CLaSH is a tool-chain/language to translate subsets of Haskell to synthesizable VHDL. Haskell modules (example) written in a hardware-oriented subset of Haskell are translated into VHDL by the library. That is, you can design hardware directly in Haskell, and generate hardware designs from GHCi.
Amongst other things, it provides a “:vhdl” extension to the GHCi command line, allowing you to generate hardware designs from the GHCi prompt. For more information about CLaSH, see the examples, sources and documentation project page, and the demo from the Haskell Workshop last year:
Other hardware-related libraries in Haskell in AUR: