Cabal install does not find locally installed dependency ("unknown package")

[danse]

Describe the bug
I have package b depending on a. a is locally installed but when trying to build b, the dependency is not found. The error looks like:

Error: cabal: Could not resolve dependencies:
[__0] trying: b-0.1.0.0 (user goal)
[__1] unknown package: a (dependency of b)
[__1] fail (backjumping, conflict set: b, a)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: b, a

To Reproduce
Move to a, run cabal install, the installation succeeds. Now move to b depending on a, run cabal install, the installation fails with the error mentioned above. These are v2- commands.

Expected behavior
After installing a, i expected it to be locally visible for resolving dependencies.

System information

• Ubuntu 20.04.5 LTS
• cabal 3.8.1.0, ghc 8.6.5

Additional context
Executables provided by a are accessible and the library it provides gets correctly imported into ghci

[andreasabel]

These are v2- commands.

In general, v2 commands do not honor packages installed locally. If you want your workflow, you have to use v1 commands.

One way to make local packages available for v2 is to add your own local package index:

• https://cabal.readthedocs.io/en/latest/config.html#local-no-index-repositories

[gbaz]

This may be resolved in master by #8607 (comment)

[danse]

Thanks @andreasabel, probably i have a simplistic mental model of caching in cabal. I thought that once installed, packages kept no track of where they came from.

The doc you referenced seems a great solution to use version two for this use case. I tried adding:

repository a
        url: file+noindex:///home/name/a/dist-newstyle/sdist/

To b.cabal, since install leaves a .tar.gz for a in that dir, but it still does not find the package.

Thanks @gbaz for the reference to what is being merged. Is there a way to figure out which release will include those improvements, so that i can test again?

[andreasabel]

@danse worte

adding ... To b.cabal

Ah, no, this has to go into cabal.project which you can put besides b.cabal.
.cabal files are for uploading to hackage, so they cannot mention things local to your machine/configuration.

[danse]

whoops, i thought that was the section about .cabal configuration. This works perfectly now, i just added a packages: b.cabal to start the cabal.project. Thanks @andreasabel!

[andreasabel]

Yes, somehow the documentation should in section https://cabal.readthedocs.io/en/latest/config.html#repository-specification state explicitly that one can also specify repositories in the cabal.project file.

Also, information about repositories should be referenced from https://cabal.readthedocs.io/en/latest/cabal-project.html# e.g. in https://cabal.readthedocs.io/en/latest/cabal-project.html#cfg-field-active-repositories or more centrally.

[ulysses4ever]

state explicitly that one can also specify repositories in the cabal.project file.

Sounds like an instance of #6319.

[josephcsible]

For reference, I asked this SO question a while back about how to do this: How do I get cabal to use a local version of a package as a dependency for a Hackage package?