Resolving dependencies...
cabal: Could not resolve dependencies:
next goal: haskell-docs (user goal)
rejecting: haskell-docs-4.2.7, haskell-docs-4.2.6, haskell-docs-4.2.5,
haskell-docs-4.2.4 (constraint from user target requires ==4.2.3)
trying: haskell-docs-4.2.3
next goal: ghc (dependency of haskell-docs-4.2.3)
rejecting: ghc-8.0.1.20161018/installed-8.0... (conflict: haskell-docs =>
ghc>=7.2 && <7.9)
Dependency tree exhaustively searched.