Is there any way to define flags for cabal dependencies?

Question

I recently ran into a Cabal issue that I only managed to solve by manually installing transformers-compat with the -f transformers3 flag in my cabal sandbox be

Answer 1

Looks like it's not possible to specify such a dependency via the build-depends field in your .cabal file. buildDepends is defined as [Dependency], where data Dependency = Dependency PackageName VersionRange. You can use cabal install --constraint="transformers-compat +transformers3", though.

Looking at the transformers-compat.cabal file, I think that the solver should be able to figure out the correct flag assignment if you constrain your dependency on transformers appropriately. E.g. build-depends: transformers >= 0.3 && < 0.4 should force the solver to choose transformers-compat +transformers3. If this doesn't work, it may be a bug in the solver.