ghcid

Either "GHCi as a daemon" or "GHC + a bit of an IDE". Unlike other Haskell development tools, ghcid is intended to be incredibly simple. In particular, it doesn't integrate with any editors, doesn't depend on GHC the library and doesn't start web servers.

Using it

Run cabal update && cabal install ghcid to install it as normal. Then run ghcid --height=10 "--command=ghci Main.hs". The height is the number of lines you are going to resize your console window to (defaults to 8), and the command is how you start this project in ghci. Personally, I always create a .ghci file at the root of all my projects, which usually reads something like:

:set -fwarn-unused-binds -fwarn-unused-imports
:load Main

If you have that, then you can pass --command=ghci (or nothing, since that is the default).

After that, resize your console and make it so you can see it while working in your editor. On Windows the console will automatically sit on top of all other windows. On Linux, you probably want to use your window manager to make it topmost or use a tiling window manager.

What you get

On every save you'll see a list of the errors and warnings in your project. It uses ghci under the hood, so even relatively large projects should update their status pretty quickly. As an example:

Main.hs:23:10:
    Not in scope: `verbosit'
    Perhaps you meant `verbosity' (imported from System.Console.CmdArgs)
Util.hs:18:1: Warning: Defined but not used: `foo'

Or, if everything is good, you see:

All good

Please report any bugs you find.