GHCID - a new GHCi based IDE (ish)

Summary: I've just released ghcid, which interactively shows the errors in your project on each save.

I'm please to announce ghcid, which is either "GHCi as a daemon" or "GHC + a bit of an IDE". I've been using it for my development for the last few days, and already find it quite valuable. 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. It's under 200 lines of fairly dull Haskell, which talks over pipes to your existing ghci.

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
:set -isrc
:load Main

With that 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 ghcid 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 a single 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

This project is only a few days old, so please report any bugs you find.

What you want

I regularly use an IDE to develop in a Haskell-like language. I find that with the IDE I'm about 25% more productive than without it. While an IDE can provide lots of neat features (go to definition, search, type tooltips) I think most of the productivity gains come from:

1. Syntax coloring.
2. A list of errors and warnings...
3. ...which is updated as you type...
4. ...and are highlighted in the text (red squiggles).

Every text editor already provides syntax coloring. With ghcid you get the list of errors and warnings. To get the final two features you need to integrate with an editor. I'm hoping that ghcid can do some of the heavy lifting by taking a directory of files to treat as overrides, and producing a list of warnings/errors to a file. The rest is editor specific, and I hope to attempt integration with Sublime Text at some point (although would love some help).