[Haskell-cafe] automatically inserting type declarations

I remember hearing about a Haskell mode for Vim, Emacs, Yi or VisualHaskell that inserts type declarations automatically (it's lazier to just check the type than to write it manually), but I can't remember any details. What editor mode / IDE was it? What do most people use with GHC on Linux? I'm more used to Vim than to Emacs. Yi sounds like something I might like. Is it stable enough to solve more problems than it would create? (I hate buggy and broken stuff)