ghcid

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.

Using it

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.

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.

Editor integration

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

• VS Code
• nvim
• Emacs

Usage tips

• 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.

FAQ

• 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 ghcid and start it again.
• I want to run my tests when files change. 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).
• I want to run arbitrary commands when arbitrary files change. This project reloads ghci when files loaded by ghci change. If you want a more general mechanism something like Steel Overseer or Watchman 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. Another option is to pipe ghcid through source-highlight (ghcid | source-highlight -s haskell -f esc).
• I'm not seeing pattern matching warnings. 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.
• I get "During interactive linking, GHCi couldn't find the following symbol". 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.
• I only see source-spans or error messages on errors/warnings after the first load. 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".
• I want to match on the file/line/column to get jump-to-error functionality in my editor. 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]*).