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 provide access to the ghci it starts, 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.

Run stack install ghcid or cabal update && cabal install ghcid to install it as normal. Then run ghcid "--command=ghci Main.hs". 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 cabal repl.

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 you may wish to pass --topmost so the console will 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.

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.

There are a few plugins that integrate Ghcid into editors, notably:

• VS Code
• nvim
• vim
• Emacs

• If you have a .ghcid file in the current folder, or a parent folder, the contents of that file will be used as command line arguments. For example, if you always want to pass --command=custom-ghci then putting that in a .ghcid file will free you from writing it each time.
• There is an article on auto-reloading threepenny-gui apps during development.
• There are a list of general tips for using Ghcid.

In general, to use ghcid, you first need to get ghci working well for you. In particular, craft a command line or .ghci file such that when you start ghci it has loaded all the files you care about (check :show modules). If you want to use --test check that whatever expression you want to use works in that ghci session. Getting ghci started properly is one of the hardest things of using ghcid, and while ghcid has a lot of defaults for common cases, it doesn't always work out of the box.

Using the ghci session that ghcid manages you can also evaluate expressions:

• You can pass any ghci expression with the --test flag, e.g. --test=:main, which will be run whenever the code is warning free (or pass --warnings for when the code is merely error free).

• If you pass the --allow-eval flag then comments in the source files such as -- $> expr will run expr after loading - see this blog post for more details. Multiline comments are also supported with the following syntax:

  {- $>
  expr1
  expr2
  ...
  exprN
  <$ -}
  

Expressions that read from standard input are likely to hang, given that Ghcid already uses the standard input to interact with Ghci.

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. Other points in the design space include:

• ghcide - a real IDE in your editor.
• reflex-ghci - like Ghcid but with more terminal UI features.
• reflex-ghcide - a full IDE in the terminal.

Yes, that's a bug in GHCi. If you see GHCi getting confused just kill ghcid and start it again.

This project reloads ghci when files loaded by ghci change. If you want a more general mechanism, consider:

• Steel Overseer (Hackage)
• Watchman
• feedback (Hackage)
• Watchexec
• entr

One option is to use Neovim or Emacs and run the terminal in a buffer whose file type is set to Haskell. Another option is to pipe ghcid through source-highlight (ghcid | source-highlight -s haskell -f esc).

Ghcid automatically appends -fno-code to the command line, which makes the reload cycle about twice as fast. Unfortunately GHC 8.0 and 8.2 suffer from bug 10600 which means -fno-code also disables pattern matching warnings. On these versions, either accept no pattern match warnings or use -c to specify a command line to start ghci that doesn't include -fno-code. From GHC 8.4 this problem no longer exists.

This problem is a manifestation of GHC bug 8025, which is fixed in GHC 8.4 and above. Ghcid automatically appends -fno-code to the command line, but for older GHC's you can supress that with --test "return ()" (to add a fake test) or -c "ghci ..." to manually specify the command to run.

Due to limitations in ghci, these flags are only set after the first load. If you want them to apply from the start, pass them on the command line to ghci with something like -c "ghci -ferror-spans -fdiagnostics-color=always".

You will variously see file:line:col:message, file:line:col1-col2:msg and file:(line1,col1)-(line2,col2):message, as these are the formats GHC uses. To match all of them you can use a regular expression such as ^(\\S*?):(?|(\\d+):(\\d+)(?:-\\d+)?|\\((\\d+),(\\d+)\\)-\\(\\d+,\\d+\\)):([^\n]*).

You can let ghcid print more with --no-height-limit. The first error message might end up outside of the console view, so you can use --reverse-errors to flip the order of the errors and warnings. Further error messages are just a scroll away. Finally if you're going to be scrolling, you can achieve a cleaner experience with the --clear flag, which clears the console on reload.

Ghcid only notices when the .hs files change. To make it respond to other files you can pass the .x and .y files to --restart, e.g. --restart=myparser.y. As long as you set the initial command to something that runs Happy/Alex (e.g. cabal repl) then when those files change everything will restart, causing the initial command to be rerun.

ghcid ... --test Main.main --setup ":set args myargs"

The Mac has a fairly low limit on the number of file handles available. You can increase it with: sudo sysctl -w fs.inotify.max_user_watches=262144; sudo sysctl -p