cabal file in project root is not detected

[roberth]

Encountered this in https://github.com/hercules-ci/arion

haskellPathsInDir only scans subdirectories: "${x}/${x}.cabal"

We should also try ${x}.cabal (regardless of parent directory name, which is impure; store path or lazy tree).

[srid]

[Trying to describe this issue in detail]

The haskellProjects.foo.packages option currently uses the haskellPathsInDir function from nipxkgs to determine its default value:

haskell-flake/flake-module.nix

Lines 119 to 122 in 380bffd

	default =
	lib.mapAttrs
	(_: value: { root = value; })
	(lib.filesystem.haskellPathsInDir self);

This works for projects with multiple local packages (like https://github.com/srid/haskell-multi-nix) where cabal files live at */*.cabal. However, we want the default value to also work for single-package haskell projects (like https://github.com/srid/haskell-template), such that the three lines here can be removed and the flake still works, because the default will recognize it.

[roberth]

haskellPathsInDir didn't really work for hercules-ci-agent either, because that project has an integration test driver in a sub-subdirectory; depth 2, which, like depth 0, isn't depth 1.

https://github.com/hercules-ci/hercules-ci-agent/blob/40d5243b4ea4ecedbf021b50272dc0937a2ad4df/flake.nix#L293-L296

So my workaround was to use lib.mkOptionDefault, to extend the default, which is quite a fragile solution that I think most users won't be able to figure out, and shouldn't have to.

The package was listed in a cabal.project file, so if we could support that, I think we could have a fairly robust solution without having to scan deeply, which could be slow in monorepos.

The algorithm would then be roughly: check for cabal.project or stack.yaml or *.cabal in the root and use that to set the packages option; if not present, require the user to set that option (or some helper option that sets it, for instance an option that specifies the location of a cabal.project file).
This change breaks projects that don't have a cabal.project and rely on the */*.cabal behavior that we have now. They'll be in the same spot that new users are who haven't really configured their haskellProjects.x yet: an empty packages, which we can handle by presenting an error message that explains which options can be used to set packages.

[srid]

The algorithm would then be roughly: check for cabal.project or stack.yaml or *.cabal in the root and use that to set the packages option; if not present, require the user to set that option (or some helper option that sets it, for instance an option that specifies the location of a cabal.project file).

This sounds good to me.

• Is there a cabal.project parser for Nix already?
• For now, I think, we could just make the default recognize *.cabal in addition to */*.cabal before getting to implementing this idea (unless the parser is already available?), as it would help Expose two templates: default and example #69

[roberth]

Haskell.nix has one, but I don't know how easily reused it is. Maybe the relevant paths can be found without much effort, I don't know.
Dream2nix works with different files it seems. Maybe those contain all the needed info, but I don't think we should require them.

[roberth]

• For now, I think, we could just make the default recognize *.cabal in addition to */*.cabal before getting to implementing this idea

Yeah, one step at a time. We want to support this case anyway, regardless of cabal.project plans.