I don't normally draw packages from EPEL, because some of them have conflicted with packages from other repositories that I use. But packages for Haskell on RHEL can be found there, and I haven't found working .RPMs elswehere. On the assumption that one is going to use those EPEL packages, gmp-devel, which is available from RHN, must be installed. To do so, as root, enter

yum install gmp-devel

There are at least a couple of ways to fetch and install the EPEL packages for Haskell. I think that the easiest is to use yum. As root, creäte a file

/etc/yum.repos.d/epel.repo

with contents

[epel]
name=RHEL 6 - epel - $releasever - $basearch
baseurl=http://download.fedoraproject.org/pub/epel/beta/6/$basearch/
gpgcheck=0
enabled=1
priority=1
exclude=*release

Then as root enter

yum install haskell-platform

(After that, as root I delete that .repo file

rm /etc/yum.repos.d/epel.repo

so as not to have those previously mentioned conflicts with packages from other repositories.)