[Haskell-cafe] Problem when installing gtk2hs.

Hi, I have met similar problem before, and I do not know what to do. # cabal install --reinstall gtk2hs-buildtools Resolving dependencies... Configuring gtk2hs-buildtools-0.9... cabal: alex is required but it could not be found. cabal: Error: some packages failed to install: gtk2hs-buildtools-0.9 failed during the configure step. The exception was: ExitFailure 1 # alex -v Alex version 2.3.3, (c) 2003 Chris Dornan and Simon Marlow -- 竹密岂妨流水过 山高哪阻野云飞