Like many people, I use Emacs to edit my Haskell code. The standard haskell-mode works fine, but I always felt like I could use a bit more help from my editor. I recently ran across scion, which is something like a Haskell IDE library that provides deep information about programs to editors. It also happens to include Emacs integration.
A few notable features:
- In-buffer error and warning highlighting
- Expression typechecking
- Completion of
LANGUAGEpragmas - Go-to-definition of symbols
It does all of this by using the Cabal library to determine your project settings and then invoking the GHC API to compile everything. The information is very accurate and compilations after the initial setup are too fast to notice. It re-checks your code every time you save, so feedback is also basically immediate.
I really like scion so far. I had one project that was fairly complex with a fancy custom build system involving a configure script -- this proved to be too complicated for scion to handle correctly. It ended up having some linking issues with some FFI calls I was using, so I split those out into a separate package so I could use scion with my main project.