Take a look at my own Haskell operating system project Hos (https://github.com/tathougies/hos). Written in Haskell 98 using the JHC compiler (a bit outdated, but makes it easy to generate ANSI C from Haskell sources). Currently supports multi-tasking, interrupts, paging, garbage collection. Has the beginnings of a simple user-space ATA driver.

Patches welcome. E-mail me at travis@athougies.net if you want help setting up a development environment!