Is it okay to use the cabal solver to conditionally define dependencies and instances like this?

Thanks for the link.

Reading it provoked quite a bit of thinking.

On reflection there’s two problems with what I’ve suggested.

1. I’ve realised in what I’ve described actually a circular dependency between alice and alice-bob-instance, in that they both depend on each other. So I don’t think that will work.
2. The other issue is that if a package depends directly on alice and bob but only indirectly on alice-bob-instance just a change in the dependencies of it’s dependencies suddenly can make it break (namely if one of it’s dependencies no longer depends on alice-bob-instance). That’s no good.

There’s does seem to be no better solution than the orphan package and having to explicitly import the orphan package module without an import list, but this does seem deeply unsatisfying.