[Haskell-cafe] .cabal: API-compatible library versions

[Imants Cekusins]

> As a maintainer I might still need to both whitelist versions I know are good and blacklist those that are not to make sure users don't get a package combination that doesn't work as expected.

of course, maintainer input would be essential. For simple utility
libraries we could get away with file compare.

However even in different versions there may be fully compatible code.
If a package only uses still compatible code, why not save users a
reinstall?

semantic versioning as it is, is very restrictive (or misleading if
versions are not upped for 'small changes').