cabal: Could not resolve dependencies:

a.k.a "how do I get Cabal to update the freeze file with the dependencies it needs, and why doesn't cabal update work?"

• issue: cabal freeze also freezes the index-state at the bottom of the file so, in some cases, cabal update has no effect
• fix:
  • delete the line starting with index-state at the bottom of the cabal.project.freeze file
  • delete lines for any dependencies you want to update
  • run cabal update/cabal freeze