If you want the most up to date (but also most untested) packages you can use the Gentoo Haskell Overlay. "Overlays" are package trees for Portage. They contain additional ebuilds for Gentoo. They are maintained by Gentoo developers, but are distributed separately from the main Portage tree. Haskell has its own dedicated overlay containing lots of haskell projects. See the Gentoo Overlays: Users' Guide for how to use overlays. However, don't forget that like with all overlays, you use the ebuilds there at your own risk. Do not report errors in overlay ebuilds to the Gentoo Bugzilla!
These details are provided for information only. No information here is legal advice and should not be used as such.