootKeys =
["fe331502606802feac15e514d9b9ea83fee8b6ffef71335479a2e68d84adc6b0","1ea9ba32c526d1cc91ab5e5bd364ec5e9e8cb67179a471872f6e26f0ae773d42","2c6c3627bd6c982990239487f1abd02e08a02e6cf16edb105a8012d444d870c3","0a5c7ea47cd1b15f01f5f51a33adda7e655bc0f0b0615baa8e271f4c3351e21d","51f0161b906011b52c6613376b1ae937670da69322113a246a09f807c62f6921"],
remoteRepoKeyThreshold = 3, remoteRepoShouldTryHttps = True}
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: act-0.1.0.0 (user goal)
[__1] unknown package: act:happy:exe.happy (dependency of act)
[__1] fail (backjumping, conflict set: act, act:happy:exe.happy)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: act, act:happy:exe.happy

Both alex and happy are available on path in the shell:

> which alex
/nix/store/kw7250df32diiis9wx0x78irinnzibim-alex-3.2.6/bin/alex
> which happy
/nix/store/3k6bs1nz6bpkj9pfsfpmyqpfy0kjw5bd-happy-1.20.0/bin/happy

I can work around this by running cabal v2-update and then cabal will download and build alex and happy by itself, but I would prefer to avoid any impurities here and have cabal pull all it's dependencies from the nix-shell.

note that haskell.nix suffers from the same issue: input-output-hk/haskell.nix#839

cc @Ericson2314 @peti @ElvishJerricco

[matthewbauer]

I think this is from switching to new-style cabal builds. A temporary workaround is to always use cabal v1- in commands.

[d-xo]

Interesting! This does work ✨ 🙏 I'm curious why that would be the case though?

[matthewbauer]

I think v2 is just stricter in a lot of places. Cabal wants the executable in the package database, but right now it doesn't get there.

[d-xo]

I see, do you know what would be required to get the executables into the package database in the nix shell? I would be interested to maybe try and make a PR here, but I'm very unfamiliar with the nix haskell infra and how cabal works in general...

[Ericson2314]

Yeah there some some issue about this before. There isn't a good way to get it in the package database. And for thing like is just an exe it doesn't make much since. It would be nicer if there was some other way to tell cabal "trust me, I'm the right version".

[d-xo]

Why shouldn't an executable go into the package db? It seems as though the v2 cabal workflows expect them to be there?

[stale]

I marked this as stale due to inactivity. → More info

[sternenseemann]

Using v1-* commands is still the only workaround known to me.

[ParetoOptimalDev]

Why shouldn't an executable go into the package db? It seems as though the v2 cabal workflows expect them to be there?

See this related issue: haskell/cabal#8434

[ScottFreeCode]

A more complete but still minimal reproduction is here: https://gist.github.com/ScottFreeCode/c1adb5681c5b61c61373615f63a4a7d2

I've collected GitHub issues relating to this problem here: https://gist.github.com/ScottFreeCode/ef9f254e2dd91544bba4a068852fc81f