Why alex can not be automatically installed?

Hi, I see the following error. Why alex can not be automatically installed? $ cabal install language-javascript Resolving dependencies... Configuring language-javascript-0.5.13.3... Failed to install language-javascript-0.5.13.3 Build log ( /Users/pengy/.cabal/logs/language-javascript-0.5.13.3.log ): Configuring language-javascript-0.5.13.3... setup-Simple-Cabal-1.22.2.0-x86_64-osx-ghc-7.10.1: The program 'alex' version

=3.1 is required but it could not be found. cabal: Error: some packages failed to install: language-javascript-0.5.13.3 failed during the configure step. The exception was: ExitFailure 1

-- Regards, Peng