Support cabal.project import statements

[TeofilC]

Describe the bug

Recent versions of cabal-install allow using import statements in cabal.project files.
Haskell.nix doesn't seem to support these and gives an error about the imported file not existing.

I imagine this is due to some overzealous cleaning.

Steps To Reproduce

Try building cabal-install using haskell.nix.
The cabal.project file imports cabal.project.lastest-ghc.
This will give an error about the imported file not existing and the cabal-plan-to-nix-pkgs derivation fails to build.

Expected behavior

This should work.

[amesgen]

It works fine if you rename the imported file to have the .cabal extension. See here for what files end up when the plan is constructed:

haskell.nix/lib/call-cabal-project-to-nix.nix

Line 112 in 9bc10fb

pkgs.lib.any (i: (pkgs.lib.hasSuffix i path)) [ ".cabal" "package.yaml" ]); };

[michaelpj]

Right, so the problem is source filtering, which is quite important to avoid spurious rebuilds. I guess included files can have any extension? Then we cant' easily do a generic filter without checking through the project file first (and then checking through the things it imports...).

I guess a cheap solution would be to make the filter configurable, so people can include such things if they want them?

[TeofilC]

I guess a cheap solution would be to make the filter configurable, so people can include such things if they want them?

This sounds good to me. Recursively looking for imports sounds tricky.

Hopefully, with time, folks will settle on a convention (say cabal.project.* or *.cabal.project) and then we can just not filter that out

[philderbeast]

Recursively looking for imports sounds tricky.

Would it help to know which files cabal parsed?

-- | ProjectConfigSkeleton is a tree of conditional blocks and imports wrapping
-- a config. It can be finalized by providing the conditional resolution info
-- and then resolving and downloading the imports
type ProjectConfigSkeleton = CondTree ConfVar [ProjectConfigPath] ProjectConfig

parseProject :: _ -> IO (ParseResult ProjectConfigSkeleton)

With haskell/cabal#9933, we may opt to carry more data about paths when parsing;

parseProjectSkeleton
  :: _
  -> IORef [(FilePath, ProjectConfigPath)]
  -- ^ The imports seen so far, used to report on cycles and duplicates
  -- and to detect duplicates that are not cycles
  -> _
  -> IO (ParseResult ProjectConfigSkeleton)