This used to be the web site for Hoopl, the Higher-Order OPtimization Library.
Although Hoopl remains a separable library, the master sources are now distrivuted with the Glasgow Haskell Compiler. For more information about GHC sources consult the GHC Developers' Wiki.

If you want to download Hoopl directly, you can get it from Hackage by

    cabal install hoopl
or if you intend to contribute to Hoopl, you can try the git repository by
  git clone http://darcs.haskell.org/libraries/hoopl.git
(Although this URL is subject to change.)