Unhelpful error when using cabal.project.local without cabal.project

[brandonchinn178]

Describe the bug
If cabal.project.local exists without cabal.project, Cabal complains with:

Warning: both implicit and explicit configuration is present.When using configuration(s) from /Users/bchinn/Desktop/foo/cabal.project.local, the following errors occurred:
The package location glob './*.cabal' does not match any files or directories.

This happens even if cabal.project.local contains a packages: line.

Also, it seems like a newline is missing after the warning message?

To Reproduce
Steps to reproduce the behavior: (fails = shows bug, succeeds = expectedly fails because cabal file is empty)

mkdir foo
touch foo/foo.cabal
cat 'packages: foo/' >> cabal.project.local

# fails
cabal build

# succeeds with just an empty cabal.project
touch cabal.project
cabal build

Expected behavior
Renaming cabal.project to cabal.project.local should have the same behavior.

System information

• Mac
• cabal: 3.10.1.0

Additional context
Add any other context about the problem here.

[andreabedini]

I agree the message is not very helpful but there are reasons:tm: behind it.

To configure a project one uses the cabal configure command. This will take v2/nix-style/project-config options and write them in cabal.project.local. This works because packages: . is implied by the missing cabal.project.

One assumption is that we use cabal configure to make ad-hoc, local changes to a cabal project (e.g. I need to use this compiler ... cabal configure -w ghc-9.4.5). Permantent changes would be done by editing the existing cabal.project or by creating one.

My understanding is that cabal is concerned about "merging" the implicit configuration (packages: .) with the one in cabal.project.local (Note: projectPackages is a list so merging is concatenation).

We can discuss if this is something to worry about or not; off the top of my head I cannot think of any bad consequences.

Just to understand, how did you get to add packages: to cabal.project.local? why not cabal.project?

[brandonchinn178]

I was contributing to the head.hackage project, and they don't commit a cabal.project file, but they have a script that generates cabal.project.local (without cabal configure).

I guess their script could be updated to generate cabal.project + gitignore cabal.project.

[andreabedini]

I was contributing to the head.hackage project, and they don't commit a cabal.project file, but they have a script that generates cabal.project.local (without cabal configure).

I think I understand now. Did you need to add an extra package? Was it something from a Hackage (or other repository), or it was something you were just writing?
Thank you <3!

[gbaz]

I would suggest that the easiest path is that they do commit a cabal.project file, and it it contains the "packages" stanza and nothing else, and then their script generates a local with the appropriate local stuff.

[brandonchinn178]

@andreabedini I needed to add a patch for a package on Hackage to work with GHC HEAD. The script that head.hackage provides unpacks the source code + adds it as an optional package to a cabal.project.local file.

@gbaz Yes, that's what I did, but I think the error message should still be improved. It took me quite a while to figure out what was happening