I've been using this hackage package "rex" without problems on two computers (linux, windows) but now I fail setting it up on a third computer which confuses me, and the error message is not really enlightening for me:

$ cabal update
Downloading the latest package list from hackage.haskell.org
$ cabal --version
cabal-install version 1.16.0.2
using version 1.16.0.3 of the Cabal library
 $ cabal install rex
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: rex-0.3.1