When using Cabal's sandboxes together with local, unpublished, packages, one ends up with an error that Cabal cannot find the corresponding package.
An example for such an error is the following:
cabal: Could not resolve dependencies: trying: pmrsabs-0.1.0.0 (user goal) next goal: multimap (dependency of pmrsabs-0.1.0.0) rejecting: multimap-1.2.1, 1.2, 1.1, 1.0 (conflict: pmrsabs => multimap>=1.3) Dependency tree exhaustively searched.
The solution for this problem is quite simple, just call the following:
$ cabal sandbox add-source path/to/your/project
And that is it!
$ cabal sandbox list-sources
you can print all the sources you added, and with
$ cabal sandbox delete-source path
you can delete if they got obsolete.