Why alex can not be automatically installed?

[Peng Yu]

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

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Regards,
Peng