Problems while installing language-javascript and yesod

While trying to install yesod using the haskell cabal package manager, I encountered these errors.

   cabal: happy is required but it could not be found
   language-javascript-0.4.4 failed during the configuration step.
   The Exception was: ExitFailure 1

What was missing was an entry in my PATH variable to point to ~/.cabal/bin. To fix the problem, I added the following two lines to my ~/.bash_profile script

   PATH=~/.cabal/bin:$PATH
   export PATH

Save, and try again.

   $ cabal install yesod