Yum Error: Nothing to do

Whenever I try to install a package, I usually get this:

$ sudo yum install haskell-platform
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
No package haskell-platform available.
Error: Nothing to do

It happens for a variety of packages, including gcc-c++ which is kind of a problem.
What am I doing wrong?