especially if you're installing stuff from the gentoo haskell overlay you'll have to unmask tons of packages (take a look at the beginning of a standard haskell file if you don't know why)
if the emerge -pv of the package lists only the required packages to be keyworded
then you can do
emerge -pv package 2>&1 | grep ^= | sudo tee -a /etc/portage/package.accept_keywords/package