Resolving dependencies...
cabal: Could not resolve dependencies:
next goal: opencv (user goal)
rejecting: opencv-0.0.2.1 (conflict: requires pkg-config package
opencv>=3.0.0, not found in the pkg-config database)
rejecting: opencv-0.0.2.0, opencv-0.0.1.1, opencv-0.0.1.0, opencv-0.0.0.0
(constraint from user target requires ==0.0.2.1)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: opencv