here is what I did:

1. makdir happstack\_01
2. cabal-dev install happstack-server
3. write the typical helloworld.hs with "import Happstack.Server (nullConf, simpleHTTP, toResponse, ok)"
4. ghc -threaded HelloWorld.hs -o helloworld

and I got:
Could not find module `Happstack.Server'

This is so obvious wrong. But what I am more surprised is that no tutorial on google for simple thing as this. 

Any intuition would be awesome!