459234db20
This predicate filters out packages that weren't created by the Cabal builder. Doing that greatly reduces the likelihood of file collisions in the generated environment, because Haskell packages tend to have a lot of propagated build inputs. For example, both zeromq 2.x and 3.x use the same names for their header files. Users of haskell-zeromq don't need those headers, so we just don't include them in the generated environment to avoid the collision that would otherwise occur when haskell-zeromq 2.x and 3.x are installed into the same environment.