Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: happy-2.1.7 (user goal)
[__1] next goal: happy-lib (dependency of happy)
[__1] rejecting: happy-lib-2.1.6 (conflict: happy => happy-lib==2.1.7)
[__1] skipping: happy-lib-2.1.5, happy-lib-2.1.4, happy-lib-2.1.3,
happy-lib-2.1.2, happy-lib-2.0.2, happy-lib-2.1.1, happy-lib-2.1,
happy-lib-2.0.1, happy-lib-2.0 (has the same characteristics that caused the
previous version to fail: excluded by constraint '==2.1.7' from 'happy')
[__1] fail (backjumping, conflict set: happy, happy-lib)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: happy, happy-lib