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Of the "official" images currently available from the Raspberry Pi Downloads page, the following versions of GHC are available
Note: GHCi does not currently work on ARM, but it may become available in GHC 7.8. See this post on haskell-cafe for the status as of December 2013.
There seemed to be a point where GHCi was merged into version 7.4.2., and it may still be possible to install it in an experimental manner, but it is not (currently) available in the Debian Raspberry Pi repository. Here is a discussion thread wherein djhuk describes compiling and installing GHC-7.4.2 via QEMU to the Raspberry Pi, with incomplete GHCi functionality.
To build a cross-compiler from Linux to Raspberry Pi (running Raspbian), see the GHC Trac.
The main issue encountered so far is the memory limitations of the devices with 256 MB RAM; GHC will execute fine, but problems will start to arise when using cabal-install to install new libraries, especially ones with lots of dependencies. (Recent model B boards have 512 MB on board , Raspberry Pi 2 Model B boards have 1GB on board.)
Potential solutions to this problem have been suggested, including using QEMU on a more powerful system and thus compiling your application there and copying the resulting binary across to your Raspberry Pi