Either "GHCi as a daemon" or "GHC + a bit of an IDE". To a first approximation, it opens
ghci and runs
:reload whenever your source code changes, formatting the output to fit a fixed height console. 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.
Acknowledgements: This project incorporates significant work from JPMoresmau, who is listed as a co-author.
cabal update && cabal install ghcid to install it as normal. Then run
ghcid --height=8 --topmost "--command=ghci Main.hs". The
height is the number of lines you are going to resize your console window to (defaults to height of the console). The
topmost is to make the window sit above all others, which only works on Windows. The
command is how you start your project in
ghci. If you omit
--command then it will default to
stack ghci if you have the
stack.yaml file and
.stack-work directory, default to
ghci if you have a
.ghci file in the current directory, and otherwise default to
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
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:
Please report any bugs you find.
There are a few tools that integrate Ghcid into editors, see the plugins folder for currently supported integrations.
- This isn't as good as full IDE. I've gone for simplicity over features. It's a point in the design space, but not necessarily the best point in the design space for you. For "real" IDEs see the Haskell wiki.
- If I delete a file and put it back it gets stuck. Yes, that's a bug in GHCi. If you see GHCi getting confused just kill
ghcidand start it again.
- I want to run my tests when files change. You can pass any
ghciexpression with the
--test=:main, which will be run whenever the code is warning free (or pass
--warningsfor when the code is merely error free).
- I want to run arbitrary commands when arbitrary files change. This project reloads
ghciwhen files loaded by
ghcichange. If you want a more general mechanism something like Steel Overseer will probably work better.
- I want syntax highlighting in the error messages. One option is to use Neovim or Emacs and run the terminal in a buffer whose file type is set to Haskell.
- I'm not seeing pattern matching warnings. Ghcid automatically appends
-fno-codeto the command line, which makes the reload cycle about twice as fast. Unfortunately GHC 8.0 suffers from bug 10600 which means
-fno-codealso disables pattern matching warnings. Until that GHC bug is fixed either accept no pattern match warnings or use
-cto specify a command line to start
ghcithat doesn't include